The development of plant root systems is sensitive to the availability and distribution of nutrients within the soil. For example, lateral roots proliferate preferentially within nitrate (NO3-)-rich soil patches. A NO3--inducible Arabidopsis gene (ANR1), was identified that encodes a member of the MADS box family of transcription factors. Transgenic plants in which ANR1 was repressed had an altered sensitivity to NO3- and no longer responded to NO3--rich zones by lateral root proliferation, indicating that ANR1 is a key determinant of developmental plasticity in Arabidopsis roots.

Root development is extremely sensitive to variations in nutrient supply, but the mechanisms are poorly understood. We have investigated the processes by which nitrate (NO 3 ؊ ), depending on its availability and distribution, can have both positive and negative effects on the development and growth of lateral roots. When Arabidopsis roots were exposed to a locally concentrated supply of NO 3 ؊ there was no increase in lateral root numbers within the NO 3 ؊ -rich zone, but there was a localized 2-fold increase in the mean rate of lateral root elongation, which was attributable to a corresponding increase in the rate of cell production in the lateral root meristem. Localized applications of other N sources did not stimulate lateral root elongation, consistent with previous evidence that the NO 3 ؊ ion is acting as a signal rather than a nutrient. The axr4 auxin-resistant mutant was insensitive to the stimulatory effect of NO 3 ؊ , suggesting an overlap between the NO 3 ؊ and auxin response pathways. High rates of NO 3 ؊ supply to the roots had a systemic inhibitory effect on lateral root development that acted specifically at the stage when the laterals had just emerged from the primary root, apparently delaying final activation of the lateral root meristem. A nitrate reductase-deficient mutant showed increased sensitivity to this systemic inhibitory effect, suggesting that tissue NO 3 ؊ levels may play a role in generating the inhibitory signal. We present a model in which root branching is modulated by opposing signals from the plant's internal N status and the external supply of NO 3 ؊ .

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SummaryLateral root (LR) formation displays considerable plasticity in response to developmental and environmental signals. The mechanism whereby plants incorporate diverse regulatory signals into the developmental programme of LRs remains to be elucidated. Current concepts of lateral root regulation focus on the role of auxin. In this study, we show that another plant hormone, abscisic acid (ABA), also plays a critical role in the regulation of this post-embryonic developmental event. In the presence of exogenous ABA, LR development is inhibited. This occurs at a specific developmental stage, i.e. immediately after the emergence of the LR primordium (LRP) from the parent root and prior to the activation of the LR meristem, and is reversible. Interestingly, this inhibition requires 10-fold less ABA than the inhibition of seed germination and is only slightly reduced in characterised abi mutants, suggesting that it may involve novel ABA signalling mechanisms. We also present several lines of evidence to support the conclusion that the ABA-induced lateral root inhibition is mediated by an auxin-independent pathway. First, the inhibition could not be rescued by either exogenous auxin application or elevated auxin synthesis. Secondly, a mutation in the ALF3 gene, which is believed to encode an important component in the auxin-dependent regulatory pathway for the post-emergence LR development, does not affect the sensitivity of LRs to ABA. Thirdly, ABA and the alf3-1 mutation do not act at the same developmental point. To summarise, these results demonstrate a novel ABA-sensitive, auxin-independent checkpoint for lateral root development in Arabidopsis at the post-emergence stage. In addition, we also present data indicating that regulation of this developmental checkpoint may require novel ABA signalling mechanisms and that ABA suppresses auxin response in the LRPs.Abbreviations: ABA, abscisic acid; LR, lateral root; LRP, lateral root primordium; abi, abscisic acid insensitive; IAA, indole-3-acetic acid; NAA, a-naphthaleneacetic acid; 2,4-D, 2,4-dichlorophenoxyacetic acid; DAG, days after germination.

SummaryThe formation of lateral roots (LR) is a major post-embryonic developmental event in plants. In Arabidopsis thaliana, LR development is inhibited by high concentrations of NO 3 ± . Here we present strong evidence that ABA plays an important role in mediating the effects of NO 3 ± on LR formation. Firstly, the inhibitory effect of NO 3 ± is signi®cantly reduced in three ABA insensitive mutants, abi4-1, abi4-2 and abi5-1, but not in abi1-1, abi2-1 and abi3-1. Secondly, inhibition by NO 3 ± is signi®cantly reduced, but not completely abolished, in four ABA synthesis mutants, aba1-1, aba2-3, aba2-4 and aba3-2. These results indicate that there are two regulatory pathways mediating the inhibitory effects of NO 3 ± in A. thaliana roots. One pathway is ABA-dependent and involves ABI4 and ABI5, whereas the second pathway is ABA-independent. In addition, ABA also plays a role in mediating the stimulation of LR elongation by local NO 3 ± applications.