Patterns and Timing in Expression of Early Auxin-Induced Genes Imply Involvement of Phospholipases A (pPLAs) in the Regulation of Auxin Responses

While it is known that patatin-related phospholipase A (pPLA) activity is rapidly activated within 3min by auxin, hardly anything is known about how this signal influences downstream responses like transcription of early auxin-induced genes or other physiological responses. We screened mutants with T-DNA insertions in members of the pPLA gene family for molecular and physiological phenotypes related to auxin. Only one in nine Arabidopsis thaliana ppla knockdown mutants displayed an obvious constitutive auxin-related phenotype. Compared to wild-type, ppla-IIIδ mutant seedlings had decreased main root lengths and increased lateral root numbers. We tested auxin-induced gene expression as a molecular readout for primary molecular auxin responses in nine ppla mutants and found delayed up-regulation of auxin-responsive gene expression in all of them. Thirty minutes after auxin treatment, up-regulation of up to 40% of auxin-induced genes was delayed in mutant seedlings. We observed only a few cases with hypersensitive auxin-induced gene expression in ppla mutants. While, in three ppla mutants, which were investigated in detail, rapid up-regulation (as early as 10min after auxin stimulus) of auxin-regulated genes was impaired, late transcriptional responses were wild-type-like. This regulatory or dynamic phenotype was consistently observed in different ppla mutants with delayed up-regulation that frequently affected the same genes. This defect was not affected by pPLA transcript levels which remained constant. This indicates a posttranslational mechanism as a functional link of pPLAs to auxin signaling. The need for a receptor triggering an auxin response without employing transcription regulation is discussed.