Several plant genes (referred to as nodulin genes) that are exclusively involved in the early stages of root-hair infection and nodule development have been identified (Radutolu et al., 2003; Limpens et al., 2003). Other nodulins are expressed at later stages of nodule functional development and include hemoglobin and enzymes involved in N assimilation. Recent analysis of plant mutants has placed emphasis on identifying the early nodulin genes. For example, NFR1/NFR5 are genes placed at the top of the hierarchical cascade because plants with mutations in these genes lack all known responses to nod factor. NFR1/NFR5 code for a transmembrane receptor-like kinase (an enzyme that acts as a "molecular switch" by covalently attaching phosphate groups to other proteins), which turns enzymatic pathways on or off. It is thought that a specific part of the kinase protein binds to the N-acetyl-glucosamine backbone of nod factor and initiates the nodulation process (Fig. 14.6). In the legume Medicago truncatula, mutants have been obtained in three other genes (DMI1 and DMI2 and DMI3) that do not exhibit most nod factor responses (no root-hair branching, nodulin gene expression, or cortical cell division). While root hairs in mutants DMI1 and DM12 have lost their ability to undergo oscillations in intracellular Ca levels in response to nod factor, Ca oscillations are unimpaired in root hairs of the DM13 mutant, suggesting that its role in nodulation is farther downstream from DMI1 and DM12. This work suggests there is a relationship between DM11 and DM12 and Ca "spiking" associated with root hair infection and that DM13 is involved in relaying this Ca signal to other parts of the nodule-forming process. Interestingly, mutants DM11, DM12, and DM13 cannot form mycorrhizal associations with the arbuscular mycorrhizal (AM) fungus
Root-hair recognition of rhizobia
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