Regulation of Arabidopsis root development by nitrate availability

When the root systems of many plant species are exposed to a localized sour ce of nitrate (NO3- they respond by proliferating their lateral roots to co lonize the nutrient-rich zone. This study reviews recent work with Arabidop sis thaliana in which molecular genetic approaches are being used to try to understand the physiological and genetic basis for this response, These st udies have led to the conclusion that there are two distinct pathways by wh ich NO3- modulates root branching in Arabidopsis, On the one hand, meristem atic activity in lateral root tips is stimulated by direct contact with an enriched source of NO3- (the localized stimulatory effect). On the other, a critical stage in the development of the lateral root (just after its emer gence from the primary root) is highly susceptible to inhibition by a syste mic signal that is related to the amount of NO3- absorbed by the plant (the systemic inhibitory effect). Evidence has been obtained that the localized stimulatory effect is a direct effect of the NO3- ion itself rather than a nutritional effect. A NO3--inducible MADS-box gene (ANR1) has been identif ied which encodes a component of the signal transduction pathway linking th e external NO3- supply to the increased rate of lateral root elongation. Ex periments using auxin-resistant mutants have provided evidence for an overl ap between the auxin and NO3- response pathways in the control of lateral r oot elongation. The systemic inhibitory effect, which does not affect later al root initiation but delays the activation of the lateral root meristem, appears to be positively correlated with the N status of the plant and is p ostulated to involve a phloem-mediated signal from the shoot.