PHOSPHITE (PHOSPHOROUS ACID): ITS RELEVANCE IN THE ENVIRONMENT AND AGRICULTURE AND INFLUENCE ON PLANT PHOSPHATE STARVATION RESPONSE

Abstract

Phosphites (H₂PO₃ ⁻; Phi) are alkali metal salts of phosphorous acid [HPO(OH)₂] that are being widely marketed either as an agricultural fungicide or as a superior source of plant phosphorus (P) nutrition. Published research conclusively indicates that Phi functions as an effective control agent for a number of crop diseases caused by various species of pathogenic pseudo fungi belonging to the genus Phytophthora. However, evidence that Phi can be directly used by plants as a sole source of nutritional P is lacking. When Phi is administered in such as way as to allow it to come into contact with bacteria, either associated with plant root systems or in the soil, then the oxidation of Phi to phosphate (HPO₄ ²⁻; Pi) may take place. By this indirect method Phi could thus become available to the plant as a P nutrient. The rates at which this occurs are slow, taking months or as much as a year, depending on the soil type. Phi is not without direct effects on plants itself, as Phi concentrations comparable to those required to control plant infection by pathogenic Phytophthora, or to restrict Phytophthora growth in sterile culture, are extremely phytotoxic to Pi-deprived, but not Pi-fertilized, plants. This is because Phi treatment negates the acclimation of plants to Pi deficiency by disrupting the induction of enzymes (e.g., acid phosphatase) and transporters (e.g., high-affinity plasmalemma Pi translocator) characteristic of their Pi starvation response. Thus, Phi intensifies the deleterious effects of P-deficiency by ‘tricking’ Pi-deprived plant cells into sensing that they are Pi-sufficient, when, in fact, their cellular Pi content is extremely low. The Phi anion appears to effectively obstruct the signal transduction pathway by which plants (and yeast) perceive and respond to Pi deprivation at the molecular level. The review concludes by citing concerns and recommendations regarding the significant input of Phi into food products and the environment that arises from its extensive use in agriculture and industry.

ACKNOWLEDGMENTS

Work in W.C.P's lab is supported by research and equipment grants from the Natural Sciences and Engineering Research Council of Canada (NSERC). We are also grateful to Prof. K.G. Ragothama (Purdue Univ.) and Dr. Julie Niere (Royal Melbourne Institute of Technology) for their helpful discussions and comments on the manuscript.

Notes

¹Although the terms phosphite and phosphonate have both been used to describe salts of phosphorous acid, HPO(OH)₂, phosphonate is also employed for the nomenclature of compounds containing a C-P bond [5]. To avoid ambiguity this review therefore uses the term phosphite for the description of alkali metal salts of phosphorous acid.

²Large amounts of hypophosphite (H₂PO₂ ⁻; Fig. 1) are being used to reduce metal ions in chemical-plating processes such as those used in compact-disk manufacturing [2]. After metal plating, wastewaters containing high concentrations of Phi have been released into the environment. Treatment of Phi-containing industrial wastes is becoming a difficult problem associated with such high-tech industries [2].