The tapetum regulatory cascade for pollen coat lipid formation
Tapetum provides nutrition for microspore development, secrets hydrolases for tetrad wall dissolution, and supply materials for pollen wall and pollen coat formation during anther development. In Arabidopsis, the genetic regulatory pathway DYT1-TDF1-AMS-MS188-MS1 is important for tapetum development and function (Gu et al., 2014; Lou et al., 2014; Lou et al., 2018; Zhu et al., 2011). In this pathway, AMS directly regulates ABCG26 for sporopollenin transportation (J. Xu et al., 2010) and MGT5 to provide the Mg2+ for microspores development (X. F. Xu et al., 2015). MS188 directly regulates CYP703A2 and other sporopollenin biosynthesis genes for sporopollenin synthesis and sexine formation (K. Wang et al., 2018; Xiong et al., 2016). Sexine is the outer pollen wall where pollen coat (including pollen coat lipids) are deposited. MS1 is a transcription factor for late tapetum development directly regulated by MS188. It regulates the expression of multiple pollen coat proteins (Lu et al., 2020). MS1 also regulates the expression of KCS7 ,KCS15 and KCS21 for pollen coat lipid synthesis (Fig. 2, 5). This pollen coat lipid may provide a matrix for assembly of pollen coat PCPs (Fig. 5b). After tapetum PCD, the endothecium-derived lipids deposited outside of the pollen coat (Fig. 5b). Together, this work reveals that following outer pollen wall formation regulated by MS188, its directly regulating transcription factor MS1 regulates the downstream gene expression for pollen coat formation. This regulatory cascade is helpful to make sure that pollen wall and pollen coat are orderly synthesized during anther development.