[Summary] [Rhizobium] [Medicago]
Nod factor directly acts as a morphogen on the plant host: it causes resumption of cell division in root cells that should be terminally differentiated, resulting in tumorigenic growth of root tissue into a nodule. We study plant cellular responses to Rhizobiumand to Nod factor by means of genetics, and through microscopic and computer analyses.
Parasites - whether pathogens or symbionts - often exploit central host cellular mechanisms in order to accomplish their goals of invasion and reproduction. Through genetic and cellular studies of Rhizobiumsymbiosis, we hope to find components and mechanisms that govern central functions in plant growth and development.
Calcium behavior in root hairs:
We previously reported that alfalfa root hairs exposed to Nod factor display sharp oscillations ("spiking") of Ca++ initiating within 3-30 minutes (video). We have found that root hairs exposed to specific Rhizobiumsignals respond with sharp oscillations in cytoplasmic calcium ("spiking"), a signal transduction motif. We have now found that Ca++ spiking occurs in response to specific Rhizobiumsignals in at least four additional species of legume including Melilotus albus, Vicia sativa, Lotus japonicus, and M. truncatula. Ethylene appears to have significant regulatory effects on this very early Nod factor response.
Mutant studies
We have begun analysis of Ca++ spiking in symbiotic mutants of the model diploid plant, Medicago truncatula,which carries out symbiosis with R. meliloti.In collaboration with the laboratories of D. Cook and J. Dénarié, we have shown that Nod- mutants, plants unable to carry out symbiosis, are also blocked for calcium spiking. Typically, these mutants are also blocked in their ability to carry out mycorrhizal associations. Pharmacological and other studies are being used to define the cellular events that initiate and sustain Ca++ spiking. We plan further analysis of plant mutants to define components required for bacterial signal recognition and for diverse plant developmental responses. We have employed fast neutron bombardment mutagenesis to generate a new series of M. truncatulamutants, and have screened these to find new plant genes, and novel alleles of known genes, that are required for symbiosis.
Morphogenesis of root hairs
We use microscopic imaging to define the immediate local growth responses of root hairs to encountering live Rhizobiumbacteria. Our initial project has aimed to define growth of unperturbed root hairs. We visualized growth of uninfected root hairs with 0.1 micron fluorescent beads as markers to follow local surface events on the plant cell wall. This reveals that root hair expansion at the tip includes several distinct patterns of behavior: at the very tip is a small zone of very low expansion; around this is a ring of rapid expansion, followed by a more basal zone of anisotropic local expansion. With this base line of characterization, we now wish to observe how Rhizobiumbacteria perturb growth, leading to root hair branching and curling, the precursor steps to infection.
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Calcium spiking in alfalfa root hair responding to Nod
factor
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Calcium spiking in alfalfa root hair responding
to Nod factor
[Links] [S. meliloti genome] [Symbiosis GeneChip] [Stanford Biology] [Stanford Home]
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