Abstract
Phosphite is a less oxidized form of phosphorus than phosphate. Phosphite is considered to be taken up by the plant through phosphate transporters. It can mimic phosphate to some extent, but it is not metabolized into organophosphates. Phosphite could therefore interfere with phosphorus signalling networks. Typical physiological and transcriptional responses to low phosphate availability were investigated and the short-term kinetics of their reversion by phosphite, compared with phosphate, were determined in both roots and shoots of Arabidopsis thaliana. Phosphite treatment resulted in a strong growth arrest. It mimicked phosphate in causing a reduction in leaf anthocyanins and in the expression of a subset of the phosphate-starvation-responsive genes. However, the kinetics of the response were slower than for phosphate, which may be due to discrimination against phosphite by phosphate transporters PHT1;8 and PHT1;9 causing delayed shoot accumulation of phosphite. Transcripts encoding PHT1;7, lipid-remodelling enzymes such as SQD2, and phosphocholine-producing NMT3 were highly responsive to phosphite, suggesting their regulation by a direct phosphate-sensing network. Genes encoding components associated with the 'PHO regulon' in plants, such as At4, IPS1, and PHO1;H1, generally responded more slowly to phosphite than to phosphate, except for SPX1 in roots and MIR399d in shoots. Two uncharacterized phosphate-responsive E3 ligase genes, PUB35 and C3HC4, were also highly phosphite responsive. These results show that phosphite is a valuable tool to identify network components directly responsive to phosphate.
Highlights
Phosphite (H2PO3, Phi) is a less oxidized form of phosphorus (P) than phosphate (H2PO4, inorganic phosphorus/phosphate/H2PO4– (Pi))
Prior to transfer or harvest, the seedlings were scanned at 600 dpi resolution to determine root and root hair length, growth rate, and lateral root number (LSM Image Browser v4.2; Carl Zeiss Microscopy GmbH, Jena, Germany)
This conclusion was drawn from a series of experiments where P-sufficient plants were transferred to Pi-containing or Pi-free media supplemented with increasing Phi concentrations, or where seeds were germinated on these media (Carswell et al, 1996; Ticconi et al, 2001; Varadarajan et al, 2002; Berkowitz et al, 2013; Eshraghi et al, 2014)
Summary
Phosphite (H2PO3–, Phi) is a less oxidized form of phosphorus (P) than phosphate (H2PO4–, Pi). Phi can be translocated, and it preferentially accumulates in sink tissues (Nartvaranant et al, 2004). Plants are not able to metabolize Phi (McDonald et al, 2001). P-limited plants are highly sensitive to Phi and display toxicity symptoms such as leaf chlorosis and stunted growth (McDonald et al, 2001; Ratjen and Gerendas, 2009; Thao and Yamakawa, 2009). Other detrimental effects caused by Phi are the arrest of primary root growth, yellowing of the leaf lamina of young leaves, and a patchy accumulation of anthocyanins in older leaves (Varadarajan et al, 2002). It was recently found that the accumulation of Phi impacts on metabolism in Arabidopsis thaliana, leading to changes in the levels of several central metabolites (Berkowitz et al, 2013)
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