Molecular characterization of a tomato purple acid phosphatase during seed germination and seedling growth under phosphate stress.

KEY MESSAGE: SlPAP1 is a phosphate starvation responsive purple acid phosphatase during tomato seed germination. Future research on its family members in tomato might improve the phosphate stress tolerance. Phosphate deficiency is a major constraint upon crop growth and yield. In response to phosphate deficiency, plants secrete acid phosphatases (APases) to scavenge organic phosphate from soil. In this study, we investigated the impact of Pi starvation on germination and seedling growth of tomato, and we then cloned and characterized a phosphate starvation responsive purple APase (SlPAP1) that expressed during tomato seedling growth. Our results showed that phosphate deficiency reduced germination and growth rates of tomato, and also increased intracellular and secretory APase activity in a concentration-dependent manner. An in-gel activity assay found that two APases of 50 and 75 kDa were secreted during conditions of phosphate deficiency. SlPAP1 is a single copy gene belonging to a small gene family. It was expressed as a cDNA of approximately 1.5 kbp encoding a secreted glycoprotein of 470 amino acids. Northern blot analysis showed that SlPAP1 was specifically expressed in root tissue during phosphate deficiency. SlPAP1 had high sequence identity (56-89%) with other plant PAPs and contained highly conserved metal-binding residues. SlPAP1 is the first PAP to be cloned and characterized from tomato. This study provides useful information for future research on PAP family members in tomato, leading to better understanding of phosphate deficiency in this important crop plant.