The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

Although desert ecosystems are predicted to be the most responsive to elevated CO₂, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (～375 ppm) and elevated (～517 ppm) CO₂ concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO₂ treatments to determine responses of primary and lateral roots to an increase in CO₂. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO₂. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO₂ and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO₂. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO₂, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO₂ for these species does not appear to be constrained by the plant's root growth or activity.     

Pattern and mortality in Colorado Desert plants

Summary We tested for intraspecific interference among Colorado Desert shrubs using an integrated analysis of spatial pattern and juvenile mortality. The data set included 7,000 woody perennials of 24 species in a mapped hectare of Joshua Tree National Monument, California. The site is dominated by Ambrosia dumosa (62.0% of the stems), with Larre tridentata a conspicuous secondary species (2.3% of the stems). Analyses of static pattern for common species showed: (1) aggregated adults and juveniles for Ambrosia dumosa, Erigonum fasciculatum, Mirabilis bigelovii, and Sphaeralcea ambigua, with more aggregation among juveniles than adults; (2) randomly distributed adults and juveniles for Krameria grayi, Opuntia rasmosissima, Simondsia chinensis, and Yucca schidigera. The summed volumes and distances between nearest conspecific neighbors were positively correlated for Ambrosia dumosa and Larrea tridentata, but not significantly correlated for eight remaining species with 100 individuals. Static pattern suggests only weak evidence for negative interactions in Ambrosia and Larrea, and little evidence for other species. Alternative mechanisms other than negative interaction that could give rise to these static patterns are discussed. Juvenile mortality was documented for four common species (Ambrosia dumosa, Eriogonum fasciculatum, Mirabilis bigelovii, and Sphaeralcea ambigua) that experienced substantial mortality. Analyses show: (1) the proportion of individuals that died was independent of the initial density of conspecifics; (2) distance to conspecific adults did not differ for juveniles that died versus those that survived; and (3) death was no more likely for juveniles that contacted other plants than for those that were isolated. The exception was a vine, Mirabilis bigelovii, whose juveniles survived better in contact with other plants. In sum, neither spatial pattern nor patterns of mortality showed clear evidence of negative intraspecific interference.     

