The presence of nodules on legume root systems can alter phenotypic plasticity in response to internal nitrogen independent of nitrogen fixation

All higher plants show developmental plasticity in response to the availability of nitrogen (N) in the soil. In legumes, N starvation causes the formation of root nodules, where symbiotic rhizobacteria fix atmospheric N₂ for the host in exchange for fixed carbon (C) from the shoot. Here, we tested whether plastic responses to internal [N] of legumes are altered by their symbionts. Glasshouse experiments compared root phenotypes of three legumes, Medicago truncatula, Medicago sativa and Trifolium subterraneum, inoculated with their compatible symbiont partners and grown under four nitrate levels. In addition, six strains of rhizobia, differing in their ability to fix N₂ in M. truncatula, were compared to test if plastic responses to internal [N] were dependent on the rhizobia or N₂‐fixing capability of the nodules. We found that the presence of rhizobia affected phenotypic plasticity of the legumes to internal [N], particularly in root length and root mass ratio (RMR), in a plant species‐dependent way. While root length responses of M. truncatula to internal [N] were dependent on the ability of rhizobial symbionts to fix N₂, RMR response to internal [N] was dependent only on initiation of nodules, irrespective of N₂‐fixing ability of the rhizobia strains.     

The soybean (Glycine max) nodulation‐suppressive CLE peptide,GmRIC1, functions interspecifically in common white bean (Phaseolus vulgaris), but not in a supernodulating line mutated in the receptor PvNARK

Legume plants regulate the number of nitrogen‐fixing root nodules they form via a process called the Autoregulation of Nodulation (AON). Despite being one of the most economically important and abundantly consumed legumes, little is known about the AON pathway of common bean (Phaseolus vulgaris). We used comparative‐ and functional‐genomic approaches to identify central components in the AON pathway of common bean. This includes identifying PvNARK, which encodes a LRR receptor kinase that acts to regulate root nodule numbers. A novel, truncated version of the gene was identified directly upstream of PvNARK, similar to Medicago truncatula, but not seen in Lotus japonicus or soybean. Two mutant alleles of PvNARK were identified that cause a classic shoot‐controlled and nitrate‐tolerant supernodulation phenotype. Homeologous over‐expression of the nodulation‐suppressive CLE peptide‐encoding soybean gene, GmRIC1, abolished nodulation in wild‐type bean, but had no discernible effect on PvNARK‐mutant plants. This demonstrates that soybean GmRIC1 can function interspecifically in bean, acting in a PvNARK‐dependent manner. Identification of bean PvRIC1, PvRIC2 and PvNIC1, orthologues of the soybean nodulation‐suppressive CLE peptides, revealed a high degree of conservation, particularly in the CLE domain. Overall, our work identified four new components of bean nodulation control and a truncated copy of PvNARK, discovered the mutation responsible for two supernodulating bean mutants and demonstrated that soybean GmRIC1 can function in the AON pathway of bean.     

The ecological significance of plasticity in root weight ratio in response to nitrogen: Opinion

We analyzed data on root weight ratio from a range of experimental studies documenting plant allocation changes in response to altered nitrogen availability. Our goal was to determine the degree to which plasticity in allocation between roots and shoots exists and to search for patterns in such plasticity among species. Our survey included 77 studies representing 206 cases and 129 species. As expected, we found that root weight ratio decreased with increased nitrogen availability in the majority of cases examined, and this response was most consistent when plants were grown individually or in intraspecific competition (versus interspecific competition). Surprisingly, however, we found no evidence to support existing hypotheses that fast-growing species adapted to high soil fertilities exhibit the highest levels of morphological plasticity, or that plasticity is positively associated with competitive ability. Rather, we found that average amounts of plasticity in root weight ratio in response to nitrogen availability were similar among species grouped by maximum relative growth rate and habitat fertility. Similar results were obtained for species categorized by life form, life history or root weight ratio itself, and plasticity in root weight ratio also had no consistent relationship with competitive ability. Numerous difficulties are associated with the attempt to search for pattern using independent studies, however our results lead to the conclusion that strong patterns in plasticity of root weight ratio in response to nitrogen availability among species do not exist. We discuss two reasons for this: (1) the costs of plasticity relative to its benefits are lower than previously predicted and (2) plasticity in traits other than root weight ratio is more important to plant foraging ability.     

