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 pPLAIII[delta], 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 pPLAIII[delta] but also distinctive functions, such as the control of seed germination. This study also provides new insights into the pathways downstream of pPLAIII[alpha].