When inorganic phosphate is limiting, Arabidopsis has the facultative abili
ty to metabolize exogenous nucleic acid substrates, which we utilized previ
ously to identify insensitive phosphate starvation response mutants in a co
nditional genetic screen. In this study, we examined the effect of the phos
phate analog, phosphite (Phi), on molecular and morphological responses to
phosphate starvation. Phi significantly inhibited plant growth on phosphate
-sufficient (2 mm) and nucleic acid-containing (2 mm phosphorus) media at c
oncentrations higher than 2.5 mm. However, with respect to suppressing typi
cal responses to phosphate limitation, Phi effects were very similar to tho
se of phosphate. Phosphate starvation responses, which we examined and foun
d to be almost identically affected by both anions, included changes in: (a
) the root-to-shoot ratio; (b) root hair formation; (c) anthocyanin accumul
ation; (d) the activities of phosphate starvation-inducible nucleolytic enz
ymes, including ribonuclease, phosphodiesterase, and acid phosphatase; and
(e) steady-state mRNA levels of phosphate starvation-inducible genes. It is
important that induction of primary auxin response genes by indole-3-aceti
c acid in the presence of growth-inhibitory Phi concentrations suggests tha
t Phi selectively inhibits phosphate starvation responses. Thus, the use of
Phi may allow further dissection of phosphate signaling by genetic selecti
on for constitutive phosphate. starvation response mutants on media contain
ing organophosphates as the only source of phosphorus.