Above-ground nitrogen dynamics following the complete pruning of a nodulated woody legume in humid tropical field conditions

Above-ground nitrogen fluxes following a complete pruning of the neotropical agroforestry tree Erythrina poeppi-giana (Walpers) O.F. Cook (Leguminosae: Phaseoleae) were studied during pruning intervals of 26 and 23 weeks in the humid tropics of Costa Rica. The experiment included four clones and a half-sib family of the species. The leaf-level N fluxes were modelled as a function of leaf age, and were scaled to the canopy level on the basis of the leaf age distribution. The N content of young leaves increased exponentially, followed by relatively small changes at maturity. Slight net N retranslocation started at the middle of leaf life span, but a major N efflux occurred only a few days before leaf shedding. It was estimated that, depending on the tree source, 29-44& of the total N accumulation in the above-ground biomass occurred during 11 weeks after pruning (WAP), before initiation of nodulation. The nodule biomass peaked at the time leaf shedding started (15–17 WAP). Considerable rapid N efflux from the foliage was observed at this time, suggesting regulation of nodulation by a feedback from the foliage. The implications of the N economy of E. poep-pigiana on its management in agroforestry systems are discussed.     

Signalling mechanisms underlying the morphological responses of the root system to nitrogen in Arabidopsis thaliana

Plants display considerable developmental plasticity in response to changing environmental conditions. The adaptations of the root system to variations in N supply are an excellent example of such developmental plasticity. In Arabidopsis, four morphological adaptations to the N supply have been characterized: (i) a localized stimulatory effect of external nitrate on lateral root elongation; (ii) a systemic inhibitory effect of high tissue nitrate concentrations on the activation of lateral root meristems; (iii) a suppression of lateral root initiation by high C:N ratios, and (iv) an inhibition of primary root growth and stimulation of root branching by external L-glutamate. These responses have provided valuable experimental systems for the study of N signalling in plants. This article will highlight some recent progress made in this direction from studies using the Arabidopsis root system. One recent development of note has been the emerging evidence of a regulatory role of nitrate transporters in some of the responses. It has been reported that the AtNRT1.1 (CHL1) dual-affinity nitrate transporter acts upstream of the ANR1 MADS box gene in mediating the stimulatory effect of a localized nitrate supply on lateral root proliferation. The AtNRT2.1 high-affinity nitrate transporter seems to be involved in the repression of lateral root initiation by high C:N ratios. The systemic inhibitory effect of high nitrate supply on lateral root development, which is mediated by abscisic acid (ABA), may be linked to the recently identified ABA receptor, FCA. The newly discovered root architectural response to external L-glutamate potentially offers a valuable experimental tool for studying the biological function of plant glutamate receptors and amino acid signalling.     

Contrasting responses of legume versus non-legume shrubs to soil water and nutrient shortages in the Mu Us Sandland

Aims Legumes and non-legumes usually differ in using soil water and nutrients. Both water and nutrients are scarce in the semi-arid Mu Us Sandland where legume and/or non-legume shrubs coexist/dominate. Here, we addressed the responses of legume versus non-legume shrubs to different soil water and nutrient conditions.Methods We conducted an experiment in which a legume (Hedysarum laeve) and a non-legume (Artemisia ordosica) were used, both of which are dominant species in the Mu Us Sandland. Seedlings of these two species were subjected to three water levels (45.0, 67.5 and 90.0 ml every 3 days) and three nutrient treatments (0, 0.1% and 0.2% nutrient solution every week) during the experiment.Important findings Interactions between water and nutrients on total biomass, root weight ratio and rain use efficiency (RUE) were detected in A. ordosica but not in H. laeve, suggesting that water effects on A. ordosica but not on H. laeve are dependent on soil nutrients. Nutrient addition alleviated drought stress and increased RUE in A. ordosica. The interspecific differences in response to soil water and nutrients may be linked to the ability of plants to fix nitrogen. In addition, under low-soil water or nutrient conditions, H. laeve produced more biomass than A. ordosica, and the opposite was the case under high-soil resources. The relationship between relative growth rate (RGR) and RUE [or nutrient use efficiency (NUE)] varied with two species. RGR of A. ordosica was positively correlated with both RUE and NUE while RGR of H. laeve was negatively correlated with NUE. The different responses may be linked to the trade-off between high-growth rate and low-resource use efficiency.     