Identity recognition and plant behavior

The ability to distinguish self from nonself allows organisms to protect themselves against attackers. Sagebrush plants use volatile cues emitted by clipped neighbors to adjust their defenses against herbivores. Recently, we reported that cues from genetically identical ‘self’ clones were more effective at reducing damage than were cues from ‘nonself’ clones. This indicates that plants can distinguish self from non-self through volatiles and respond differentially. Identity recognition may be an essential step in enabling plants to behave cooperatively. Emission of cues which enable other plant tissues (on the same or other individual) to respond appropriately to herbivore risk may have evolved if cues are aimed primarily at self tissue.Key words: communication, eavesdropping, herbivory, kin recognition, self/nonself, volatilesThe ability to recognize self from nonself is a fundamental property of individuals of all multicellular organisms. Distinguishing between molecules that are part of one''s own tissues and those of an invader provides a first step towards the evolution of a functioning immune system. An immune system responds differently towards self and nonself tissues, destroying the later. In addition to immune responses, many other sophisticated behaviors have been described for animals that differentiate self from non-self and even kin from strangers.¹ For example, social behaviors including altruism can be favored by natural selection when animals are able to first distinguish kin from non-kin and respond differently to individuals in these two categories.² Although plant behavior is far less well studied, plants too display many sophisticated and context-dependent behaviors.³Plant biologists have described various situations in which plants exhibit different behaviors based on identity. It has been known for some years that many angiosperms choose mates based on genetic identity.⁴ Numerous mechanisms have been described, primarily involving differential germination of pollen, growth of pollen tubes through stigmatic tissue, and production of competent zygotes. More recently, several workers have found that plants may differentiate self from non-self and alter their morphologies in response to cues from these two types of sources. Plants appeared to recognize their own roots and to grow fewer and shorter roots when they contacted self roots compared to non-self roots (reviewed in references ^5–7). A common feature of these experimental studies is that roots only showed self-recognition when they were physically attached. These experimental studies may be subject to alternate explanations.^8,9Recently we reported that sagebrush plants induced resistance more effectively against their herbivores in response to the volatile cues emitted by self clones compared to the cues of non-self clones.¹⁰ We had previously found that experimental clipping to branches caused systemic induced resistance within an individual against herbivores only when volatile cues were transmitted.¹¹ To evaluate self/non-self discrimination we first produced clones of 60 parent plants in the field by root crown division. These potted clones were propagated and then placed back in the field near either their genetically identical parent (self treatment) or a genetically different parent (nonself treatment). The potted clones were experimentally clipped in spring for both treatments and the damage that accumulated over the growing season was recorded for parents near self and non-self clones. We found that plants near clipped self clones received approximately 42% less damage by their herbivores than plants near clipped non-self clones (Fig. 1, One-way ANOVA, F_1,58 = 8.72, p = 0.005).Open in a separate windowFigure 1The mean number of leaves that were damaged by herbivores (grasshoppers, caterpillars and deer) on assay branches of sagebrush (±1 se). Cuttings were either genetically identical (self) or different (non-self) from the assay branch; assay branches were within 5 cm of potted cuttings but not in physical contact. Cuttings were experimentally clipped to simulate herbivory in May and herbivore damage accumulated on the assay branches until season''s end in September when damage was assayed.This result is novel in several ways. Past results showing self/nonself recognition between roots required that they be in physical contact for discrimination to occur; physical contact was not required in this case. In addition, this is the first identity study to measure responses in terms of damage by herbivores rather than plant morphology or reproduction. This result is more robust than the changes in root morphology because changes attributed to self or non-self volatiles cannot be explained by alternative hypotheses involving potentially confounding differences in resource availability or pot size.^8,9 The ability of plants to differentiate self from non-self is important because it may enable differential treatment towards ramets that share genes.Recent work has also suggested that plants may be able to discriminate between kin and strangers. Cakile edentula and Impatiens pallida changed their morphologies depending upon whether their roots contacted kin or strangers.^12,13 These altered morphologies were consistent with the notion that kin cooperated and non-kin competed. Examination of self/non-self recognition and kin/stranger recognition patterns in Arabidopsis thaliana indicated that these two forms of identity discrimination were affected differently by inhibitors and therefore suggested that they may involve different signaling mechanisms.¹⁴Plants that emit volatile cues that other individuals can use to adjust their defenses (eavesdropping) may be at a selective disadvantage.¹⁵ Why should a plant dispense information that allows its neighbors to fine tune their defenses against herbivory? One possible answer to this conundrum may be that plants emit volatile cues to coordinate their own defenses since volatile cues are active over relatively short distances. A second possible answer is that greater sensitivity to self volatiles reduces the cost of eaves-dropping. In designing our sagebrush experiment we cloned plants as a means of producing physically separate pairs of plants that were either genetically identical or different. Early genetic work indicated that populations of sagebrush were highly structured genetically.¹⁶ In other words, relatedness decreased as a function of the distance between individuals, also known as population viscosity. Recent genetic analyses of microsatellites indicate that vegetative reproduction by rhizomes also occurs in this species and some neighbors in nature are genetically identical (Ishizaki, et al. in review). Population viscosity has been considered to increase the likelihood of cooperation, in part because neighbors already share genes.^2,17 Applying similar logic, communication is facilitated by kin recognition if relatives are better able to communicate than non-kin. Communication may be favored if the tissue emitting cue is surrounded by primarily self tissue or if the exchange of cues is more effective and likely to occur between self tissues. In conclusion, plant communication using volatile cues may have evolved because individual plants were communicating primarily with themselves.     

