Improving phosphorus use
Phosphorus, like nitrogen, is an essential nutrient and limits crop growth in nearly 40% of agricultural land, but estimates suggest this could be as much as 70%. Orthophosphate is the only bioavailable form for plants and is a nonrenewable resource obtained from mining. Because of its reactivity with soils rich in iron and aluminum oxides and conversion by soil microbes to forms inaccessible to plants, crops capture <30% of phosphorus applied as fertilizer. Moreover, phosphate excesses are a primary contributor to eutrophication. Recent progress toward the development of plants with improved phosphorus use will help maintain supplies of this resource and could limit overuse.
Traditional breeding and QTL analysis identified a rice variety with a protein kinase [phosphorus-starvation tolerance 1 (PSTOL1)] that acts as an enhancer of early root growth and provides tolerance to phosphorus deficiency. PSTOL1 overexpression enhanced grain yield in phosphorus-deficient soil through altered root architecture that allows for better nutrient uptake. Variations in root structure that enhance phosphorus mobilization could also be combined with improved knowledge of key plant-fungal interactions. For example, the endophytic fungus Colletotrichum tofieldiae, which associates with Arabidopsis thaliana in the wild, transfers phosphorus to shoots to promote plant growth and increases plant fertility under phosphorus-limited conditions.
Complementing these efforts, our molecular understanding of how phosphorus is sensed was advanced by the identification of the SPX inositol polyphosphate-binding domain shared by phosphate transporters, signal transduction proteins, and inorganic polyphosphate polymerase. This work suggests that association of SPX domain proteins with transcription factors modulates phosphorus starvation responses. There are also efforts to alter the form of phosphorus fertilizer used by plants, with the added benefit of weed control. Transgenic plants expressing a bacterial protein that oxidizes phosphite to orthophosphate can grow with 30 to 50% less phosphorus input. Moreover, because orthophosphate does not discriminate between crops and weeds, use of phosphite fertilizer allowed transgenic plants to grow with up to 10-fold greater biomass in competition with weeds than when provided with orthophosphate. Deciphering the phosphorus response network in plants promises to reveal new targets for improving nutrient use.
