Shoot-derived auxin is essential for early lateral root emergence in Arabidopsis seedlings

Lateral root formation is profoundly affected by auxins. Here we present data which indicate that light influences the formation of indole-3-acetic acid (IAA) in germinating Arabidopsis seedlings. IAA transported from the developing leaves to the root system is detectable as a short-lived pulse in the roots and is required for the emergence of the lateral root primordia (LRP) during early seedling development. LRP emergence is inhibited by the removal of apical tissues prior to detection of the IAA pulse in the root, but this treatment has minimal effects on LRP initiation. Our results identify the first developing true leaves as the most likely source for the IAA required for the first emergence of the LRP, as removal of cotyledons has only a minor effect on LRP emergence in contrast to removal of the leaves. A basipetal IAA concentration gradient with high levels of IAA in the root tip appears to control LRP initiation, in contrast to their emergence. A significant increase in the ability of the root system to synthesize IAA is observed 10 days after germination, and this in turn is reflected in the reduced dependence of the lateral root emergence on aerial tissue-derived auxin at this stage. We propose a model for lateral root formation during early seedling development that can be divided into two phases: (i) an LRP initiation phase dependent on a root tip-localized IAA source, and (ii) an LRP emergence phase dependent on leaf-derived IAA up to 10 days after germination.     

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.     

Chloroplast redox homeostasis is essential for lateral root formation in Arabidopsis

Redox regulation based on dithiol-disulphide interchange is an essential component of the control of chloroplast metabolism. In contrast to heterotrophic organisms, and non-photosynthetic plant tissues, chloroplast redox regulation relies on ferredoxin (Fd) reduced by the photosynthetic electron transport chain, thus being highly dependent on light. The finding of the NADPH-dependent thioredoxin reductase C (NTRC), a chloroplast-localized NTR with a joint thioredoxin domain, showed that NADPH is also used as source of reducing power for chloroplast redox homeostasis. Recently we have found that NTRC is also in plastids of non-photosynthetic tissues. Because these non-green plastids lack photochemical reactions, their redox homeostasis depends exclusively on NADPH produced from sugars and, thus, NTRC may play an essential role maintaining the redox homeostasis in these plastids. The fact that redox regulation occurs in any type of plastids raises the possibility that the functions of chloroplasts and non-green plastids, such as amyloplasts, are integrated to harmonize the growth of the different organs of the plant. To address this question, we generated Arabidopsis plants the redox homeostasis of which is recovered exclusively in chloroplasts, by leaf-specific expression of NTRC in the ntrc mutant, or exclusively in amyloplasts, by root-specific expression of NTRC. The analysis of these plants suggests that chloroplasts exert a pivotal role on plant growth, as expected because chloroplasts constitute the major source of nutrients and energy, derived from photosynthesis, for growth of heterotrophic tissues. However, NTRC deficiency causes impairment of auxin synthesis and lateral root formation. Interestingly, recovery of redox homeostasis of chloroplasts, but not of amyloplasts, was sufficient to restore wild type levels of lateral roots, showing the important signaling function of chloroplasts for the development of heterotrophic organs.     

Osmotic regulation of root system architecture

Although root system architecture is known to be highly plastic and strongly affected by environmental conditions, we have little understanding of the underlying mechanisms controlling root system development. Here we demonstrate that the formation of a lateral root from a lateral root primordium is repressed as water availability is reduced. This osmotic-responsive regulatory mechanism requires abscisic acid (ABA) and a newly identified gene, LRD2. Mutant analysis also revealed interactions of ABA and LRD2 with auxin signaling. Surprisingly, further examination revealed that both ABA and LRD2 control root system architecture even in the absence of osmotic stress. This suggests that the same molecules that mediate responses to environmental cues can also be regulators of intrinsic developmental programs in the root system.     

