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New light on nodulation
With my long-standing interest in the cell cycle, my eye was inevitably drawn to the paper by Gentili et al. (Umeå, Sweden and Bernal, Argentina, pp. 309–315) on cell division and nodulation in Alnus incana following inoculation by the actinomycete Frankia. The Alnus–Frankia relationship is a rare example of a non-rhizobial N-fixing symbiosis; it is the early stages of development of the symbiosis that are discussed in this paper. The authors developed a light microscope technique that enabled them to observe intact roots and thus to show that the earliest changes in cell division patterns occurred in the cortex: at 2 d after inoculation, anticlinal divisions were seen in the outer cortical layer in the region of the root where root hairs had started to become deformed. Root hair deformation and cortical cell division continued; by 6 d, there were dividing cells in the first three cortical layers. The development of this cortical cell division area (CCDA), which has not been described before, preceded the initiation of nodule primordia in the pericycle. The exact function of the CCDA is not clear but it may represent a ‘reservoir’ of dividing cells from which additional or replacement nodules may be formed. The authors went on to investigate the effects of P and N, given at different combinations of concentrations. The key findings were that at a ‘medium’ concentration of P (0.1 mm), he size of the CCDA and the number of nodule primordia were increased without any effect on root growth. Increasing the N concentration from low (0.71 mm) to high (6.45 mm) inhibited the early cortical cell divisions and counteracted the P-mediated increase in the number of nodule primordia. Thus, as the authors point out, the effects of N and P, and especially the latter, are evident very early in the establishment of the Alnus–Frankia symbiosis, before the formation of nodule primordia.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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