| In angiosperms, pollen wall pattern formation is determined by primexinedeposition on the microspores. In this study, we show that an auxin response factor,ARF17plays an essential role for primexine formation and pollen wall developmentin Arabidopsis thaliana. The knock out mutant of arf17exhibited a male sterilephenotype with normal vegetative growth. ARF17was expressed in microsporocytesand microgametophytes from meiosis to the bicellular microspore stage. Semi-thincross sections and transmission electron microscopy (TEM) observation showed thatprimexine was absent in the arf17mutant, which leads to pollen wall patterningdefects and pollen degradation. In addition, ARF17mutation caused reduced malegametophyte transmission efficiency. In vitro and in vivo pollen geimination analysisshowed that pollen tube growth waa slower that that in wild type. Callose deposition,as well as he expression of CALLOSE SYNTHASE5(CalS5) was significantly reduced inthe arf17mutan. ARF proteins can regulate the expression of genes with an AuxRE(TGTCTC/GAGACA) in their promoter. The CalS5promoter contains one AuxRE (-863to-858). Both Chromatin immunoprecipitation (ChIP) and Electrophoretic MobilityShift Assay (EMSA) showed that ARF17can directly bind to the CalS5promoter.Plants expressing the chimeric ARF17repressor under ARF17native promoter(ARF17-SRDX) mimics phenotype of arf17at both cytological and molecular level,suggesting ARF17functions as a transcriptional activator. As indicated by expressionof the DR5::GFP auxin reporter, auxin signaling appeared to be specifically impairedin arf17anthers. Taken together, our results suggest that ARF17is essential for pollenwall patterning in Arabidopsis by modulating primexine formation at least partiallythrough direct regulation of CalS5gene expression. Our study provides importantevidence that auxin signaling pathway plays an important role in pollen walldevelopment and new insight for pollen wall pattern formation through ARF17mediated signaling pathway. |
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