To elucidate the cellular functions of phospholipase A 2 in plants, an Arabidopsis cDNA encoding a secretory low molecular weight phospholipase A 2 ( At sPLA 2 ␤ ) was isolated. Phenotype analyses of transgenic plants showed that overexpression of At sPLA 2 ␤ promotes cell elongation, resulting in prolonged leaf petioles and inflorescence stems, whereas RNA interferencemediated silencing of At sPLA 2 ␤ expression retards cell elongation, resulting in shortened leaf petioles and stems. At sPLA 2 ␤ is expressed in the cortical, vascular, and endodermal cells of the actively growing tissues of inflorescence stems and hypocotyls. At sPLA 2 ␤ then is secreted into the extracellular spaces, where signaling for cell wall acidification is thought to occur. At sPLA 2 ␤ -overexpressing or -silenced transgenic plants showed altered gravitropism in inflorescence stems and hypocotyls. At sPLA 2 ␤ expression is induced rapidly by auxin treatment and in the curving regions of inflorescence stems undergoing the gravitropic response. These results suggest that At sPLA 2 ␤ regulates the process of cell elongation and plays important roles in shoot gravitropism by mediating auxin-induced cell elongation.

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Plant secretory phospholipases A 2 (sPLA 2 s) probably play important roles in phospholipid signaling based on the data reported from other organisms, but their functions are poorly understood because of the lack of cloned sPLA 2 genes. In this study, we cloned and characterized an Arabidopsis secretory phospholipase A 2 -Q Q (AtsPLA 2 -Q Q) cDNA, and examined its enzymatic properties. The recombinant protein of AtsPLA 2 -Q Q showed maximal enzyme activity at pH 8.0, and required Ca 2þ for activity. Moreover, AtsPLA 2 -Q Q showed sn-2 position speci¢city but no prominent acyl preference, though it showed head group speci¢city to phosphatidylethanolamine rather than to phosphatidylcholine. AtsPLA 2 -Q Q was found to predominate in the mature £ower rather than in other tissues, and subcellular localization analysis con¢rmed that AtsPLA 2 -Q Q is secreted into the intercellular space.