Pollen coat lipids derived from both tapetum and endothecium
Pollen coat lipids are major part of pollen coat which constitutes the outer layer of pollen. Previous investigation suggests that some pollen coat lipids are derived from endothecium (Zhan et al., 2018). It was generally considered that pollen coat lipids are mainly derived from tapetum (Hernández-Pinzón et al., 1999). In this work, the reduced pollen coat lipids in the triple mutant of tapetum expressed genes (KCS7 , KCS15 and KCS21 ) suggest that theseKCS s play a role in tapetum to provide pollen coat lipids (Fig. 3, 5). KCS7, KCS15 and KCS21 were expressed in tapetum from stage 8 to 10 while tapetum programmed cell death (PCD) occurred after stage 10 (Fig. 1). It is likely that lipids begin to deposit to the outside of developing microspores before tapetum PCD. With tapetum PCD, all tapetum compounds including lipids deposit to the pollen coat. The expression of KCS6, CER2, and CER2L2 in endothecium is much later than the expression of KCS7, KCS15 and KCS21 in tapetum (Fig. 5a). This further supports the previous prediction that lipids from endothecium deposit outside of the lipids from tapetum (Zhan et al., 2018). KCSs catalyze the synthesis of C20 to C28 lipids, while CER2 and CER2L2 are required for the production of C28 to C34 lipids (Fiebig et al., 2000; Haslam et al., 2015). This indicates that the inner pollen coat lipid from tapetum is likely medium- and long-chain fatty acids (< 28 carbon atoms), while outer pollen coat lipid from endothecium is very-long-chain fatty acids (> 30 carbon atoms).