Nodulins Involved in Early Stages of Pea Root Nodule Development

Abstract

In nodules formed on the roots of pea plants at least 25 plant genes are specifically expressed, the nodulin genes. The majority of these genes are first expressed 13 days after sowing and inoculation with Rhizobium leguminosarum (Govers et al 1985). Histological examination of longitudinal sections of infected roots of 8, 10 and 13 day old plants showed that a complete nodule structure is already present at day 10 (Figure 1A). From the apical meristem infected as well as uninfected cells, the two cell types that are present in the symbiotic zone of the mature nodule, have been formed (Figure 1A, B). Vascular bundles are present at the periphery of the nodule, an uninfected cortex surrounds the central part of the nodule and in the zone with the differentiating cells a great number of infection threads is present. At day 8 this differentiation process has started, but the difference between infected and uninfected cells is not yet clear (Figure 1B), and at 13 days the infected cells are fully packed with rhizobia (Figure 1B). Since the complete nodule structure is already present before the majority of the nodulin genes is expressed these nodulin genes cannot be involved in the morphogenesis of this new organ.