Environmental stress and the costs of whole-organism phenotypic plasticity in tadpoles

Costs of phenotypic plasticity are important for the evolution of plasticity because they prevent organisms from shaping themselves at will to match heterogeneous environments. These costs occur when plastic genotypes have relatively low fitness regardless of the trait value expressed. We report two experiments in which we measured selection on predator-induced plasticity in the behaviour and external morphology of frog tadpoles (Rana temporaria). We assessed costs under stressful and benign conditions, measured fitness as larval growth rate or competitive ability and focused analysis on aggregate measures of whole-organism plasticity. There was little convincing evidence for a cost of phenotypic plasticity in our experiments, and costs of canalization were nearly as frequent as costs of plasticity. Neither the magnitude of the cost nor the variation around the estimate (detectability) was sensitive to environmental stress.     

Variation in the degree and costs of adaptive phenotypic plasticity among Rana temporaria populations

Adaptive phenotypic plasticity in the form of capacity to accelerate development as a response to pond drying risk is known from many amphibian species. However, very little is known about factors that might constrain the evolution of this type of plasticity, and few studies have explored to what degree plasticity might be constrained by trade-offs dictated by adaptation to different environmental conditions. We compared the ability of southern and northern Scandinavian common frog (Rana temporaria) larvae originating from 10 different populations to accelerate their development in response to simulated pond drying risk and the resulting costs in metamorphic size in a factorial laboratory experiment. We found that (i) northern larvae developed faster than the southern larvae in all treatments, (ii) a capacity to accelerate the response was present in all five southern and all five northern populations tested, but that the magnitude of the response was much larger (and less variable) in the southern than in the northern populations, and that (iii) significant plasticity costs in metamorphic size were present in the southern populations, the plastic genotypes having smaller metamorphic size in the absence of desiccation risk, but no evidence for plasticity costs was found in the northern populations. We suggest that the weaker response to pond drying risk in the northern populations is due to stronger selection on large metamorphic size as compared with southern populations. In other words, seasonal time constraints that have selected the northern larvae to be fast growing and developing, may also constrain their innate ability for adaptive phenotypic plasticity.     

Signaling mechanisms integrating root and shoot responses to changes in the nitrogen supply

During their life cycle, plants must be able to adapt to wide variations in the supply of soil nitrogen (N). Changes in N availability, and in the relative concentrations of NO₃ ⁻and NH₄ ⁺, are known to have profound regulatory effects on the N uptake systems in the root, on C and N metabolism throughout the plant, and on root and shoot morphology. Optimising the plant’s responses to fluctuations in the N supply requires co-ordination of the pathways of C and N assimilation, as well as establishment of the appropriate allocation of resources between root and shoot growth. Achieving this integration of responses at the whole plant level implies long-distance signaling mechanisms that can communicate information about the current availability of N from root-to-shoot, and information about the C/N status of the shoot in the reverse direction. In this review we will discuss recent advances which have contributed to our understanding of these long-range signaling pathways.     

Plant responses to simulated warming and drought: a comparative study of functional plasticity between congeneric mid and high elevation species

Aims Effects of climate change, especially changes in temperatures and precipitation patterns, are particularly pronounced in alpine regions. In response, plants may exhibit phenotypic plasticity in key functional traits allowing short-term adjustment to novel conditions. However, little is known about the degree of phenotypic plasticity of high elevation species relative to mid elevation congeners.Methods We transplanted 14 herbaceous perennial species from high elevation into two common gardens (1050 and 2000 m.a.s.l.) in the Swiss Alps, and we examined plastic responses in key functional traits to changes in temperature and soil water availability. This design was replicated with 14 congeneric species from mid elevation to assess if the degree of phenotypic plasticity differs between mid and high elevation species. Survival was assessed across two growing seasons, while aboveground biomass and specific leaf area (SLA) were measured after the first growing season, and biomass allocation to belowground and reproductive structures after the second. Moreover, a phenotypic plasticity index was calculated for the functional traits to compare the degree of plasticity between mid and high elevation species.Important findings Aboveground biomass was higher in mid elevation species relative to high elevation congeners in all treatments, yet decreased for both with elevation and drought. Similarly, SLA decreased with elevation and drought. Root mass fraction (RMF) was generally higher in high elevation species, and decreased with drought at the lower site. Drought increased the allocation to reproductive structures, especially when plants were grown at their elevation of origin. Interestingly, no difference was found in the degree of phenotypic plasticity averaged across mid and high elevation species for any of the studied functional traits. These results indicate that phenotypic plasticity in the focal traits did not depend on the elevation of origin of the species. Plasticity was not related to environmental heterogeneity, nor constrained by selective pressures at high elevation. However, both species groups showed a remarkable capacity for short-term acclimation to a prospective climate through rapid adjustments in key functional traits.     

