The ZmMYB84-ZmPKSB regulatory module controls male fertility through modulating anther cuticle-pollen exine trade-off in maize anthers

Anther cuticle and pollen exine are two crucial lipid layers that ensure normal pollen development and pollen-stigma interaction for successful fertilization and seed production in plants. Their formation processes share certain common pathways of lipid biosynthesis and transport across four anther wall layers. However, molecular mechanism underlying a trade-off of lipid-metabolic products to promote the proper formation of the two lipid layers remains elusive. Here, we identified and characterized a maize male-sterility mutant pksb, which displayed denser anther cuticle but thinner pollen exine as well as delayed tapetal degeneration compared with its wild type. Based on map-based cloning and CRISPR/Cas9 mutagenesis, we found that the causal gene (ZmPKSB) of pksb mutant encoded an endoplasmic reticulum (ER)-localized polyketide synthase (PKS) with catalytic activities to malonyl-CoA and midchain-fatty acyl-CoA to generate triketide and tetraketide alpha-pyrone. A conserved catalytic triad (C171, H320 and N353) was essential for its enzymatic activity. ZmPKSB was specifically expressed in maize anthers from stages S8b to S9-10 with its peak at S9 and was directly activated by a transcription factor ZmMYB84. Moreover, loss function of ZmMYB84 resulted in denser anther cuticle but thinner pollen exine similar to the pksb mutant. The ZmMYB84-ZmPKSB regulatory module controlled a trade-off between anther cuticle and pollen exine formation by altering expression of a series of genes related to biosynthesis and transport of sporopollenin, cutin and wax. These findings provide new insights into the fine-tuning regulation of lipid-metabolic balance to precisely promote anther cuticle and pollen exine formation in plants.

Automated summary

Anther cuticle and pollen exine are two crucial lipid layers in plants that are important for normal pollen development and successful fertilization. The molecular mechanisms underlying the formation of these lipid layers have been unclear. In this study, we identified a maize mutant called pksb, which had a denser anther cuticle, thinner pollen exine, and delayed tapetal degeneration compared to the wild type. We found that the causal gene of the pksb mutant (ZmPKSB) encoded a polyketide synthase (PKS) that plays a role in lipid biosynthesis. The ZmPKSB gene was specifically expressed in maize anthers and was regulated by a transcription factor called ZmMYB84. Loss of function of ZmMYB84 resulted in similar phenotypes as the pksb mutant. The ZmMYB84-ZmPKSB regulatory module controlled the formation of the anther cuticle and pollen exine by regulating the expression of genes related to sporopollenin, cutin, and wax biosynthesis and transport. These findings provide new insights into the regulation of lipid metabolism in plants.