Patterns of variability in the diameter of lateral roots in the banana root system

The relative importance of root system structure, plant carbon status and soil environment in the determination of lateral root diameter remains unclear, and was investigated in this study. Banana (Musa acuminata) plants were grown at various moderate levels of soil compaction in two distinct experiments, in a field experiment (FE) and in a glasshouse experiment (GE). Radiant flux density was 5 times lower in GE. The distribution of root diameter was measured for several root branching orders. Root diameters ranged between 0.09 and 0.52 mm for secondary roots and between 0.06 and 0.27 mm for tertiary roots. A relationship was found between the diameter of the parent bearing root and the median diameter of its laterals, which appears to be valid for a wide range of species. Mean lateral root diameter increased with distance to the base of the root and decreased with branching density [number of lateral roots per unit length of bearing root (cm(-1))]. Typical symptoms of low light availability were observed in GE. In this case, lateral root diameter variability was reduced. Although primary root growth was affected by soil compaction, no effects on lateral root diameter were observed.     

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.     

Maximum axial growth pressures of the lateral roots of pea and eucalypt

Although lateral roots may contribute significantly towards growth and nourishment of plants, the mechanics of their elongation behaviour in strong soils is not well known. The aim of this study is to report maximum axial growth pressures (p) and maximum elongation rates (E) of the lateral roots of an annual herbaceous plant (pea) and a woody perennial (eucalypt). As such measurements have not been reported previously, measurements of P and E for lateral roots were compared with the primary roots of pea for which reports are widespread. Values of P were estimated from the measured maximum values of axial force and root diameter on single, intact roots of seedlings in the laboratory. Additional measurements of both P and E were made for the lateral roots of pea when the growth of the remaining root axes was stopped (with removal of tips) to determine the overall effects of root-growth-inhibition on P and E of single roots.Values of P and E for lateral roots of pea were significantly greater than those for the lateral roots of eucalypt. Although root diameter for the primary roots of pea were similar to those for the lateral roots of eucalypt, the former exerted nearly twice as much pressure as the latter. The lateral roots of pea elongated significantly slower than the primary roots; however, P of lateral roots was significantly lower than the primary roots when elongation of all other roots was inhibited during the measurements. Production and/or development of lateral roots increased when elongation of the remaining roots (both primary and lateral roots) of pea seedlings was restricted due to the removal of root tips and exposure of one of the lateral roots to high strength. In general, maximum axial force exerted by primary and lateral roots was similar for roots of <1 mm diameter. However, primary roots exerted greater maximum axial force than the lateral roots when root diameter was >1 mm. As axial pressure of lateral roots was independent of root diameter, thickening of root tips is less likely to assist penetration of lateral roots in strong soils.     

Plant-derived smoke solutions stimulate the growth of Lycopersicon esculentum roots in vitro

Aqueous extracts of smoke, derived from Themeda triandra, a fire-climax grass, and Passerina vulgaris, a fynbos plant, stimulated the growth of primary root sections of tomato roots in suspension culture. The optimal dilution for both extracts was 1:2000. Several of the fractions obtained from TLC separation of the Themeda and the Passerina extracts significantly promoted primary root growth. The auxins naphthaleneacetic acid (NAA), indolebutyric acid (IBA) and indoleacetic acid (IAA) were found to stimulate the growth of the primary root axis, with IAA and NAA significantly promoting lateral root number. Similarly, the naturally occurring cytokinins, zeatin and its derivatives (zeatin-O-glucoside; dihydrozeatin and zeatin riboside) stimulated primary root length. Zeatin and dihydrozeatin promoted secondary root growth, but only at very low concentrations.     