Borate promotes the formation of a complex between legume AGP‐extensin and Rhamnogalacturonan II and enhances production of Rhizobium capsular polysaccharide during infection thread development in Pisum sativum symbiotic root nodules

The capacity to bind to biomolecules is considered to be the basis for any physiological role of boron (B). Legume arabinogalactan protein‐extensin (AGPE), a major component of the infection thread matrix of legume nodules is a potential B‐ligand. Therefore, its role in infection threads development was investigated in Pisum sativum grown under B deficiency. Using the AGPE‐specific antibody MAC265, immunochemical analysis revealed that a 175 kDa MAC265 antigen was abundant in +B but much weaker in –B nodule extracts. A B‐dependent complex involving AGPE and rhamnogalacturonan II (RGII) could be co‐purified using anti‐RGII antiserum. Following fractionation of –B nodules, MAC265 antigens were mostly associated with the bacterial pellet. Immunogold staining confirmed that AGPE was closely associated with the surface of rhizobia in the lumen of threads in ?B nodules whereas in +B nodules, AGPE was separated from the bacterial surface by a sheath of capsular polysaccharide. Interestingly, colonies of rhizobia grown in free‐living culture without B developed low capsule production. Therefore, we propose that B could be important for apical growth of infection threads by strengthening thread wall through a B‐dependent AGPE‐RGII interaction and by promoting bacterial advance through a B‐dependent production of a stable rhizobial capsule that prevents AGPE attachment.     

Rapid adaptation to a novel light environment: The importance of ontogeny and phenotypic plasticity in shaping the visual system of Nicaraguan Midas cichlid fish (Amphilophus citrinellus spp.)

Colonization of novel habitats is typically challenging to organisms. In the initial stage after colonization, approximation to fitness optima in the new environment can occur by selection acting on standing genetic variation, modification of developmental patterns or phenotypic plasticity. Midas cichlids have recently colonized crater Lake Apoyo from great Lake Nicaragua. The photic environment of crater Lake Apoyo is shifted towards shorter wavelengths compared to great Lake Nicaragua and Midas cichlids from both lakes differ in visual sensitivity. We investigated the contribution of ontogeny and phenotypic plasticity in shaping the visual system of Midas cichlids after colonizing this novel photic environment. To this end, we measured cone opsin expression both during development and after experimental exposure to different light treatments. Midas cichlids from both lakes undergo ontogenetic changes in cone opsin expression, but visual sensitivity is consistently shifted towards shorter wavelengths in crater lake fish, which leads to a paedomorphic retention of their visual phenotype. This shift might be mediated by lower levels of thyroid hormone in crater lake Midas cichlids (measured indirectly as dio2 and dio3 gene expression). Exposing fish to different light treatments revealed that cone opsin expression is phenotypically plastic in both species during early development, with short and long wavelength light slowing or accelerating ontogenetic changes, respectively. Notably, this plastic response was maintained into adulthood only in the derived crater lake Midas cichlids. We conclude that the rapid evolution of Midas cichlids’ visual system after colonizing crater Lake Apoyo was mediated by a shift in visual sensitivity during ontogeny and was further aided by phenotypic plasticity during development.     

Correlated responses to artificial body size selection in growth, development, phenotypic plasticity and juvenile viability in yellow dung flies