A Histone H3 Lysine-27 Methyltransferase Complex Represses Lateral Root Formation in Arabidopsis thaliana

Root branching or lateral root formation is crucial to maximize a root system acquiring nutrients and water from soil. A lateral root （LR） arises from asymmetric cell division of founder cells （FCs） in a pre-branch site of the primary root, and FC establishment is essential for lateral root formation. FCs are known to be specified from xylem pole pericycle cells, but the molecular genetic mechanisms underlying FC establishment are unclear. Here, we report that, in Arabidopsis thaliana, a PRC2 （for Polycomb repressive complex 2） histone H3 lysine-27 （H3K27） methyltransferase complex, functions to inhibit FC establishment during LR initiation. We found that functional loss of the PRC2 subunits EMF2 （for EMBRYONIC FLOWER 2） or CLF （for CURLY LEAF） leads to a great increase in the number of LRs formed in the primary root. The CLF H3K27 methyltransferase binds to chromatin of the auxin efflux carrier gene PIN FORMED 1 （PIN1）, deposits the repres- sive mark H3K27me3 to repress its expression, and functions to down-regulate auxin maxima in root tissues and inhibit FC establishment. Our findings collectively suggest that EMF2-CLF PRC2 acts to down-regulate root auxin maxima and show that this complex represses LR formation in Arabidopsis.     

Relationship between tissue sugar content, phloem import and lateral root initiation in wheat

Split‐root experiments were conducted to test the hypothesis that adjustments in lateral root initiation, as might occur in response to localized soil conditions, are determined by the sugar content of the root and do not depend on changes in the import of phloem‐translocated phytohormones. Wheat ( Triticum aesticum L. cv. Alexandria) seedlings were grown in hydroponics with their seminal roots divided between two compartments within the culture vessel. Two seminal roots of treated plants were supplied with standard nutrient solution supplemented with 50 m M glucose, whilst the remaining three roots received nutrient solution without glucose. Control plants had their roots divided in the same ratio, but both 'halves' received nutrient solution without glucose. Feeding glucose to one 'half' of the root system increased the frequency (number per unit length) of lateral root primordia in the fed axes. The increase was first observed 15 h after the start of treatment and was located within the apical 30 mm of root. At this time there was no significant treatment effect on the frequency of primordia in non‐fed axes. The enhanced initiation of lateral roots in glucose‐fed root tips was associated with an increase in their concentration of glucose and sucrose plus low molecular mass fructans. In contrast, there was a reduction in partitioning of ¹⁴C‐photosynthate to these root tips compared to the non‐fed roots of treated plants and controls. The results indicate that lateral root initiation can be stimulated by sugars in the absence of an increase in phloem translocation. It is proposed that proliferation of lateral roots in response to localized soil conditions, such as nutrient patches, may be signalled by an increase in sugar content of the tissue, rather than an altered flux of phytohormones or other material co‐transported with sucrose in the phloem.     

Regulatory components involved in altering lateral root development in response to localized iron: Evidence for natural genetic variation

On the search for sparingly available nutrients, plants may alter their root architecture to improve soil exploration. So far, the examples for root system modifications induced by a heterogeneous availability of nutrients have been reported for the macronutrients nitrogen (N) and phosphorous (P). In an attempt to extend this type of knowledge to other nutrients, we recently provided evidence that Arabidopsis roots are able to sense a local availability of the micronutrient iron (Fe) and to respond with lateral root elongation into the Fe-containing patch. This specific root response was caused by enhanced elongation of cells leaving the root meristem and was dependent on an AUX1-mediated auxin accumulation in the lateral root apices. In this report, we compare mechanisms underlying this response with those known for other nutrients and show that a substantial genotypic variation exists among accessions of A. thaliana in the responsiveness of lateral roots toward localized Fe supplies.     

The Xerobranching Response Represses Lateral Root Formation When Roots Are Not in Contact with Water

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Intrinsic and environmental response pathways that regulate root system architecture

Root system development is an important agronomic trait. The right architecture in a given environment allows plants to survive periods of water of nutrient deficit, and compete effectively for resources. Root systems also provide an optimal system for studying developmental plasticity, a characteristic feature of plant growth. This review proposes a framework for describing the pathways regulating the development of complex structures such as root systems: intrinsic pathways determine the characteristic architecture of the root system in a given plant species, and define the limits for plasticity in that species. Response pathways co-ordinate environmental cues with development by modulating intrinsic pathways. The current literature describing the regulation of root system development is summarized here within this framework. Regulatory pathways are also organized based on their specific developmental effect in the root system. All the pathways affect lateral root formation, but some specifically target initiation of the lateral root, while others target the development and activation of the lateral root primordium, or the elongation of the lateral root. Finally, we discuss emerging approaches for understanding the regulation of root system architecture.     

