The Regulation of Sporopollenin Biosynthesis Genes for Rapid Pollen Wall Formation

Abstract

Sporopollenin is the major component of the outer pollen wall (sexine). It is synthesized using a pathway of approximately eight genes in Arabidopsis (<i>Arabidopsis thaliana</i>). MALE STERILITY188 (MS188) and its direct upstream regulator ABORTED MICROSPORES (AMS) are two transcription factors essential for tapetum development. Here, we show that all the sporopollenin biosynthesis proteins are specifically expressed in the tapetum and are secreted into anther locules. MS188, a MYB transcription factor expressed in the tapetum, directly regulates the expression of <i>POLYKETIDE SYNTHASE A</i> (<i>PKSA</i>), <i>PKSB</i>, <i>MALE STERILE2</i> (<i>MS2</i>), and a CYTOCHROME P450 gene (<i>CYP703A2</i>). By contrast, the expression of <i>CYP704B1</i>, <i>ACYL-COA SYNTHETASE5</i> (<i>ACOS5</i>), <i>TETRAKETIDE a-PYRONE REDUCTASE1</i> (<i>TKPR1</i>) and <i>TKPR2</i> are significantly reduced in <i>ams</i> mutants but not affected in <i>ms188</i> mutants. However, MS188 but not AMS can activate the expression of <i>CYP704B1</i>, <i>ACOS5</i>, and <i>TKPR1</i> In <i>ms188</i>, dominant suppression of MS188 homologs reduced the expression of these genes, suggesting that MS188 and other MYB family members play redundant roles in activating their expression. The expression of some sporopollenin synthesis genes (<i>PKSA</i>, <i>PKSB</i>, <i>TKPR2</i>, <i>CYP704B1</i>, and <i>ACOS5</i>) was rescued when <i>MS188</i> was expressed in <i>ams</i> Therefore, MS188 is a key regulator for activation of sporopollenin synthesis, and AMS and MS188 may form a feed-forward loop that activates the expression of the sporopollenin biosynthesis pathway for rapid pollen wall formation.