Most life history traits are positively influenced by body size, whereas disadvantages of large body size are poorly documented. To investigate presumed intrinsic costs of large size in the yellow dung fly (Scathophaga stercoraria; Diptera: Scathophagidae), we established two replicates each of three body size laboratory selection lines (small, control and large; selection on males only), and subjected flies of the resulting extended body size range to various abiotic stresses. Response to selection was symmetrical in the small and large lines (realized h(2) = 0.16-0.18). After 24 generations of selection body size had changed by roughly 10%. Female size showed a correlated response to selection on male size, whereas sexual size dimorphism did not change. Development time also showed a correlated response as, similar to food limited flies, small line flies emerged earlier at smaller body size. At the lowest larval food limit possible, flies of all lines emerged at the same small body size after roughly the same development time; so overall phenotypic plasticity in body size and development time strongly increased following selection. Juvenile mortality increased markedly when food was extremely limited, large line flies showing highest mortality. Winter frost disproportionately killed large (line) flies because of their longer development times. Mortality at high temperatures was high but size-selective effects were inconsistent. In all environments the larger males suffered more. Initial growth rate was higher for males and at unlimited food. Small line individuals of both sexes grew slowest at unlimited larval food but fastest at limited larval food, suggesting a physiological cost of fast growth. Overall, extension of the natural body size range by artificial selection revealed some otherwise cryptic intrinsic juvenile viability costs of large size, mediated by longer development or faster growth, but only in stressful environments.     

Genotype–environment interaction and the ontogeny of diet-induced phenotypic plasticity in size and shape of Melanoplus femurrubrum (Orthoptera: Acrididae)

The developmental origin of phenotypic plasticity in morphological shape can be attributed to environment-specific changes in growth of overall body size, localized growth of a morphological structure or a combination of both. I monitored morphological development in the first four nymphal instars of grasshoppers (Melanoplus femurrubrum) raised on two different plant diets to determine the ontogenetic origins of diet-induced phenotypic plasticity and to quantify genetic variation for phenotypic plasticity. I measured diet-induced phenotypic plasticity in body size (tibia length), head size (articular width and mandible depth) and head shape (residual articular width and residual mandible depth) for grasshoppers from 37 full-sib families raised on either a hard plant diet (Lolium perenne) or a soft plant diet (Trifolium repens). By the second to third nymphal instar, grasshoppers raised on a hard plant diet had significantly smaller mean tibia length and greater mean residual articular width (distance between mandibles adjusted for body size) compared with full-sibs raised on a soft plant diet. However, there was no significant phenotypic plasticity in mean unadjusted articular width and mandible depth, and in mean residual mandible depth. At the population level, development of diet-induced phenotypic plasticity in grasshopper head shape is mediated by plastic changes in allocation to tissue growth that maintain growth of head size on hard, low-nutrient diets while reducing growth of body size. Within the population, there was substantial variation in the plasticity of growth trajectories since different full-sib families developed phenotypic plasticity of residual articular width through different combinations of head and body size growth. Genetic variation for diet-induced phenotypic plasticity of residual articular width, residual mandible depth and tibia length, as estimated by genotype–environment interaction, exhibited significant fluctuation through ontogeny (repeated measures MANOVA , family × plant × instar, P < 0.01). For example, there was significant genetic variation for phenotypic plasticity of residual articular width in the third nymphal instar, but not earlier or later in ontogeny. The observed patterns of genetic variation are discussed with reference to short-term constraints and the evolution of phenotypic plasticity.     

Morphological and physiological responses to sediment type and light availability in roots of the submerged plant Myriophyllum spicatum

BACKGROUND AND AIMS: Both sediment and light are essential factors regulating the growth of submerged macrophytes, but the role of these two factors in regulating root morphology and physiology is far from clear. The responses of root morphology and physiology to sediment type and light availability in the submerged plant Myriophyllum spicatum were studied and the hypothesis was tested that a trade-off exists in root growth strategy between internal aeration and nutrient acquisition. METHODS: Plants were grown on two types of sediment (fertile mud and an infertile mixture of mud and sandy loam) and under three levels of light availability (600, 80 and 20 micro mol m(-2) s(-1)) in a greenhouse. KEY RESULTS: The significantly higher alcohol dehydrogenase (ADH) activity in root tissues indicated that oxygen deficiency existed in the plants growing in fertile mud and low (or high) light environments. Significantly, low plant N and P concentrations indicated that nutrient deficiency existed in the mixed sediment and high light environment. As a response to anoxia, plants did not change the porosity of the main roots. The effect of sediment type on root morphology was insignificant under higher light environments, whereas root diameter generally decreased but specific root length (SRL) increased with decreasing light availability. Both low light and fertile mud jointly led to lack of second-order laterals. More biomass was allocated to lateral roots in infertile environments, whereas mass fractions of laterals were lower in low light and mud environments. CONCLUSIONS: These data indicate that this plant can achieve the trade-off between internal aeration and nutrient acquisition by adjusting the structure of the root system and the pattern of biomass allocation to different root orders rather than root morphology and root porosity.