ABSTRACT

Phosphate (Pi) is a crucial but limiting macronutrient for plant growth and metabolism. Agricultural Pi deficiency is alleviated by the massive application of Pi fertilizers. However, Pi assimilation by fertilized crops is quite inefficient and unsustainable, as a large proportion of the applied Pi becomes immobile in the soil or may run off into, and thereby pollute, nearby surface waters. The projected depletion of global rock-Pi reserves has prompted plant scientists to develop strategies and molecular tools for engineering Pi-efficient transgenic crops. However, for this to succeed we need to achieve a thorough understanding of the intricate molecular and biochemical adaptations of Pi-deprived (−Pi) plants, which include the de novo synthesis of purple acid phosphatases (PAPs). Purple acid phosphatases likely function in −Pi plants to recycle and scavenge Pi from intra- and extracellular organic Pi-esters. Three Pi-starvation inducible (PSI) PAP isozymes demonstrating distinctive physical and kinetic characteristics have been recently purified and characterized from −Pi tomato suspension cell cultures. Two are secreted monomeric PAPs having Mr s of 84 and 57 kDa, whereas the third is a 142 kDa heterodimeric intracellular 216PAP composed of 63 and 57 kDa subunits. The three PSI tomato PAPs efficiently hydrolyzed Pi from a wide variety of Pi-esters under acidic conditions and are multifunctional proteins exhibiting significant alkaline peroxidase activity, indicating a potential additional function in the metabolism of reactive oxygen species. Time-course immunoblot studies of tomato cells and seedlings undergoing a transition from Pi sufficiency to Pi deficiency revealed a close relationship between total acid phosphatase activity and relative amounts of the three antigenic PSI PAPs. These results corroborated recent transcript profiling studies suggesting that PSI proteins are subject to both temporal and tissue-specific syntheses in −Pi plants. The discovery of intracellular and secreted PSI tomato PAP isozymes may lead to biotechnological strategies to increase Pi efficiency in tomato plants grown on Pi-limited soils. The aim of this chapter is to present an overview of the properties and functions of PSI PAPs in tomato Pi nutrition.