Ammonium sensing by plant roots

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The sensing of localized nutrient patches in soils by plant roots is not only part of a strategy to increase the competitiveness of plants in natural habitats but also important in agricultural production systems to improve fertilizer use efficiency of crops. Due to the high responsiveness of lateral roots to the localized supply of certain nutrients, lateral root formation has been established as a morphological read-out for nutrient sensing. During the first funding period, we have shown that localized ammonium supply triggers higher-order lateral root branching whereas nitrate stimulates lateral root elongation. In Arabidopsis, this morphological response is mediated by the ammonium transporter AMT1;3. The aim of the present proposal is i) to investigate whether AMT1;3 acts as an ammonium transceptor coupling the ammonium transport process with a signaling event leading to lateral root branching; ii) to determine the role of the phytohormone auxin in ammonium-induced lateral root branching, and iii) to identify genes in the ammonium signaling pathway downstream of the ammonium transport process from a set of genes identified in transcriptome approaches. Our approach includes AMT promoter swapping experiments for expression in a quadrouple amt knock-out line (qko) and root system analysis on split-agar plates with localized ammonium supplies. Moreover, qko lines with reconstituted expression of individual AMTs will be transformed with auxin reporter constructs and analyzed for expression of candidate genes. The ultimate goal is to identify molecular components in Arabidopsis roots integrating the information on a locally restricted ammonium source into the root developmental program.