Nonself help: how immunology might reframe the Enlightenment

The classical immunological paradigm is predicated on the body's ability to recognize and eliminate “nonself.” However, the “self–nonself” model has yet to facilitate any resolution of the field's major concerns, and may thus prove to be of limited use. Merely discarding it is no solution, as the juxtaposition of “self” and “nonself” persists in research, in clinical settings, and in everyday practice despite the best efforts of theoretical immunologists. Instead, the very conception of “selfhood” may prove to be key. Replacing immunology's prior and persistent “self” with less static concepts derived from non-Western contexts not only resolves immunology's famous paradoxes but also offers a new and more accurate model that allows immunology to reframe what may become an outmoded Enlightenment construct of “self.” In such a new paradigm, immunology's well-known system of protection and defense is replaced with a view in which nonself becomes less the body's enemy than its primary mechanism for the creative assimilation of difference. This incorporative model—in which the “immune system” functions more as a search engine than as an expeller of difference—both resolves outstanding paradoxes, and complies more accurately with contemporary knowledge and research practice.     

Vulnerability to cavitation in populations of two desert species, Hymenoclea salsola and Ambrosia dumosa, from different climatic regions

Seeds from different populations of two desert species, Hymenocleasalsola Torr. and A. Gray and Ambrosia dumosa (A. Gray) Payne,were collected along a climatic gradient, germinated in a greenhouseand the plants tested for their vulnerability to cavitationby the air-pressure method. Differences among populations wereevident in A. dumosa, but not in H. salsola. Greenhouse treatmentssimulating regimes in temperature and relative humidity encounteredin different desert environments did not cause appreciable changesin vulnerability to cavitation. It is suggested that a homeostaticmechanism may have helped in maintaining a constant water potentialdrop in the xylem with little need for an adjustment in theresistance to cavitation. Different plant organs had differentvulnerabilities to cavitation, with roots being the most andwoody stems the least susceptible. Young green twigs were intermediate.A simulation model confirmed that low water potentials are mostlikely to cause runaway cavitation in the roots, not in theother organs. It is hypothesized that green twigs are adaptedto the favourable water conditions of the growing season, whilewoody stems are adapted to endure prolonged periods of droughtstress. Key words: Cavitation, xylem embolism, hydraulic conductance, Hymenoclea salsola, Ambrosia dumosa     

Relationship between root elongation rate and diameter and duration of growth of lateral roots of maize

The objective of this work was to describe the relationship between elongation rate and diameter of maize roots and to estimate the length and growth duration of lateral roots of maize. Diameters and elongation rates of roots were measuredin situ on plants grown 5 weeks in small rhizotrons under greenhouse conditions. At the end of the experimental period the roots were harvested and diameters of axile and lateral roots were measured. The frequency distribution of diameters of harvested roots was bimodal with a minimum at 0.6 mm; 97% of axile roots were larger than this value and 98% of the lateral roots were smaller. Root elongation per day increased as diameter increased but the slope of the relationship with lateral roots was about 2.5 times that with axile roots when separate linear regressions were fitted to the two populations. The length of lateral roots found on axillary roots between the base and about 30 cm from the apex was approximately 2.2 cm. All of the data was consistent with the hypothesis that the lateral roots grew for about 2.5 days and then ceased growing. The axillary roots continued to grow throughout the experimental period at a rate of about 3 cm day⁻¹. Contribution from the Department of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Agronomy paper No. 1661. This research is part of the program of the Center for Root-Soil Research.     

Elevated auxin and reduced cytokinin contents in rootstocks improve their performance and grafting success

Plant grafting is an important technique for horticultural and silvicultural production. However, many rootstock plants suffer from undesirable lateral bud outgrowth, low grafting success rates or poor rooting. Here, we used a root‐predominant gene promoter (SbUGT) to drive the expression of a tryptophan‐2‐monooxygenase gene (iaaM) from Agrobacterium tumefaciens to increase auxin levels in tobacco. The transgenic plants, when used as a rootstock, displayed inhibited lateral bud outgrowth, enhanced grafting success rate and improved root initiation. However, root elongation and biomass of SbUGT::iaaM transgenic plants were reduced compared to those of wild‐type plants. In contrast, when we used this same promoter to drive CKX (a cytokinin degradation gene) expression, the transgenic tobacco plants displayed enhanced root elongation and biomass. We then made crosses between the SbUGT::CKX and SbUGT::iaaM transgenic plants. We observed that overexpression of the CKX gene neutralized the negative effects of auxin overproduction on root elongation. Also, the simultaneous expression of both the iaaM and CKX genes in rootstock did not disrupt normal growth and developmental patterns in wild‐type scions. Our results demonstrate that expression of both the iaaM and CKX genes predominantly in roots of rootstock inhibits lateral bud release from rootstock, improves grafting success rates and enhances root initiation and biomass.     