Role of cytokinin and auxin in shaping root architecture: regulating vascular differentiation, lateral root initiation, root apical dominance and root gravitropism

BACKGROUND AND AIMS: Development and architecture of plant roots are regulated by phytohormones. Cytokinin (CK), synthesized in the root cap, promotes cytokinesis, vascular cambium sensitivity, vascular differentiation and root apical dominance. Auxin (indole-3-acetic acid, IAA), produced in young shoot organs, promotes root development and induces vascular differentiation. Both IAA and CK regulate root gravitropism. The aims of this study were to analyse the hormonal mechanisms that induce the root's primary vascular system, explain how differentiating-protoxylem vessels promote lateral root initiation, propose the concept of CK-dependent root apical dominance, and visualize the CK and IAA regulation of root gravitropiosm. KEY ISSUES: The hormonal analysis and proposed mechanisms yield new insights and extend previous concepts: how the radial pattern of the root protoxylem vs. protophloem strands is induced by alternating polar streams of high IAA vs. low IAA concentrations, respectively; how differentiating-protoxylem vessel elements stimulate lateral root initiation by auxin-ethylene-auxin signalling; and how root apical dominance is regulated by the root-cap-synthesized CK, which gives priority to the primary root in competition with its own lateral roots. CONCLUSIONS: CK and IAA are key hormones that regulate root development, its vascular differentiation and root gravitropism; these two hormones, together with ethylene, regulate lateral root initiation.     

Auxin reflux between the endodermis and pericycle promotes lateral root initiation

Lateral root (LR) formation is initiated when pericycle cells accumulate auxin, thereby acquiring founder cell (FC) status and triggering asymmetric cell divisions, giving rise to a new primordium. How this auxin maximum in pericycle cells builds up and remains focused is not understood. We report that the endodermis plays an active role in the regulation of auxin accumulation and is instructive for FCs to progress during the LR initiation (LRI) phase. We describe the functional importance of a PIN3 (PIN‐formed) auxin efflux carrier‐dependent hormone reflux pathway between overlaying endodermal and pericycle FCs. Disrupting this reflux pathway causes dramatic defects in the progress of FCs towards the next initiation phase. Our data identify an unexpected regulatory function for the endodermis in LRI as part of the fine‐tuning mechanism that appears to act as a check point in LR organogenesis after FCs are specified.     

Auxin-induced inhibition of lateral root initiation contributes to root system shaping in Arabidopsis thaliana

The hormone auxin is known to inhibit root elongation and to promote initiation of lateral roots. Here we report complex effects of auxin on lateral root initiation in roots showing reduced cell elongation after auxin treatment. In Arabidopsis thaliana, the promotion of lateral root initiation by indole-3-acetic acid (IAA) was reduced as the IAA concentration was increased in the nanomolar range, and IAA became inhibitory at 25 nM. Detection of this unexpected inhibitory effect required evaluation of root portions that had newly formed during treatment, separately from root portions that existed prior to treatment. Lateral root initiation was also reduced in the iaaM-OX Arabidopsis line, which has an endogenously increased IAA level. The ethylene signaling mutants ein2-5 and etr1-3, the auxin transport mutants aux1-7 and eir1/pin2, and the auxin perception/response mutant tir1-1 were resistant to the inhibitory effect of IAA on lateral root initiation, consistent with a requirement for intact ethylene signaling, auxin transport and auxin perception/response for this effect. The pericycle cell length was less dramatically reduced than cortical cell length, suggesting that a reduction in the pericycle cell number relative to the cortex could occur with the increase of the IAA level. Expression of the DR5:GUS auxin reporter was also less effectively induced, and the AXR3 auxin repressor protein was less effectively eliminated in such root portions, suggesting that decreased auxin responsiveness may accompany the inhibition. Our study highlights a connection between auxin-regulated inhibition of parent root elongation and a decrease in lateral root initiation. This may be required to regulate the spacing of lateral roots and optimize root architecture to environmental demands.     

Relationships between root diameter,root length and root branching along lateral roots in adult,field-grown maize

