Predicted functions, subcellular localizations, and expression patterns of genes encoding secretory proteins associated with pine pollen germination

Pollen germination represents the transition from mature to germinated pollen, which is a critical event in seed plant reproduction. Cell wall–related processes represent many of the cellular activities during this transition but information on the genes involved is limited, particularly in gymnosperms. Yeast secretion trap (YST) is employed in the current study to isolate cDNAs encoding secretory proteins associated with in vitro germinated pollen of loblolly pine (Pinus taeda). YST is a functional screen and when coupled with computational prediction provides insights on the diversity of populations, subcellular localizations, and functions of the encoded proteins. Based on 100 confirmed YST clones, our results identified 21 known, 4 unknown, and 10 hypothetical genes encoding secretory proteins, which are mostly predicted to be localized in the plasma membrane or extracellular space. Based on the known sequences, pollen germination involves genes associated with cell wall degradation, biosynthesis and remodeling, stress and defense responses, signaling, and protein processing. This study characterizes further 10 highly expressed cDNAs based on their temporal (mature pollen vs germinated pollen) and spatial (germinated pollen, young stem, and needle tissue) expression patterns, as well as sequence features such as the presence of transmembrane α-helix, glycosylphosphatidylinositol anchor, and conserved domains. It appears that pine pollen germination involves genes with static and dynamic expression profiles including those with germination-specific expressions. This study confirms the distinct expression profiles of mature and germinated pollen, and expands our understanding of the likely molecular players and processes associated with pine pollen germination, particularly pollen tube wall formation.