BIOCHEMICAL AND MORPHOLOGICAL EFFECTS OF TEMPERATURE ON TYPHA LATIFOLIA L. (TYPHACEAE) ORIGINATING FROM DIFFERENT ENDS OF A THERMAL GRADIENT. I. CONTROLLED ENVIRONMENTAL STUDIES

T. latifolia grows along most of the shoreline of Par Pond, a cooling reservoir for the reactors at the Savannah River Plant in Aiken, South Carolina. A thermal gradient exists because of the movement of hot water from one end of the pond to the other. This study was undertaken to determine the biochemical and morphological effects of temperature on Typha latifolia clones originating from different ends of this thermal gradient. The data indicated that both “hot” and “cold” end plants react similarly to temperature. Forty-eight rhizome plants from both ends of Par Pond were transplanted into tanks at 20 C and 30 C. Total levels of malic dehydrogenase activity, proportions of mitochondrially bound MDH and growth rates were significantly different in plants grown in the two tanks, but no significant differences were observed within each tank between individuals from the “hot” and “cold” end. The relation of these results to hypotheses concerning quantitative strategies of enzyme adaptation are discussed in the paper.     

Effects of physical connection and genetic identity of neighbouring ramets on root-placement patterns in two clonal species

Root-placement patterns were examined in the clonal species Glechoma hederacea and Fragaria vesca when grown with different types of neighbours. Three different patterns were predicted as consequences of different types of interactions between roots: the avoidance pattern if root growth decreases in the presence of neighbouring roots; the intrusive pattern if root growth increases towards neighbouring roots; and the unresponsive pattern if root growth is unaffected by neighbouring roots. Experiments were conducted in which physical connection between ramets, and the genetic identity of neighbouring ramets, were manipulated. The patterns of distribution of entire root systems and elongation rates of individual roots were measured. Root systems and individual roots of G. hederacea avoided contact with roots of neighbouring ramets, irrespective of connection to the neighbour and its genetic or specific identity. In contrast, F. vesca roots grew equally towards and away from intraspecific ramet neighbours and their elongation was stimulated by contact with roots of G. hederacea ramets. These results demonstrate that root-placement patterns of plants grown with different types of neighbours vary between species, and suggest that factors additional to resource depletion could be involved in their development.     

Primary and Lateral Root Development of Dark- and Light-Grown Cotton Seedlings under Salinity Stress*

Primary root growth dynamics and lateral root development of dark- and light grown cotton seedlings (Gossypium hirsutum L., cv. Acala SJ-2) were studied under control and salinity stress conditions. The seedlings were grown by two methods: A) in paper-lined, vermiculite-filled beakers with the plants growing between the paper and the glass wall (Gladish and Rost, 1993), and B) in hydroponics after germination and initial growth in germination paper rolls saturated with the treatment solutions (Kent and Läuchli, 1985). After germination, daily primary root elongation rate gradually incrased to a maximum, then gradually declined to close to zero for dark-grown seedlings, or to sustained rates of about 10 mm per day for light-grown control plants. Salinity stress delayed primary root growth and reduced peak elongation rates, without changing the general primary root growth pattern. These results suggest that salinity changed the time-scale, but did not modify the normal developmental sequence. Lateral root growth was more inhibited by salinity than primary root growth. In addition, elongation of lateral roots was more inhibited by salinity than their initiation and emergence. Light exposure of the shoot favored both sustained primary root growth from 7 days after planting, and lateral root emergence and growth. Salinity effects were more severe on seedlings germinated and grown in hydroponics (method B) than on vermiculite-grown plants (method A). These results emphasize the importance of growing conditions for the NaCl-induced effects on cotton root development. In addition, the differential effects of salinity on primary and lateral roots became evident, pointing to diverse control mechanisms for the development of these root types.     