Background and Aims Root diameter, especially apical diameter, plays an important role in root development and function. The variation in diameter between roots, and along roots, affects root structure and thus the root system’s overall foraging performance. However, the effect of diameter variation on root elongation, branching and topological connections has not been examined systematically in a population of high-order roots, nor along the roots, especially for mature plants grown in the field.Methods A method combining both excavation and analysis was applied to extract and quantify root architectural traits of adult, field-grown maize plants. The relationships between root diameter and other root architectural characteristics are analysed for two maize cultivars.Key Results The basal diameter of the lateral roots (orders 1–3) was highly variable. Basal diameter was partly determined by the diameter of the bearing segment. Basal diameter defined a potential root length, but the lengths of most roots fell far short of this. This was explained partly by differences in the pattern of diameter change along roots. Diameter tended to decrease along most roots, with the steepness of the gradient of decrease depending on basal diameter. The longest roots were those that maintained (or sometimes increased) their diameters during elongation. The branching density (cm^–1) of laterals was also determined by the diameter of the bearing segment. However, the location of this bearing segment along the mother root was also involved – intermediate positions were associated with higher densities of laterals.Conclusions The method used here allows us to obtain very detailed records of the geometry and topology of a complex root system. Basal diameter and the pattern of diameter change along a root were associated with its final length. These relationships are especially useful in simulations of root elongation and branching in source–sink models.     

The relationships between static and dynamic variables in the description of root growth. Consequences for field interpretation of rooting variability

Field root investigations are often limited by the static nature of classical observations, resulting in the need to develop alternative methodologies that allow dynamic interpretation of root architecture variability on the basis of static measurements. The objectives of this work were (i) to evaluate the use of selected morphological indicators, namely root apical diameter (Da) and the length of the apical unbranched zone (LAUZ), in predicting primary and lateral root growth patterns in banana trees, (ii) to propose a field methodology for the assessment of root dynamics based on static measurements. Banana trees (Musa acuminata cv `Grande Naine') were grown in 5 rhizotrons as well as in field conditions, respectively on pouzzolane and Mollic Andosols. In rhizotrons, root growth analysis was carried out by reporting root elongation, Da and LAUZ, three times a week. In field conditions, 4 series of excavations were made at three-week intervals. Apart from root growth rate, measurements were the same as those in the rhizotrons. LAUZ was confirmed as a stable and good predictor of root growth rate for the different types of roots. In the rhizotrons, the root growth of lateral roots was found to be well correlated to the product of Da and the growth rate of the bearing root. Evaluation in field conditions from static observations attested consistent relationships between measured and predicted root length for lateral roots (slopes close to 1:1). The apical diameter can be considered as a good indicator of root growth potential, while actual lateral root growth depends on the bearing root elongation rate. Morphological static indicators calibrated from growth dynamics in rhizotrons are of major interest in explaining growth variability in field conditions. Especially the `growth rate-LAUZ' relationship can be considered a useful tool in interpreting field patterns of growing roots in relation to various soil conditions.     

Control of lateral root initiation by DA3 in Arabidopsis

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Cadmium stress suppresses lateral root formation by interfering with the root clock

A biological clock activated by oscillating signals, known as root clock, has been linked to lateral root (LR) formation and is essential for regular LR spacing along the primary root. However, it remains unclear how this internal mechanism is influenced by environmental factors known to affect the LR pattern. Here, we report that excessive cadmium (Cd) inhibits LR formation by disrupting the lateral root cap (LRC)‐programmed cell death (PCD)‐regulated root clock. Cd restricts the frequency of the oscillating signal rather than its amplitude. This could be attributed to the inhibition on meristematic activity by Cd, which resulted in decreased LRC cell number and LRC‐PCD frequency. Genetic evidence further showed that LRC cell number is positively correlated with root resistance to Cd. Our study reveals root cap dynamics as a novel mechanism mediating root responses to Cd, providing insight into the signalling pathways of the root clock responding to environmental cues.     

A growth chamber for idealized studies of seedling root growth dynamics and structure

A root growth chamber is described which allows seedling root growth dynamics and structure to be monitored continuously under a variety of conditions for several weeks. The chamber consists of two cells with inner dimensions 18×20×0.12 cm. To simulate the soil matrix, each cell was filled with spherical glass beads of 0.1 cm diameter. Given the 0.12 cm width of each cell, the glass bead matrix was approximately one bead layer thick. Roots were therefore grown in a quasi -two-dimensional and transparent environment. This enabled root images of high spatial and temporal resolution to be collected and analysed quantitatively using standard image analysis techniques. The chamber was constructed such that the root environment could be manipulated with regard to nutrient distribution, `soil' matrix structure and other perturbations to the system. Preliminary experiments of the growth dynamics of lentil roots (Lens culinaris L. cv. Verte du Puy) in the chamber were conducted. The majority of the primary and lateral roots followed a similar growth pattern with high growth rates between days 5 and 9 and days 14 and 18 separated by a period of low growth rate between days 10 and 12 after seeding in the chamber. Thus, primary and lateral root growth was to a certain extent synchronized. Lateral roots developed after 3 to 8 days on the outer curve (convex side) of the primary root. The roots shared many of the characteristics of roots developed in three-dimensional systems indicating that the chamber did not induce artificial root behaviour. Thus, the idealized and quantitative studies that can be conducted in the chamber may enable many aspects of the complex interactions between the root system and environment to be studied.     

