| Summary: | A <i>PACLOBUTRAZOL-RESISTANCE</i> (<i>PRE</i>) gene family, consisting of six genes in <i>Arabidopsis thaliana</i>, encodes a group of helix-loop-helix proteins that act in the growth-promoting transcriptional network. To delineate the specific role of each of the <i>PRE</i> genes in organ growth, we took a reverse genetic approach by constructing high order <i>pre</i> loss-of-function mutants of <i>Arabidopsis thaliana</i>. In addition to dwarf vegetative growth, some double or high order <i>pre</i> mutants exhibited defective floral development, resulting in reduced fertility. While <i>pre2pre5</i> is normally fertile, both <i>pre2pre6</i> and <i>pre5pre6</i> showed reduced fertility. Further, the reduced fertility was exacerbated in the <i>pre2pre5pre6</i> mutant, indicative of the redundant and critical roles of these PREs. Self-pollination assay and scanning electron microscopy analysis showed that the sterility of <i>pre2pre5pre6</i> was mainly ascribed to the reduced cell elongation of anther filament, limiting access of pollens to stigma. We found that the expression of a subset of flower-development related genes including <i>ARGOS</i>, <i>IAA19</i>, <i>ACS8,</i> and <i>MYB24</i> was downregulated in the <i>pre2pre5pre6</i> flowers. Given these results, we propose that PREs, with unequal functional redundancy, take part in the coordinated growth of floral organs, contributing to successful autogamous reproduction in <i>Arabidopsis thaliana.</i>
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