Arabidopsis SNAREs SYP121 and SYP122 mediate the secretion of distinct cargo 2 subsets

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins drive vesicle fusion and contribute to homoeostasis, pathogen defence, cell expansion, and growth in plants. In Arabidopsis thaliana, the two homologous Qa-SNAREs, SYP121 and SYP122 facilitate the majority of secretory traffic to the plasma membrane. In the absence of stress, the single mutants are indistinguishable from wild-type plants, implying a redundancy in their functions. Nonetheless, several studies suggest differences among the secretory cargo of these SNAREs. To address this issue, we conducted an analysis of the proteins secreted by cultured wild-type, syp121 and syp122 mutant Arabidopsis seedlings. Here, we report that a number of cargo proteins are associated differentially with traffic mediated by SYP121 and SYP122. The data also point to important overlaps between the SNAREs. The findings lead us to conclude that the two Qa-SNAREs mediate distinct, but complementary secretory pathways during vegetative plant growth.