ABSTRACT

Nodulation of legumes is significant as the nitrogen fixing symbiosis adds critical available nitrogen to the biosphere. Often major crop plants are either legumes (c.f., soybean, peanut, bean, medics, clovers, chickpea), or they rely on rotational practices involving legumes. Nodule formation occurs predominantly on roots after mitogenic stimulation of cortical and pericycle cells by the Rhizobium-secreted lipo-oligosaccharide Nod-Factor. To date, the timing, the signal, its receptor (a LysM receptor kinase complex), major parts of the down-stream response cascade (involving ion channels, a calcium calmodulin dependent protein kinase, a cytokinin receptor and numerous transcription factors) and the target cell types are known1,2 (Fig. 1A). As a result, nodule ontogeny is an elegant and mature experimental system to analyse plant development, associated signaling and behavior.3 Genetic, genomic, physiological and biochemical tools are currently being combined to optimise the role of legumes in diverse agricultural and silvicultural systems to facilitate more sustainable production of food, feed and biofuels.

Mutant and Transgenic Analysis using Model Legumes

Ethylene gas exposure, or growth in the presence of ethylene precursors such as ACC, limits nodule numbers per plant through reduced infection by ‘rhizobia’4-6 and decreased ability to initiate cell divisions in target plant zones behind the growing root tip (the ‘zone of nodulation’ characterized by emerging root hairs). The laboratory of Doug Cook and Varma Penmetsa was first to describe legume mutants with increased insensitivity for the classical ‘triple response’ during dark germination.⁷ Specifically, a Medicago truncatula mutant, called ‘sickle’, because of the characteristic root deformation caused by excessive nodule initiation, was described. Of symbiotic significance, this recessive mutant displayed increased nodule initiation and rhizobial infection. Its symbiotic phenotype was controlled by the root as shown by reciprocal grafts. Recently, the affected gene was cloned and shown to be EIN2,8 a known member of the signal response chain to ethylene in Arabidopsis thaliana. EIN2 is presumed to encode a nuclear membrane component, possibly transmitting a cytoplasmic response in CTR1 to nuclear transcription and response factors.

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