The plant auxin indole-3-acetic acid (IAA) plays critical roles in plant growth and development. There are two main strategies proposed for plant synthesis of IAA: the Trp-dependent (TD) and the Trp-independent (TI) pathways. Four TD pathways, namely the indole-3-acetamide pathway, the indole-3-pyruvic acid pathway, the tryptamine pathway and the indole-3-acetaldoxime pathway, have been postulated, identified and extensively studied. On the other hand, neither genes nor mutants involved in the TI pathway have been identified to date. Interestingly, some bacteria have auxin synthesis pathways that are similar to those in plants, indicating conserved biosynthetic mechanisms. Over the past few years, genetic, biochemical and molecular studies have greatly advanced our understanding of auxin biosynthesis. This review both summarizes recent advances in genetic and molecular knowledge and addresses the unsolved questions regarding auxin biosynthesis pathways in plants.
Summary
Ammonium (NH4+) is toxic to root growth in most plants, even at moderate concentrations. Transcriptional regulation is one of the most important mechanisms in the response of plants to NH4+ toxicity, but the nature of the involvement of transcription factors (TFs) in this regulation remains unclear.
Here, RNA‐seq analysis was performed on Arabidopsis roots to screen for ammonium‐responsive TFs. WRKY46, the member of the WRKY transcription factor family most responsive to NH4+, was selected. We defined the role of WRKY46 using mutation and overexpression assays, and characterized the regulation of NUDX9 and indole‐3‐acetic acid (IAA)‐conjugating genes by WRKY46 via yeast one‐hybrid and electrophoretic mobility shift assays and chromatin immunoprecipitation‐quantitative real‐time polymerase chain reaction (ChIP‐qPCR).
Knockout of WRKY46 increased, while overexpression of WRKY46 decreased, NH4+‐suppression of the primary root. WRKY46 is shown to directly bind to the promoters of the NUDX9 and IAA‐conjugating genes (GH3.1, GH3.6, UGT75D1, UGT84B2) and to inhibit their transcription, thus positively regulating free IAA content and stabilizing protein N‐glycosylation, leading to an inhibition of NH4+ efflux in the root elongation zone (EZ).
We identify TF involvement in the regulation of NH4+ efflux in the EZ, and show that WRKY46 inhibits NH4+ efflux by negative regulation of NUDX9 and IAA‐conjugating genes.
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