Signal Transduction and Endocytosis of Rhizobia in the Host Cells

Abstract

Root nodule organogenesis proceeds through many early morphological responses of the host to rhizobia invasion. These include deformation and curling of root hairs, formation of infection threads, de-differentiation of cortical cells, formation of nodule primodia and endocytosis of rhizobia. Recent advances in molecular techniques have allowed observations of early physiological and biochemical changes of the host accompanying the above morphological changes. This became possible by identification and purification of Nod factors. These factors have provided a new tool for studying nodule initiation, morphogenesis as well as signal recognition [1]. Nod factors are a group of lipo-oligosaccharide molecules that are secreted by rhizobia in response to signals from the host-plant and are active in nanomolar concentrations in inducing nodule-like structures on legume roots. One of the observations on early host responses has demonstrated that Nod factors induce a rapid depolarization of membrane potential in legume root hairs [2]. This depolarization response is specific because preparations from a Nod-rhizobium strain failed to induce this response and roots of non-legume plants did not respond to purified Nod factors. Since signal transduction in mammalian cells often couples with a membrane potential depolarization, these results suggest that the host responds to Nod factors by transducing a signal across the plasma membrane. Intracellular Ca⁺⁺ levels increase rapidly following application of Nod factors (W. Broughton, personal communication), suggesting possible involvement of Ca⁺⁺ ions in this process. Recent studies on expression of early nodulin genes using PCR methodology and promoter-reporter system have also provided new insights into early host responses. Expression of Enod5 and Enod12 is induced specifically after application of Nod factors 131. The promoter of Enod12 gene is activated in epidermal cells of the root hair zone by addition of Nod factors in nanomolar concentrations, demonstrating spatial distribution of host response [4]. However, it is not yet clear how Nod signal is transduced and initiates the nodule morphogenesis cascade [1].