Autoreactive T‐cell receptor (Vβ/D/Jβ) sequences in diabetes are homologous to insulin,glucagon, the insulin receptor,and the glucagon receptor

The hypervariable (Vβ/D/Jβ) regions of T‐cell receptors (TCR) have been sequenced in a variety of autoimmune diseases by various investigators. An analysis of some of these sequences shows that TCR from both human diabetics and NOD mice mimic insulin, glucagon, the insulin receptor, and the glucagon receptor. Such similarities are not found in the TCR produced in other human autoimmune diseases. These data may explain how insulin, glucagon, and their receptors are targets of autoimmunity in diabetes and also suggest that TCR mimicking insulin and its receptor may be targets of anti‐insulin autoantibodies. Such intra‐systemic mimicry of self‐proteins also raises complex questions about how “self” and “nonself” are regulated during TCR production, especially in light of the complementarity of insulin for its receptor and glucagon for its receptor. The data presented here suggest that some TCR may be complementary to other TCR in autoimmune diseases, a possibility that is experimentally testable. Such complementarity, if it exists, could either serve to down‐regulate the clones bearing such TCR or, alternatively, trigger an intra‐immune system civil war between them. Copyright © 2008 John Wiley & Sons, Ltd.     

Root halotropism: Salinity effects on Bassia indica root

Abstract

Plant roots are responsible for the acquisition of nutrients and water from the soil and have an important role in plant response to soil stress conditions. The direction of root growth is gravitropic in general. Gravitropic responses have been widely studied; however, studies about other root tropisms are scarce. Soil salinity is a major environmental response factor for plants, sensed by the roots and affecting the whole plant. Our observations on root architecture of Kochia (Bassia indica) indicated that salinity may cue tropism of part of the roots toward increasing salt concentrations. We termed this phenomenon “positive halotropism”. It was observed that Kochia individuals in the field developed horizontal roots, originating from the main tap root, which was growing toward saline regions in the soil. Under controlled conditions in greenhouse experiments, Kochia plants were grown in pots with artificial soil salinity gradients, achieved by irrigation with saline and fresh water. It was shown that plants grown in low‐salt areas developed a major horizontal root toward the higher salt concentration region in the gradient. In regions of high salinity and in the absence of a salinity gradient, roots grew vertically without a major horizontal root. The novel finding of “positive halotropism” is discussed.     

Plastic,inquisitive roots and intelligent plants in the light of some new vistas in plant biology

Abstract

Metaphors, such as those used in the title of this article, are often useful for the comprehension of specialised topics in plant biology. A brief attempt is made to elucidate one of these metaphors, plant “intelligence”, as it relates to the plastic responses of roots and root systems to their environment. Tropisms and nastic movements of root apices are two expressions of an inherent plasticity of form exhibited by roots. In soil, roots are exposed to multiple stimuli, many of which can potentially elicit such movements. Hence, a key question is how roots respond to and process the different stimuli which simultaneously reach their surfaces. Assuming that roots always use the same site along their length to express their movement responses, and that they also use an auxin‐based information‐transduction pathway, the most evident choices for the processing of stimuli are that roots either prioritise the various incoming stimuli and respond only to the strongest or they amalgamate stimuli and mount an averaged compromise response to all of them. The proposal that plants may be “intelligent”, especially in respect to their plastic growth responses, is one that draws upon knowledge of this faculty from animal biology. Also implied is that plants and animals are sufficiently similar to share usage of this term “intelligence”. But an alternative view is that plants and animals are sufficiently different and so intelligence is an unfitting term. Following the line of enquiry into creative evolution initiated by Henri Bergson, plants can be viewed differently to animals. The tendency of plants is towards instinctive behaviour rather than intelligent behaviour.     

