PCP‐B class pollen coat proteins are key regulators of the hydration checkpoint in Arabidopsis thaliana pollen–stigma interactions

Abstract

Summary The establishment of pollen–pistil compatibility is strictly regulated by factors derived from both male and female reproductive structures. Highly diverse small cysteine‐rich proteins (CRPs) have been found to play multiple roles in plant reproduction, including the earliest stages of the pollen–stigma interaction. Secreted CRPs found in the pollen coat of members of the Brassicaceae, the pollen coat proteins (PCPs), are emerging as important signalling molecules that regulate the pollen–stigma interaction.Using a combination of protein characterization, expression and phylogenetic analyses we identified a novel class of Arabidopsis thaliana pollen‐borne CRPs, the PCP‐Bs (for pollen coat protein B‐class) that are related to embryo surrounding factor (ESF1) developmental regulators. Single and multiple PCP‐B mutant lines were utilized in bioassays to assess effects on pollen hydration, adhesion and pollen tube growth.Our results revealed that pollen hydration is severely impaired when multiple PCP‐Bs are lost from the pollen coat. The hydration defect also resulted in reduced pollen adhesion and delayed pollen tube growth in all mutants studied.These results demonstrate that At PCP‐Bs are key regulators of the hydration ‘checkpoint’ in establishment of pollen–stigma compatibility. In addition, we propose that interspecies diversity of PCP‐Bs may contribute to reproductive barriers in the Brassicaceae.

Live-cell imaging of early events following pollen perception in self-incompatibleArabidopsis thaliana

“…Thus, inhibition of the incompatible pollen acts very early, within the first minutes following stigma contact, blocking pollen hydration. As previously described, hydration appears as the first check point controlling pollen rejection in SI species with dry stigma (Zuberi and Dickinson, 1985; Dickinson, 1995; Samuel et al , 2009; Hiroi et al , 2013; Safavian and Goring, 2013; Wang et al , 2016). In this study, the hydration threshold required to trigger germination was found to correspond to a pollen L/W ratio below 1.4.…”

“…5C). We calculated the slopes of the hydration curves by linear regression, as previously described (Wang et al , 2016), and found that early and intermediate germinating pollen grains had the highest hydration rates compared with late and non germinating pollen grains (Fig 5C). Non germinating pollen exhibited a very slow hydration rate (slope – 0.043) associated with a poor water uptake at 10 min, deduced from the high pollen ratio (mean of 1.77).…”

“…Because the increase of water content in pollen grain is correlated with change of its shape from ellipsoid to spheroid (Zuberi and Dickinson, 1985; Hiroi et al , 2013; Wang et al , 2016), for each pollen grain, we recorded every two minutes the ratio between the long and the wide axis (L/W) to estimate the pollen hydration rate until germination. Slopes were determined by linear regression of the hydration curve based on pollen ratio evolution during the first 10 minutes period (5 data points), using the curve f = ax + b.…”

Identification of a role for an E6-like 1 gene in early pollen-stigma interactions in Arabidopsis thaliana

“…Interestingly, both of these secreted glycoproteins were also demonstrated to bind to small Brassica class-A pollen coat proteins (PCPs; Doughty et al 1993;Hiscock et al 1995;Takayama et al 2000). More recently, class-B PCPs were demonstrated to be required for pollen adhesion and hydration on stigmas as pollen grains from a triple pcp-b mutant were impaired in these processes (Wang et al 2017).…”

Characterization of the role of several COPI complex isoforms during the early acceptance of compatible pollen grains inArabidopsis thaliana

“…During foot formation, it is hypothesized that the Brassicaceae-specific compatible pollen recognition occurs, although the full mechanism is not yet clearly understood (Hulskamp et al 1995;Pruitt 1999). There are two stigma-specific Brassica oleracea glycoproteins that play a role in this; S-Locus Glyco-protein and S-Locus Related1 (SLR1) (Luu et al 1997;Luu et al 1999) which may interact with two pollen coat specific proteins, PCP-A1 and SLR1-BP, respectively (Doughty et al 1998;Takayama et al 2000;Wang et al 2017). At this same moment, self-incompatible pollen grains can be recognized and deactivation of the basal pollen response beings while simultaneously activating the self-incompatibility signaling pathway (for review see, (Dresselhaus and Franklin-Tong 2013;Sawada et al 2014).…”