Allan Downie
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Current ResearchRhizosphere molecular microbiology.
The expression of "nodulation" genes in the bacteria is activated by signals from plant roots and as a result the bacteria synthesise chemical signals (Nod-factors) that induce a nodule meristem and enable the bacteria to enter this meristem via a plant-made infection thread. However in addition to Nod factors, the bacteria make other chemical signals (acyl homoserine lactones, AHLs) that enable individual bacterial cells to sense how many other bacteria are surround them. In this way they can determine whether there are enough bacteria, i.e. a quorum, to initiate the change towards acting in a multicellular fashion. This is known as 'quorum sensing' (see www.nottingham.ac.uk/quorum/ for further information). Such gene regulatory systems play an important role in optimising the potential of Rhizobium cells to compete in the rhizosphere and therefore to take the opportunity to infect legume roots to form nodules. Rhizobium genes induced by quorum-sensing regulation affect several characteristics including the transfer of nodulation and nitrogen fixation capacity to other bacteria, legume infection, attachment and biofilm formation on roots, and the ability to survive stresses. Our current research in this area includes understanding how the network of quorum-sensing regulation is coordinated, understanding root attachment and biofilm formation in relation to various bacterial surface polysaccharides and secreted proteins, and understanding how rhizobia respond to different extracellular stresses regulated by a specialised group of RNA polymerase subunits known as extracytoplasmic sigma factors. This involves collaborations with Philip Poole's group. Nod-factors made by Rhizobium bacteria activate a signalling cascade in root cells leading to the activation of gene expression. As a consequence cells in the root start to divide eventually forming a growing meristem that develops into a root nodule. In parallel, the rhizobia initiate infections, inducing the plant to produce specialised infection structures (infection threads) along which the bacteria grow, so that they can reach and infect the dividing plant cells of the nodule meristem. We have demonstrated that the Nod factors induce two distinct calcium responses in root hairs, a rapid calcium influx and oscillations in intracellular calcium, particularly around the nuclear region. We are interested in understanding how the plant cells generate these responses and then how the changes in calcium are interpreted leading to induction of gene expression. In this work we use the model legumes Lotus japonicus and Medicago truncatula and pea. This involves collaborations with Giles Oldroyd, Trevor Wang and Noel Ellis.
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