Penetration of very strong soils by seedling roots of different plant species

The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2 kg kg^–1 water content and mean penetrometer resistance of 4.2 MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).Differences in both root elongation and root diameter were observed among plant species. Generally, the roots of dicotyledons (with large diameters) penetrated the strong medium more than graminaceous monocotyledons (with smaller diameters). There was a significant positive correlation (r=0.78, p<0.05) between root diameter and elongation over all the species in the stressed plants. The species were ranked according to the relative root elongation and relative root thickening. Based on this ranking, lupin (Lupinus angustifolius), medic (Medicago scutelata) and faba bean (Vicia faba) were the species with the greatest thickening and elongation while wheat (Triticum aestivum), rhodesgrass (Chloris gayana) and barley (Hordeum vulgare) had the least. The weight of the seeds did not seem to influence either the thickening or elongation of the roots.     

Why are laterals less affected than main axes by homogeneous unfavourable physical conditions? A model-based hypothesis

When plants develop in strong soils, growth of the root system is generally depressed. However, branching and elongation of branches are often less affected than growth of the main axes, whenever the whole root system encounters even-impeded conditions. On the basis of a model simulating root growth and architecture as related to assimilate availability, we propose a simple hypothesis to explain such behaviour. In the model, growth of each root depends on its own elongation potential, which is estimated by its apical diameter. The potential elongation rate–apical diameter relationship is the same for all the roots of the system and is described by a monomolecular function. Our hypothesis is that the effect of soil strength can be simulated by introducing an impedance factor in the definition of root maximum potential elongation rate, common to the whole root system. When such impedance factor is applied, it affects more the potential of larger roots (main axes) than that of thinner roots (secondary and tertiary branches). Simulations provided in high impedance conditions led to root systems characterised by short taproots, whereas growth of secondary roots was unaffected and growth of tertiary roots was enhanced. Actual branching density was also higher, although branching rules have been unchanged. Such simulated systems where similar to that observed in strong soils. Friction laws or pore size can be involved in the larger reduction of the potential growth of main axes. Moreover, when growth of main axes is restricted, assimilate availability becomes higher for branches and that could explain that their growth could be increased in a homogeneous strong soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Periodicity in the development of the root system of young rubber trees (Hevea brasiliensis Müell. Arg.): relationship with shoot development

The growth pattern of the root system of young rubber trees (Hevea brasiliensis) was studied in relation to shoot development over a period of 3 months. Temporal and spatial variations in elongation and branching processes were examined for the different root types, by means of root observation boxes. Shoot growth was typically rhythmic. Root development was periodic and related to leaf expansion. Root elongation was depressed during leaf growth, whereas branching was enhanced. Consequently, highly branched areas with vigorous secondary roots alternated along the taproot with poorly branched areas with shorter roots. Root types were not affected to the same degree by shoot competition: during leaf expansion, taproot growth was just depressed but remained continuous, the emergence and elongation rates of secondary roots were significantly affected and the elongation rates of tertiary roots fell to zero. These results were consistent with the hypothesis that root growth is related to competition for assimilates and to the sink strength of the different root types, whereas root branching appeared to be promoted by leaf development.     

Quantitative aspects of T-cell recognition: from within the antigen-presenting cell to within the T cell

T lymphocytes circulate continually throughout the peripheral lymphoid organs, where they scrutinize the surface of cells to detect the presence of nonself protein fragments. During the last years, many facets of T-cell function have been unravelled. After being bound by major histocompatibility complex (MHC) molecules, peptides derived from nonself as well as from self proteins are delivered to the cell surface. A few copies of a nonself peptide “presented” at the cell surface in the context of an MHC molecule can be detected by specific T cells, and suffice to trigger T-cell activation. This paper reviews the requirements imposed on T cells to fulfill this exquisite sensitivity. BioEssays 20 :412–422, 1998.© 1998 John Wiley & Sons Inc.     

