Abstract
BackgroundThe coordination of pollen tube (PT) growth, guidance and timely growth arrest and rupture mediated by PT-pistil interaction is crucial for the PT to transport sperm cells into ovules for double fertilization. The plasma membrane (PM) represents an important interface for cell–cell interaction, and PM proteins of PTs are pioneers for mediating PT integrity and interaction with pistils. Thus, understanding the mechanisms underlying these events is important for proteomics.ResultsUsing the efficient aqueous polymer two-phase system and alkali buffer treatment, we prepared high-purity PM from mature and germinated pollen of rice. We used iTRAQ quantitative proteomic methods and identified 1,121 PM-related proteins (PMrPs) (matched to 899 loci); 192 showed differential expression in the two pollen cell types, 119 increased and 73 decreased in abundance during germination. The PMrP and differentially expressed PMrP sets all showed a functional skew toward signal transduction, transporters, wall remodeling/metabolism and membrane trafficking. Their genomic loci had strong chromosome bias. We found 37 receptor-like kinases (RLKs) from 8 kinase subfamilies and 209 transporters involved in flux of diversified ions and metabolites. In combination with the rice pollen transcriptome data, we revealed that in general, the protein expression of these PMrPs disagreed with their mRNA expression, with inconsistent mRNA expression for 74% of differentially expressed PMrPs.ConclusionsThis study identified genome-wide pollen PMrPs, and provided insights into the membrane profile of receptor-like kinases and transporters important for pollen tube growth and interaction with pistils. These pollen PMrPs and their mRNAs showed discordant expression. This work provides resource and knowledge to further dissect mechanisms by which pollen or the PT controls PMrP abundance and monitors interactions and ion and metabolite exchanges with female cells in rice.
Highlights
The coordination of pollen tube (PT) growth, guidance and timely growth arrest and rupture mediated by PT-pistil interaction is crucial for the PT to transport sperm cells into ovules for double fertilization
Preparation and purity detection of plasma membranes from pollen To identify PM proteins, we first prepared PM vesicles from rice mature pollen grains (MPGs) (Fig. 1a) and germinated pollen grains (GPGs) (Fig. 1b) by using the aqueous two-phase partition system followed by a high pH carbonate buffer wash (Fig. 1c)
PMA2 was detected as two different isoforms in rice pollen: one showed increased abundance from the entire cell (EC) lysate to carbonate buffer-washed PM (CPM) and one was almost undetectable in EC lysates but was most abundant in CPM (Fig. 2a)
Summary
The coordination of pollen tube (PT) growth, guidance and timely growth arrest and rupture mediated by PT-pistil interaction is crucial for the PT to transport sperm cells into ovules for double fertilization. The tube further journeys within the pistil and arrives in the synergids of embryo sacs, where it arrests growth and ruptures to release sperm cells for double fertilization This process requires maintenance of PT integrity and PT–pistil interaction to guide the PT toward the embryo sac and regulate the timely growth arrest of the PT and rupture for immobile gamete release, thereby precisely guaranteeing the one PT to one ovule relationship [1]. Additional RLKs are involved in sensing these female factors
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