SEC1A is a major Arabidopsis Sec1/Munc18 gene in vesicle trafficking during pollen tube tip growth

Abstract

SUMMARY Pollen tubes (PTs) grow by the targeted secretion of new cell wall material to their expanding tip region. Sec1/Munc18 (SM) proteins promote membrane fusion through regulation of the SNARE complex. We have previously shown that disruption of protein glycosylation in the Arabidopsis thaliana hpat1 hpat3 double mutant leads to PT growth defects that can be suppressed by reducing secretion. Here, we identified five point mutant alleles of the SM protein SEC1A as hpat1/3 suppressors. The suppressors increased seed set, reduced PT growth defects and reduced the rate of glycoprotein secretion. In the absence of the hpat mutations, sec1a reduced pollen germination and PT elongation producing shorter and wider PTs. Consistent with a defect in membrane fusion, sec1a PTs accumulated secretory vesicles. Though sec1a had significantly reduced male transmission, homozygous sec1a plants maintained full seed set, demonstrating that SEC1A was ultimately dispensable for pollen fertility. However, when combined with a mutation in another SEC1‐like SM gene, keule, pollen fertility was totally abolished. Mutation in sec1b, the final member of the Arabidopsis SEC1 clade, did not enhance the sec1a phenotype. Thus, SEC1A is the major SM protein promoting pollen germination and tube elongation, but in its absence KEULE can partially supply this activity. When we examined the expression of the SM protein family in other species for which pollen expression data were available, we found that at least one Sec1‐like protein was highly expressed in pollen samples, suggesting a conserved role in pollen fertility in other species.

Phospholipase C6 Regulates HydroxyprolineO-arabinosyltransferase-mediated Pollen Tube Growth inArabidopsis

“…plc6-4 also decreased pollen germination, as does plc6-2 and plc6-3 (in the hpat1/3 background), consistent with a role of PLC6 in promoting secretion. Because these phenotypes are similar to what we previously observed in the hpat1/3;exo70a2-2 and other secretion mutant suppressors (Beuder et al, 2020, 2022), and PLCs are known to regulate PI(4,5)P 2 availability at the plasma membrane, we reasoned that PLC6 may influence secretion by regulating exocyst binding site availability and affecting apical secretion; however, there are important phenotypic differences to note between the plc6-4 and the other known hpat1/3 suppressor mutations.…”

“…The GF(EXT3)P reporter is Hyp O- arabinosylated in an HPAT-dependent manner; therefore, our data suggested that Hyp O- arabinosylation may regulate the rate of EXO70A2-mediated extensin secretion. We also observed that loss of function mutations in SEC15A suppressed hpat1/3 PT growth defects (Beuder et al, 2022). Sec15 is another member of the exocyst complex and is required for PT germination, growth, and fertility (Hála et al, 2008).…”

“…hpat1/3 PTs also germinated at a slightly lower frequency compared to WT, and this was not rescued in frh3 PTs (Figure 2B). Because PT germination requires secretion of cell wall materials, and we previously showed that knocking down the secretory pathway rescued hpat1/3 PT growth defects (Beuder et al, 2020, 2022), we wanted to examine how the secretion of HPAT-modified cargos was affected in frh3 PTs. To do this, we crossed the GF(EXT3)P reporter into frh3 and measured the amount of fluorescent signal secreted into the cell wall (as described in Beuder et al, 2020).…”

“…In the plc6-4 single mutant background, GF(EXT3)P secretion was not affected, but it was decreased in exo70a2 and sec1a mutants (Figure 2D, Beuder et al, 2020, 2022). Compared to plc6-4 , PT germination is decreased to a larger extent in exo70a2 and sec15a PTs (Figure 3D; (Beuder et al, 2020).…”

“…hpat1/3 pollen fertility defects were also strongly suppressed by multiple independent mutations in SEC1A , which encodes a Sec1/Munc18 protein that regulates the formation of SNARE complexes between vesicle and target membranes prior to fusion (Beuder et al, 2022; Südhof & Rothman, 2009). Sec1a and Keule (a Sec1p ortholog) function redundantly in pollen and are required for PT germination and fertility (Beuder et al, 2022). sec1a-1 strongly rescued PT growth and seed set in hpat1/3 , and also decreased GF(EXP3)P secretion and pollen germination, similar to the effects caused by exo70a2 .…”