Links between root developmental traits and foraging performance

We designed a simple dynamic and stochastic architectural model with six parameters to link the foraging performance of root systems to their developmental processes. Soil foraging was quantified by the volume enveloping the roots until a given uptake distance. Many simulated architectures were obtained by combining four different values for each parameter. The rate of soil colonization was mainly defined by individual root elongation rates and interbranch distances. Less intuitively, we showed that differentiation of elongation rates among the roots increased this colonization rate. Uptake efficiency--the ratio of the actual colonized volume to the volume of a unique cylinder with the same length and a radius corresponding to the uptake distance--declined with root system size. Nevertheless, large variations in efficiency existed among root systems for a given size, typically in a 4- to 10-fold range. Therefore, the 'efficiency gain' was defined as the deviation from the average trend in efficiency versus size. Between-root differentiation in elongation rates increased this gain. The level of hierarchy between mother and lateral roots, as well as the variation of elongation rates among lateral roots, was also shown to contribute to this optimization. Several parameter combinations could lead to similar efficiency gains.     

Degenerate T-cell recognition of peptides on MHC molecules creates large holes in the T-cell repertoire

The cellular immune system screens peptides presented by host cells on MHC molecules to assess if the cells are infected. In this study we examined whether the presented peptides contain enough information for a proper self/nonself assessment by comparing the presented human (self) and bacterial or viral (nonself) peptides on a large number of MHC molecules. For all MHC molecules tested, only a small fraction of the presented nonself peptides from 174 species of bacteria and 1000 viral proteomes ([Formula: see text]0.2%) is shown to be identical to a presented self peptide. Next, we use available data on T-cell receptor-peptide-MHC interactions to estimate how well T-cells distinguish between similar peptides. The recognition of a peptide-MHC by the T-cell receptor is flexible, and as a result, about one-third of the presented nonself peptides is expected to be indistinguishable (by T-cells) from presented self peptides. This suggests that T-cells are expected to remain tolerant for a large fraction of the presented nonself peptides, which provides an explanation for the "holes in the T-cell repertoire" that are found for a large fraction of foreign epitopes. Additionally, this overlap with self increases the need for efficient self tolerance, as many self-similar nonself peptides could initiate an autoimmune response. Degenerate recognition of peptide-MHC-I complexes by T-cells thus creates large and potentially dangerous overlaps between self and nonself.     

Root elongation rate is accounted for by intercepted PPFD and source-sink relations in field and laboratory-grown sunflower

The existence of relationships between intercepted photo-synthetic photon flux density (PPFD) and growth of individual organs is somewhat controversial. We have tested whether such relationships could account for the natural variability in elongation rates of taproot and secondary roots of sunflower (from 2 to 135 mm d⁻¹), in field and laboratory conditions. Elongation of taproot and secondary roots was recorded daily through windows in the field. A range of PPFD was obtained by following day-to-day natural fluctuation for three contrasting growing periods, and by shading part of the plants under study. A parallel experiment was carried out in a growth chamber with contrasting light intensities and with a ¹⁴CO₂ labelling experiment. After the two-leaf stage, i.e. when the contribution of photosynthetic carbon became appreciable in root growth, daily root elongation rate was closely linked to the PPFD intercepted from 36 to 12 h before the measurement of root elongation. Curvilinear relationships applied to plants grown in the field as well as in a growth chamber, and to shaded plants as well as to plants subjected to day-to-day changes in intercepted PPFD. For a given intercepted PPFD, the taproot elongated faster than secondary roots, and secondary roots originating near the base of the taproot elongated faster than those originating near the apex. The elongation rate of any secondary root apex was accounted for (r= 0.77) by the ratio of intercepted PPFD to the distance between the apex and the base of the taproot. No relationships between intercepted PPFD and elongation rate were observed before the two-leaf stage, when the CO₂ labelling experiment suggests that carbon essentially originates from the seed. Therefore, this study suggests a role for source-sink relations in the distribution of elongation between apices and a role for carbon nutrition in day-to-day variations of root elongation rate. Precise mechanisms explaining this behaviour remain to be investigated.