Ae. Mcdonald et al., Phosphite (phosphorous acid): Its relevance in the environment and agriculture and influence on plant phosphate starvation response, J PLANT NUT, 24(10), 2001, pp. 1505-1519
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 fo
r a number of crop diseases caused by various species of pathogenic pseudo
fungi belonging to the genus Phytophthora. However, evidence that Phi can b
e directly used by plants as a sole source of nutritional P is lacking. Whe
n Phi is administered in such as way as to allow it to come into contact wi
th bacteria, either associated with plant root systems or in the soil, then
the oxidation of Phi to phosphate (HPO42-; Pi) may take place. By this ind
irect 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 i
tself, 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-ferti
lized, plants. This is because Phi treatment negates the acclimation of pla
nts to Pi deficiency by disrupting the induction of enzymes (e.g., acid pho
sphatase) 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 respo
nd to Pi deprivation at the molecular level. The review concludes by citing
concerns and recommendations regarding the significant input of Phi into f
ood products and the environment that arises from its extensive use in agri
culture and industry.