| The jasmonates are a group of oxylipin plant hormones synthesized by the octadecanoid pathway from alpha-linolenic acid. Around 20 naturally occurring jasmonates have been described; and jasmonic acid (JA), the JA derivatives methyl jasmonate (MeJA) and JA-isoleucine (JA-Ile), and the JA precursor 12-oxophytodienoic acid (OPDA) all have demonstrated roles in signaling. In Arabidopsis , JA has a critical reproductive role and is essential for stamen filament elongation, anther dehiscence, and pollen viability. While much is known about JA biosynthesis via the octadecanoid pathway, very little is known about the signal transduction events and downstream transcription networks that respond to this hormone, especially those relating to reproduction. A genomics approach using Affymetrix ATH1 full genome arrays was taken to help elucidate JA-responsive transcription networks in stamens. Gene expression was measured in JA treated stamens beginning at flower developmental stage 12, the stage at which JA is required, and at four subsequent timepoints over the course of 22 hours. Broad comparisons indicate these genes encompass a largely stamen-specific transcriptome, most of which responds to JA in sporophytic tissues. Stringent filtering criteria were applied to the data to identify transcriptional regulators that respond to JA and control anther dehiscence and pollen maturation. This investigation identified two genes encoding closely related transcription factors induced by JA in stage 12 stamens that are solely expressed in reproductive tissues, MYB21 and MYB24 . Further studies indicate that MYB21 is required for filament elongation, proper timing of anther dehiscence, and acts in pollen maturation. A novel eight member gene family without previously ascribed function was also identified in this transcript profiling investigation, where eight of these genes were rapidly induced by JA. These were named the JASMONATE EARLY RESPONSE (JER) genes. JER proteins have conserved domains in common with the ZIM subfamily of zinc finger transcription factors, but no recognizable DNA binding domain. Further experiments with JER proteins support a model where they are targets of the SCFCOI1 complex for ubiquitination and subsequent degradation via the 26S proteasome. Significantly, a dominant-negative construct of one JER proteins strongly suggests that JER proteins act as transcriptional repressors that are targeted by SCF COI1. Thus, the JER proteins provide us with opportunities to ask important new questions in JA biology that were previously difficult, if not impossible, to address. |
