Legume Agp-Extensins in Rhizobium Infection
Plant cell-wall remodelling is a fundamental aspect of the Rhizobium-invasion process (Brewin, 2004). Some symbiotically defective legume mutants show abnormalities in composition of the infection thread wall and others show modifications of the extracellular matrix. For example, the pea mutant RisFixV (sym42) has enlarged infection thread walls that contain callose, whereas SGEFix--2 (sym33) develops infection threads with thick walls but without callose. Peroxide-driven cross-linking is apparently another aspect of the infection process. For example, in developing pea nodules, the inside (luminal face) of the infection thread wall is bounded by a girdle of peroxide that co-localises with diamine oxidase, the probable source of this peroxide. As the infection thread matures, the peroxide-associated zone progressively expands through the wall and into the lumen of the infection thread. In mutant lines where the infection process aborts prematurely, there is often an increased abundance of peroxide in infection threads and symbiosomes. Interestingly, however, in mutant SGEFix--2 (sym33), which does not release bacteroids from infection droplets, there is no peroxide detectable. These examples point to the need for a systematic analysis of the structure and development of infection threads using molecular probes that recognise components of the plant extracellular matrix.
KeywordsDiamine Oxidase Infection Thread Rhizobium Infection Infection Droplet Luminal Face
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