Secretory low molecular weight phospholipase A₂ plays important roles in cell elongation and shoot gravitropism in Arabidopsis

To elucidate the cellular functions of phospholipase A₂ in plants, an Arabidopsis cDNA encoding a secretory low molecular weight phospholipase A₂ (AtsPLA₂β) was isolated. Phenotype analyses of transgenic plants showed that overexpression of AtsPLA ₂β promotes cell elongation, resulting in prolonged leaf petioles and inflorescence stems, whereas RNA interference-mediated silencing of AtsPLA₂β expression retards cell elongation, resulting in shortened leaf petioles and stems. AtsPLA₂β is expressed in the cortical, vascular, and endodermal cells of the actively growing tissues of inflorescence stems and hypocotyls. AtsPLA₂β then is secreted into the extracellular spaces, where signaling for cell wall acidification is thought to occur. AtsPLA₂β-overexpressing or -silenced transgenic plants showed altered gravitropism in inflorescence stems and hypocotyls. AtsPLA₂β expression is induced rapidly by auxin treatment and in the curving regions of inflorescence stems undergoing the gravitropic response. These results suggest that AtsPLA₂β regulates the process of cell elongation and plays important roles in shoot gravitropism by mediating auxin-induced cell elongation.