LBD29-Involved Auxin Signaling Represses NAC Master Regulators and Fiber Wall Biosynthesis.

NAM, ATAF1/2 and CUC2 (NAC) domain transcription factors function as master switches in regulating secondary cell wall (SCW) biosynthesis in Arabidopsis () stems. Despite the importance of these NACs in fiber development, the upstream signal is still elusive. Using a large-scale mutant screening, we identified a dominant activation-tagging mutant, (), showing defective SCW development in stem fibers, similar to that of the () double mutant. Overexpression of () is responsible for the mutant phenotypes. By contrast, loss-of-function of , either in the dominant repression transgenic lines or in the transfer-DNA (T-DNA) insertion mutant , enhanced SCW development in fibers. Genetic analysis and transgenic studies demonstrated depends on master regulators in mediating SCW biosynthesis, specifically (), 2, and 3. Increasing indole-3-acetic acid (IAA) levels, either in stem tissues above a -1-naphthylphthalamic acid-treated region or in plants directly sprayed with IAA, inhibits fiber wall thickening. The inhibition effect of naphthylphthalamic acid treatment and exogenous IAA application depends on a known auxin signaling pathway involving AUXIN RESPONSE FACTOR7 (ARF7)/ARF19 and LBD29. These results demonstrate auxin is upstream of LBD29 in repressing NAC master regulators, and therefore shed new light on the regulation of SCW biosynthesis in Arabidopsis.