A putative transporter is essential for integrating nutrient and hormone signaling with lateral root growth and nodule development in Medicago truncatula

Legume root architecture involves not only elaboration of the root system by the formation of lateral roots but also the formation of symbiotic root nodules in association with nitrogen‐fixing soil rhizobia. The Medicago truncatula LATD/NIP gene plays an essential role in the development of both primary and lateral roots as well as nodule development. We have cloned the LATD/NIP gene and show that it encodes a member of the NRT1(PTR) transporter family. LATD/NIP is expressed throughout the plant. pLATD/NIP‐GFP promoter–reporter fusions in transgenic roots establish the spatial expression of LATD/NIP in primary root, lateral root and nodule meristems and the surrounding cells. Expression of LATD/NIP is regulated by hormones, in particular by abscisic acid which has been previously shown to rescue the primary and lateral root meristem arrest of latd mutants. latd mutants respond normally to ammonium but have defects in responses of the root architecture to nitrate. Taken together, these results suggest that LATD/NIP may encode a nitrate transporter or transporter of another compound.     

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.     

OsORC3 is required for lateral root development in rice

The origin recognition complex (ORC) is a pivotal element in DNA replication, heterochromatin assembly, checkpoint regulation and chromosome assembly. Although the functions of the ORC have been determined in yeast and model animals, they remain largely unknown in the plant kingdom. In this study, Oryza sativa Origin Recognition Complex subunit 3 (OsORC3) was cloned using map‐based cloning procedures, and functionally characterized using a rice (Oryza sativa) orc3 mutant. The mutant showed a temperature‐dependent defect in lateral root (LR) development. Map‐based cloning showed that a G→A mutation in the 9th exon of OsORC3 was responsible for the mutant phenotype. OsORC3 was strongly expressed in regions of active cell proliferation, including the primary root tip, stem base, lateral root primordium, emerged lateral root primordium, lateral root tip, young shoot, anther and ovary. OsORC3 knockdown plants lacked lateral roots and had a dwarf phenotype. The root meristematic zone of ORC3 knockdown plants exhibited increased cell death and reduced vital activity compared to the wild‐type. CYCB1;1::GUS activity and methylene blue staining showed that lateral root primordia initiated normally in the orc3 mutant, but stopped growing before formation of the stele and ground tissue. Our results indicate that OsORC3 plays a crucial role in the emergence of lateral root primordia.     

Nitrate and phosphate availability and distribution have different effects on root system architecture of Arabidopsis

Plant root systems can respond to nutrient availability and distribution by changing the three-dimensional deployment of their roots: their root system architecture (RSA). We have compared RSA in homogeneous and heterogeneous nitrate and phosphate supply in Arabidopsis. Changes in nitrate and phosphate availability were found to have contrasting effects on primary root length and lateral root density, but similar effects on lateral root length. Relative to shoot dry weight (DW), primary root length decreased with increasing nitrate availability, while it increased with increasing phosphate supply. Lateral root density remained constant across a range of nitrate supplies, but decreased with increasing phosphate supply. In contrast, lateral root elongation was suppressed both by high nitrate and high phosphate supplies. Local supplies of high nitrate or phosphate in a patch also had different effects. Primary root growth was not affected by a high nitrate patch, but growth through a high phosphate patch reduced primary root growth after the root left the patch. A high nitrate patch induced an increase in lateral root density in the patch, whereas lateral root density was unaffected by a high phosphate patch. However, both phosphate- and nitrate-rich patches induced lateral root elongation in the patch and suppressed it outside the patch. This co-ordinated response of lateral roots also occurs in soil-grown plants exposed to a nutrient-rich patch. The auxin-resistant mutants axrl, axr4 and aux1 all showed the wild-type lateral root elongation responses to a nitrate-rich patch, suggesting that auxin is not required for this response.