Novel Players Determining the Responsiveness of Root Systems to Nutrient Availabilities in Arabidopsis thaliana

Changes in the plant nutritional status and external nutrient supply modulate root system architecture (RSA). To identify novel molecular players involved with RSA responses to nutrient availabilities, we are employing GWA mapping and transcriptomics. Regarding nitrogen (N), we were particularly interested in the stimulatory response induced by mild deficiency since it reflects a foraging strategy, for which the underlying molecular mechanism remains largely unknown. Significant variation in the magnitude of this response was found among 200 natural accessions of Arabidopsis thaliana. We exploited this variation to identify candidate loci associated with N deficiency-induced root elongation. Allelic variation in one member of the brassinosteroid-signaling kinases (BSKs) was most strongly associated with primary root length. Analysis of associated SNPs in the coding sequence of the identified BSK detected one non-synonymous polymorphism in the kinase domain. In bsk mutants, the response of primary root to mild N deficiency is completely abolished, while that of LRs is partially attenuated. We complemented a bsk quadrupole mutant with alleles derived from contrasting accessions and found that BSK allelic variation modulates the sensitivity of roots to brassinosteroids thereby determining the magnitude of root elongation to mild N deficiency. Thus, our results revealed a previously unknown role of brassinosteroid signaling in RSA responses to N availability. Furthermore, the characterization of candidate genes from our transcriptome analysis identified a novel cytochrome that regulates root development according to phosphorus (P) availability. The primary root elongation of loss-of-function mutants is strongly inhibited by low P and associated with rapid meristem exhaustion and arrested cell elongation. In root tips, the newly identified cytochrome is strongly induced by P deficiency and confined to the root cap. We are currently characterizing the molecular function of the identified cytochrome and how it relates to the known mechanism that adjusts root growth to P availability.