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 AtsPLA₂β 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.

Plant thioredoxins (Trxs) participate in two redox systems found in different cellular compartments: the NADP-Trx system (NTS) in the cytosol and mitochondria and the ferredoxin-Trx system (FTS) in the chloroplast, where they function as redox regulators by regulating the activity of various target enzymes. The identities of the master regulators that maintain cellular homeostasis and modulate timed development through redox regulating systems have remained completely unknown. Here, we show that proteins consisting of a single cystathionine β-synthase (CBS) domain pair stabilize cellular redox homeostasis and modulate plant development via regulation of Trx systems by sensing changes in adenosine-containing ligands. We identified two CBS domain–containing proteins in Arabidopsis thaliana, CBSX1 and CBSX2, which are localized to the chloroplast, where they activate all four Trxs in the FTS. CBSX3 was found to regulate mitochondrial Trx members in the NTS. CBSX1 directly regulates Trxs and thereby controls H2O2 levels and regulates lignin polymerization in the anther endothecium. It also affects plant growth by regulating Calvin cycle enzymes, such as malate dehydrogenase, via homeostatic regulation of Trxs. Based on our findings, we suggest that the CBSX proteins (or a CBS pair) are ubiquitous redox regulators that regulate Trxs in the FTS and NTS to modulate development and maintain homeostasis under conditions that are threatening to the cell.

The phospholipase A₂ (PLA₂) superfamily of lipolytic enzymes is involved in a number of essential biological processes, such as inflammation, development, host defense, and signal transduction. Despite the proven involvement of plant PLA₂s in many biological functions, including senescence, wounding, elicitor and stress responses, and pathogen defense, relatively little is known about plant PLA₂s, and their genes essentially remain uncharacterized. We characterized three of four Arabidopsis thaliana PLA₂ paralogs (PLA₂-β, -γ, and -δ) and found that they (1) are expressed during pollen development, (2) localize to the endoplasmic reticulum and/or Golgi, and (3) play critical roles in pollen development and germination and tube growth. The suppression of PLA₂ using the RNA interference approach resulted in pollen lethality. The inhibition of pollen germination by pharmacological PLA₂ inhibitors was rescued by a lipid signal molecule, lysophosphatidyl ethanolamine. Based on these results, we propose that plant reproduction, in particular, male gametophyte development, requires the activities of the lipid-modifying PLA₂s that are conserved in other organisms.