Mutations in the Diageotropica (Dgt) gene uncouple patterned cell division during lateral root initiation from proliferative cell division in the pericycle

In angiosperms, root branching requires a continuous re-initiation of new root meristems. Through some unknown mechanism, in most eudicots pericycle cells positioned against the protoxylem change identity and initiate patterned division, leading to formation of lateral root primordia that further develop into lateral roots. This process is auxin-regulated. We have observed that three mutations in the Diageotropica (Dgt) gene in tomato prevent primordium formation. Detailed analysis of one of these mutants, dgt1-1, demonstrated that the mutation does not abolish the proliferative capacity of the xylem-adjacent pericycle in the differentiated root portion. Files of shortened pericycle cells found in dgt1-1 roots were unrelated to primordium formation. Auxin application stimulated this unusual proliferation, leading to formation of a multi-layered xylem-adjacent pericycle, but did not rescue the primordium formation. In contrast to wild type, auxin could not induce any cell divisions in the pericycle of the most distal dgt1-1 root-tip portion. In wild-type roots, the Dgt gene promoter was expressed strongly in lateral root primordia starting from their initiation, and on auxin treatment was induced in the primary root meristem. Auxin level and distribution were altered in dgt1-1 root tissues, as judged by direct auxin measurements, and the tissue-specific expression of an auxin-response reporter was altered in transgenic plants. Together, our data demonstrate that the Dgt gene product, a type-A cyclophilin, is essential for morphogenesis of lateral root primordia, and that the dgt mutations uncouple patterned cell division in lateral root initiation from proliferative cell division in the pericycle.     

Consequences of insect herbivory on grape fine root systems with different growth rates

Herbivory tolerance has been linked to plant growth rate where plants with fast growth rates are hypothesized to be more tolerant of herbivory than slower-growing plants. Evidence supporting this theory has been taken primarily from observations of aboveground organs but rarely from roots. Grapevines differing in overall rates of new root production, were studied in Napa Valley, California over two growing seasons in an established vineyard infested with the sucking insect, grape phylloxera (Daktulosphaira vitifoliae Fitch). The experimental vineyard allowed for the comparison of two root systems that differed in rates of new root tip production (a 'fast grower', Vitis berlandieri x Vitis rupestris cv. 1103P, and a slower-growing stock, Vitis riparia x Vitis rupestris cv. 101-14 Mgt). Each root system was grafted with a genetically identical shoot system (Vitis vinifera cv. Merlot). Using minirhizotrons, we did not observe any evidence of spatial or temporal avoidance of insect populations by root growth. Insect infestations were abundant throughout the soil profile, and seasonal peaks in phylloxera populations generally closely followed peaks in new root production. Our data supported the hypothesis that insect infestation was proportional to the number of growing tips, as indicated by similar per cent infestation in spite of a threefold difference in root tip production. In addition, infested roots of the fast-growing rootstock exhibited somewhat shorter median lifespans (60 d) than the slower-growing rootstock (85 d). Lifespans of uninfested roots were similar for the two rootstocks (200 d). As a consequence of greater root mortality of younger roots, infested root populations in the fast-growing rootstock had an older age structure. While there does not seem to be a trade-off between potential growth rate and relative rate of root infestation in these cultivars, our study indicates that a fast-growing root system may more readily shed infested roots that are presumably less effective in water and nutrient uptake. Thus, differences in root tip production may be linked to differences in the way plants cope with roots that are infested by sucking insects.