Organic Anion Exudates from Various Root Types of Wheat and Influence on Root-Colonizing Microbial Communities

Plants release a large amount of photosynthates into the soil as root exudates, and these root exudates modify the chemistry of the rhizosphere and influence the microbiome. Both these changes can affect how plants cope with various environmental stresses. In acid soils, aluminium toxicity is one of the major factors limiting plant root growth and aluminium-resistant genotypes of wheat release malate and citrate anions from their roots which protects the sensitive root apices from toxic Al³⁺ cations. Whilst these organic anion exudates have been measured from the seminal roots of young (4-5 day old) hydroponically-grown wheat seedlings, whether or not similar exudates occur from nodal and lateral roots on older plants is unknown, because maintaining axenic conditions for large plants is technically difficult. In this study we developed a novel fully-enclosed hydroponic growth chamber that can maintain growth of wheat under sterile conditions without detectable contaminations for more than four weeks. Using this system we measured malate and citrate exudation from seminal, nodal and lateral roots of 28 day old aluminium-resistant wheat varieties. We found Al³⁺ activated malate efflux from all root types, whereas citrate efflux was constitutive from seminal and nodal roots but was not released from lateral roots. We tested how these exudates influenced the microbiome around different root types in near-isogenic wheat lines that varied in organic anion exudation. Results will be presented on the microbiomes characterised at the proximal and distal sections of seminal and nodal roots from plants grown in soils with contrasting pH and Al-toxicity. The results will demonstrate how the root microbiome is influenced by specific root exudates in various soils. This information will help us understand how, in the future, we can attempt to manipulate the rhizosphere to benefit plant performance.