Figure 8.

Model of the action of type III (At5g62080) and non-type III LTPs in exine formation. Exine begins to appear at stage 8, and the formation is complete at stage 11. In tapetum cells, secretory vesicles derived from the ER-TGN system contain LTPs bound or unbound to ER-produced exine precursors. LTPs and exine precursors are released into the locule, where unbound LTPs may or may not bind to free exine precursors. LTPs and exine precursors, unbound or bound, move to the microspore (MSP) surface, where exine is assembled. Type III LTPs, but not other LTPs, remain in exine as permanent constituents. Other exine precursors produced by cytosolic enzymes in tapetum cells are transported to the locule via plasma membrane ABCG26 and, with or without binding to LTPs in the locule, move to the exine for assembly.