Phospholipase pPLAIIIα Increases Germination Rate and Resistance to Turnip Crinkle Virus when Overexpressed

Patatin-related phospholipase As (pPLAs) are major hydrolases acting on acyl-lipids and play important roles in various plant developmental processes. pPLAIII group members, which lack a canonical catalytic Ser motif, have been less studied than other pPLAs. We report here the characterization of pPLAIII alpha in Arabidopsis (Arabidopsis thaliana) based on the biochemical and physiological characterization of pPLAIII alpha knockouts, complementants, and overexpressors, as well as heterologous expression of the protein. In vitro activity assays on the purified recombinant protein showed that despite lack of canonical phospholipase motifs, pPLAIII alpha had a phospholipase A activity on a wide variety of phospholipids. Overexpression of pPLAIII alpha in Arabidopsis resulted in a decrease in many lipid molecular species, but the composition in major lipid classes was not affected. Fluorescence tagging indicated that pPLAIII alpha localizes to the plasma membrane. Although Arabidopsis pplaIII alpha knockout mutants showed some phenotypes comparable to other pPLAIIIs, such as reduced trichome length and increased hypocotyl length, control of seed size and germination were identified as distinctive pPLAIII alpha-mediated functions. Expression of some PLD genes was strongly reduced in the pplaIII alpha mutants. Overexpression of pPLAIII alpha caused increased resistance to turnip crinkle virus, which associated with a 2-fold higher salicylic acid/jasmonic acid ratio and an increased expression of the defense gene pathogenesis-related protein1. These results therefore show that pPLAIII alpha has functions that overlap with those of other pPLAIIIs but also distinctive functions, such as the control of seed germination. This study also provides new insights into the pathways downstream of pPLAIII alpha.