Abstract
Sporophytic pollen coat proteins (sPCPs) derived from the anther tapetum are deposited into pollen wall cavities and function in pollen-stigma interactions, pollen hydration, and environmental protection. In Arabidopsis, 13 highly abundant proteins have been identified in pollen coat, including seven major glycine-rich proteins GRP14, 16, 17, 18, 19, 20, and GRP-oleosin; two caleosin-related family proteins (AT1G23240 and AT1G23250); three lipase proteins EXL4, EXL5 and EXL6, and ATA27/BGLU20. Here, we show that GRP14, 17, 18, 19, and EXL4 and EXL6 fused with green fluorescent protein (GFP) are translated in the tapetum and then accumulate in the anther locule following tapetum degeneration. The expression of these sPCPs is dependent on two essential tapetum transcription factors, MALE STERILE188 (MS188) and MALE STERILITY 1 (MS1). The majority of sPCP genes are up-regulated within 30 h after MS1 induction and could be restored by MS1 expression driven by the MS188 promoter in ms188, indicating that MS1 is sufficient to activate their expression; however, additional MS1 downstream factors appear to be required for high-level sPCP expression. Our ChIP, in vivo transactivation assay, and EMSA data indicate that MS188 directly activates MS1. Together, these results reveal a regulatory cascade whereby outer pollen wall formation is regulated by MS188 followed by synthesis of sPCPs controlled by MS1.
Highlights
In higher plants, pollen is essential for sexual reproduction and for plant survival
We examined the subcellular localization of six sporophytic pollen coat protein (sPCP) (GRP14, GRP17, GRP18, GRP19, EXL4, and EXL6) using translational green fluorescent protein (GFP) fusions and determined that all were initially synthesized in the tapetum, but were later observed to fill the whole locule and surround the microspores
Studies showed that GRP17 fused with GFP was located in the tapetum (Mayfield et al, 2001; Suzuki et al, 2013), and that a GRP19– GFP protein fusion derived from the tapetum is deposited onto the pollen grain surface (Lévesque-Lemay et al, 2016), which indicates that GRP17 and GRP19 are both sporophytic in origin
Summary
Pollen is essential for sexual reproduction and for plant survival. The pollen coat has multiple roles, in pollen–stigma interactions, pollen grain hydration, and providing protection from harsh environmental conditions (Preuss et al, 1993; Hülskamp et al, 1995; Pacini and Hesse, 2002) It is mainly composed of saturated acyl groups, proteins, and non-polar esters; the exact composition of the sporopollenin structure has, been elusive, partly due to its inert nature. Pollen from the triple mutant of these (pcp-bα/β/γ) displays a substantially reduced hydration rate on stigmas, delayed pollen tube growth, as well as weakened anchoring to the stigmatic surface (Wang et al, 2017).Various pollen coat components such as proteins and fatty acids are derived from both the sporophyte and gametophyte, but how the pollen coat genes function and their regulatory relationships are still largely unknown
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