Citrate efflux from wheat roots affects the root microbiome and enhances nodal root development

Kawasaki A1, Dennis PG2, Raghavendra AKH2, Delhaize E1, Richardson AE1, Mathesius U3, Gilliham M4, Watt M5 and Ryan PR1

  1. CSIRO Agriculture and Food, Canberra, ACT, Australia.
  2. School of Earth and Environmental Sciences, Faculty of Sciences, The University of Queensland, St Lucia, QLD, Australia.
  3. Research School of Biology, Australian National University, Canberra, ACT, Australia.
  4. ARC Centre of Excellence in Plant Energy Biology, School of Agriculture, Food and Wine, Waite Research Institute, University of Adelaide, Glen Osmond, SA, Australia.
  5. Forschungszentrum Juelich GmbH, Juelich, Germany.

Plants release a wide range of organic compounds from the roots. These exudates alter the physical, chemical and biological properties of the rhizosphere and sometimes these changes can benefit plant survival. A well-studied example is the release of malate and citrate anions from the root apices of some wheat genotypes in acidic soils. Malate release is controlled by TaALMT1 gene and citrate release is controlled by TaMATE1B gene. These organic anions protect the growing root tips from toxic Al3+ cations prevalent in acidic soils and serve as carbon sources which can influence the growth and composition of soil microbes. We tested the effect of citrate efflux on the root-colonizing microbes using near-isogenic wheat lines that vary in citrate efflux (Citrate_line and Null_line). These lines were grown in contrasting soils and the bacterial communities analysed at different positions of seminal and nodal roots. We found that the root colonizing bacterial community was influenced by soil type (acidic or non-acidic), root type (seminal or nodal) and position on the root (tip or base). Citrate efflux from the root tips also affected the microbiome but only in the non-acidic soil. These results demonstrate that the “root microbiome” is not fixed but instead depends on environment, root type and even position along the root. Furthermore we show that the microbiome can be manipulated by specific root exudates. In other experiments we observed a link between the TaMATE1B locus (controlling citrate efflux) and nodal root development in acidic soils. The Citrate_line produced 30-40% more nodal roots and longer nodal roots than the Null_line in both acidic soil and in hydroponics with toxic concentrations of Al3+. These differences were smaller or absent in non-acidic soil or in hydroponics without Al3+ toxicity. We are currently assessing the value of the TaMATE1B locus to wheat production in acidic soils.