Organic anion exudation from roots influences mineral nutrition, root microbiome composition and root architecture

Ryan PR

CSIRO Agriculture and Food, Canberra.

Plants release an estimated 10-20% of fixed carbon into the soil. Whether released as a consequence of growth or in response to specific triggers and stresses, these exudates modify the chemistry of the rhizosphere which can benefit growth and survival. For example, the release of organic anions from roots enables some plants to cope with the toxic aluminium (Al3+) cations prevalent in acidic soils. Some of the genes controlling the organic anion release have been identified in crop species including wheat, barley, rice and maize. In wheat, the release of malate and citrate anions is controlled by an anion channel encoded by TaALMT1 and a co-transporter encoded by TaMATE1B, respectively. These transporters had only been characterised in young wheat seedlings so more recent experiments examined their physiology in 30 day-old plants with mature root systems. Malate release was detected from the seminal, nodal and lateral roots but citrate release only occurred from seminal and nodal roots. We further showed that citrate release affected the structure and composition of the microbiome near the root apices. These genes therefore provide strategies for manipulating the root microbiome. More surprisingly, the TaMATE1B locus was associated changes in root architecture because when wheat was grown in acid soil or hydroponic solutions with toxic levels of aluminium, lines with the TaMATE1B locus developed 30-50% more nodal roots than near-isogenic lines without this locus. Finally, the constitutive release of malate anions from roots of transgenic rice lines altered the subcellular compartmentation of manganese in leaves. These experiments demonstrate that the release of simple organic anions such as malate and citrate from roots can influence plant growth and physiology in multiple ways.