Pollen coat lipids function in pollen-stigma interaction
The pollen-stigma interaction includes pollen adhesion, hydration and pollen tube growth. The pollen coat lipids may display a semi-solid state and waterproof the pollen grain since from its dispersal until its capture on a compatible stigma (Wheeler, Franklin-Tong, & Franklin, 2001). Following the pollen-stigma interactions, a ‘foot-like’ structure is established to enhance pollen-stigma adhesion (Elleman & Dickinson, 1990). During ‘foot’ formation, the lipids may create a capillary system to facilitate water transfer from the stigma cell to desiccated pollen, and pollen hydration occurs. However, it is difficult to clearly separate the processes of pollen adhesion and hydration in most reports. The very-long-chain fatty acids from endothecium is predicted to locate outer pollen coat (Zhan et al., 2018). In the mutant of kcs6 andcer2cer2l2 , the pollen could not hydrate on stigma and the plants are male sterility (Fiebig et al., 2000; Haslam et al., 2015; Zhan et al., 2018). This hydration defects might result from adhesion defect. The delayed hydration in the triple mutant of kcs7/15/21 suggest that the tapetum-derived lipids might assist in establishing a water gradient between the pollen and stigma after pollen adhesion (Fig. 4), and its mutation only postpone the pollen hydration (Fig. 5b). We propose that the endothecium-derived lipid might function early in pollen adhesion, while tapetum-derived lipid might function later in pollen hydration. This hypothesis need to be further studied. Hydration defects have also been reported in mutants of pollen coat proteins, such as EXL4, GRP17, PCP-B (Updegraff, Zhao, & Preuss, 2009; L. Wang et al., 2017; Wolters-Arts, Lush, & Mariani, 1998), indicating that lipids and proteins of pollen coat may work cooperatively to facilitate pollen hydration.