INTRODUCTION
Pollen-pistil interaction is critical for the successful fertilization
of flowering plants. This interaction is a crucial step in preventing
inbreeding and maintaining species identity, thus contributing to
angiosperm diversity. Pollen-pistil interaction consists of multiple
selective steps, including pollen adhesion, hydration, germination and
polarized tube growth (Bedinger, Broz, Tovar-Mendez, & McClure, 2017;
Edlund, Swanson, & Preuss, 2004; Zheng, Lin, Liang, Wang, & Chen,
2018). Pollen adhesion and hydration are two earliest events in
pollination among species with dry stigmas. These highly regulated
processes require proteins and lipids deriving from pollen wall (Elleman
& Dickinson, 1990; Safavian & Goring, 2013).
The mature pollen wall of flowering plants includes three main layers,
intine, exine and pollen coat. Intine is produced by microspores and is
mainly composed of pectin, cellulose and hemicellulose. Exine is
constituted of sexine and nexine (Lou et al., 2014). The main
composition of sexine is sporopollenin. Its precursors are synthesized
by multiple enzymes regulated by a MYB transcription factor MS188 in
tapetum (K. Wang et al., 2018; Z. B. Zhang et al., 2007). Pollen coat
contains many sticky and heterogenous material composed of lipids,
proteins, carotenoids and polysaccharides (Hernández-Pinzón, Ross,
Barnes, Damant, & Murphy, 1999; Piffanelli, Ross, & Murphy, 1998),
which is crucial to pollen protection from abiotic stresses, successful
pollen contact, hydration and subsequent pollen germination on dry
stigma (Dickinson, 1995; Elleman & Dickinson, 1990). The pollen coat
contains various components including pollen coat proteins (PCP) and
pollen coat lipids. PCPs are derived from tapetum after its programmed
cell death (PCD) alongside gametophytically derived proteins. In
Arabidopsis, several lipases and lipid-binding oleosin proteins were
identified in PCPs (Mayfield, Fiebig, Johnstone, & Preuss, 2001). Male
sterility1 (MS1) is a PHD transcription factor for late tapetum
development (Wilson, Morroll, Dawson, Swarup, & Tighe, 2001). These
sporophyte-derived PCPs are regulated by MS1 in tapetum (Lu et al.,
2020). They may play roles in the early events of pollen-stigma
recognition including pollen adhesion and hydration. S-locus
cysteine-rich protein (SCR), SLR1-BP1/2, PCP-A1(PCP7) and PCP-Bs have
been identified as gametophyte-derived PCPs (Doughty et al., 1998;
Nasrallah & Nasrallah, 2014; Takayama et al., 2000; L. Wang et al.,
2017). They are likely to play important roles in pollen germination and
pollen tube growth.
Lipids are one of the major subcellular components, and comprise
different combinations and positional distributions of fatty acids. They
play important roles in plants by forming membrane structures, acting as
storage lipids, signaling molecules, and surface coverings (Li-Beisson,
Nakamura, & Harwood, 2016). The pollen coat lipids display a semi-solid
state and may guide the water transfer from the stigma to pollen during
pollen-stigma interaction (Edlund et al., 2004). In lipid metabolism,
3-ketoacyl CoA synthase (KCS) are involved in fatty acid elongation or
very long chain fatty acid (VLCFA) synthesis (Haslam & Kunst, 2013;
Joubès et al., 2008). Eceriferum 2 (Cer2) and cer2-like 2 (Cer2l2)
encode BAHD acyltransferase and enhance the elongation of VLCFA from 28
to 30 carbon atoms catalyzed by KCS6 (Fiebig et al., 2000; Haslam et
al., 2015). The kcs6 single mutant and cer2cer2l2 double
mutant can produce mature pollen. However, they fail to hydrate (Fiebig
et al., 2000; Haslam et al., 2015; Hulskamp, Schneitz, & Pruitt, 1995;
Preuss, Lemieux, Yen, & Davis, 1993). KCS6, CER2 and CER2L2 express in
endothecium suggesting that endothecium plays a role to synthesize
pollen coat lipids for pollen hydration (Zhan, Xiong, Wang, & Yang,
2018). It was generally considered that pollen coat lipids are derived
from tapetum (Hernández-Pinzón et al., 1999). It is not clear how
tapetum contributes to pollen coat lipids synthesis.
The KCS family in Arabidopsis contains 21 KCS members
(Costaglioli et al., 2005; Joubès et al., 2008). In this study, we
reported KCS7, KCS15 and KCS21 were expressed in tapetum. They act
downstream of MS1, a regulator for late tapetum development. The reduced
pollen coat lipid in the triple mutant (kcs7/15/21 ) and defective
pollen hydration demonstrate that they play redundant roles for pollen
coat lipid synthesis for pollen hydration.