White lupin: A model system for understanding plant adaptation to low phosphorus availability

Abstract

Phosphorus (P), in its ionized form (Pi), is one of the most limiting nutrients for plant growth and development. White lupin is a dinitrogen (N2)-fixing legume that can increase Pi availability in soils. Under Pi deficiency, white lupin develops cluster roots, also known as proteoid roots. Cluster roots are densely packed lateral roots, resembling bottlebrushes. The resulting increase in root surface, together with coordinated biochemical responses, releases bound Pi and makes it available for plant uptake. The most noticeable biochemical responses that increase Pi availability include excretion of organic anions and phosphatases. As a consequence, white lupin can grow without addition of Pi fertilizer, and its ability to fix N2 is less inhibited by Pi deficiency, compared to other legumes. However, formation of cluster roots requires additional carbon (C) and nitrogen (N). Thus, white lupin needs to carefully balance C use with the formation of cluster roots and nodules. High-throughput approaches, particularly RNA-seq, have revealed many of the genes involved in cluster root formation and function and are beginning to reveal networks that regulate Pi acquisition and use. A better understanding of white lupin's responses to low nutrients may help to overcome inhibition of N2 fixation by low Pi availability in other legumes.