Deregulation of a Ca2+/calmodulin-dependent kinase leads to spontaneous nodule development

Abstract

Induced development of a new plant organ in response to rhizobia is the most prominent manifestation of legume root-nodule symbiosis with nitrogen-fixing bacteria. Here we show that the complex root-nodule organogenic programme can be genetically deregulated to trigger de novo nodule formation in the absence of rhizobia or exogenous rhizobial signals. In an ethylmethane sulphonate-induced snf1 (spontaneous nodule formation) mutant of Lotus japonicus, a single amino-acid replacement in a Ca2+/calmodulin-dependent protein kinase (CCaMK) is sufficient to turn fully differentiated root cortical cells into meristematic founder cells of root nodule primordia. These spontaneous nodules are genuine nodules with an ontogeny similar to that of rhizobial-induced root nodules, corroborating previous physiological studies. Using two receptor-deficient genetic backgrounds we provide evidence for a developmentally integrated spontaneous nodulation process that is independent of lipochitin-oligosaccharide signal perception and oscillations in Ca2+ second messenger levels. Our results reveal a key regulatory position of CCaMK upstream of all components required for cell-cycle activation, and a phenotypically divergent series of mutant alleles demonstrates positive and negative regulation of the process.

Dating in the dark: how roots respond to fungal signals to establish arbuscular mycorrhizal symbiosis

“…Gain-offunction mutations of the corresponding gene eliminate the need of the upstream SYM component for both nodulation and mycorrhization [ 36 and 37 ]. Interestingly, dominant forms of this kinase induce nodulation in the absence of bacteria or Nod factors [38]. Therefore, why has spontaneous nodulation never been observed in non-legume plants, where DMI3 is also present, in response to Myc-LCOs?…”

A Small GTPase of the Rab Family Is Required for Root Hair Formation and Preinfection Stages of the Common Bean–RhizobiumSymbiotic Association

“…Perception of Nod factor by the plant triggers different responses responsible for changes in young epidermal cells and, at the same time, is necessary for the molecular events that will lead to nodule organogenesis (Oldroyd and Downie, 2008). These responses can occur independently, as was shown by the phenotype of different mutants, in which bacterial infection, and related epidermal responses, is observed in the absence of nodule formation and vice versa (Gleason et al, 2006;Tirichine et al, 2006aTirichine et al, , 2006bTirichine et al, , 2007Murray et al, 2007). After the initial chemical communication, the physical association begins with the attachment of the bacteria to the surface of the root through plant lectins and polysaccharides present on the surface of the bacteria (Smit et al, 1992;Diaz et al, 1995).…”

Does SUNN-SYMRK Crosstalk occur in Medicago truncatula for regulating nodule organogenesis?

“…23 Accordingly, the proposition is also in consistence with autoregulated spontaneous nodule formation under deregulated activation of CCaMK (calcium / calmodulin dependent protein kinase) or LHK1 (cytokinin receptor) in snf1 and snf2 mutants respectively of L. japonicus where the proposed cross talk between the corresponding receptors is expected to keep the nodule number under control. 26,27 Interestingly, SYMRK's role in regulating nodule numbers was already evident when SYMRK mutants were identified as suppressors of hypernodulation in L. japonicus 28 and the proposed crosstalk between SUNN-SYMRK receptors could be a possible mechanism by which SYMRK exercises its control over nodule number. At this point, it may be relevant to mention that the pattern of rhizobia induced nodule development in SUNN mutants resemble the pattern of spontaneous nodulation in SYMRK-kd transformed roots.…”