General Patterns and Species-Specific Differences in the Organization of the Tubulin Cytoskeleton in Indeterminate Nodules of Three Legumes

Cytoskeleton as a roadmap navigating rhizobia to establish symbiotic root nodulation in legumes

Legumes enter into symbiotic associations with soil nitrogen-fixing rhizobia, culminating in the creation of new organs, root nodules. This complex process relies on chemical and physical interaction between legumes and rhizobia, including early signalling events informing the host legume plant of a potentially beneficial microbe and triggering the nodulation program. The great significance of this plant-microbe interaction rests upon conversion of atmospheric dinitrogen not accessible to plants into a biologically active form of ammonia available to plants. The plant cytoskeleton consists in a highly dynamic network and undergoes rapid remodelling upon sensing various developmental and environmental cues, including response to attachment, internalization, and accommodation of rhizobia in plant root and nodule cells. This dynamic nature is governed by cytoskeleton-associated proteins that modulate cytoskeletal behaviour depending on signal perception and transduction. Precisely localized cytoskeletal rearrangements are therefore essential for the uptake of rhizobia, their targeted delivery, and establishing beneficial root nodule symbiosis. This review summarizes current knowledge about rhizobia-dependent rearrangements and functions of the cytoskeleton in legume roots and nodules. General patterns and nodule type-, nodule stage-, and species-specific aspects of actin filaments and microtubules remodelling are discussed. Moreover, emerging evidence is provided about fine-tuning the root nodulation process through cytoskeleton-associated proteins. We also consider future perspectives on dynamic localization studies of the cytoskeleton during early symbiosis utilizing state of the art molecular and advanced microscopy approaches. Based on acquired detailed knowledge of the mutualistic interactions with microbes, these approaches could contribute to broader biotechnological crop improvement.

Rapid Changes to Endomembrane System of Infected Root Nodule Cells to Adapt to Unusual Lifestyle

Symbiosis between leguminous plants and soil bacteria rhizobia is a refined type of plant-microbial interaction that has a great importance to the global balance of nitrogen. The reduction of atmospheric nitrogen takes place in infected cells of a root nodule that serves as a temporary shelter for thousands of living bacteria, which, per se, is an unusual state of a eukaryotic cell. One of the most striking features of an infected cell is the drastic changes in the endomembrane system that occur after the entrance of bacteria to the host cell symplast. Mechanisms for maintaining intracellular bacterial colony represent an important part of symbiosis that have still not been sufficiently clarified. This review focuses on the changes that occur in an endomembrane system of infected cells and on the putative mechanisms of infected cell adaptation to its unusual lifestyle.

Tubulin Cytoskeleton Organization in Cells of Determinate Nodules

Plant cell differentiation is based on rearrangements of the tubulin cytoskeleton; this is also true for symbiotic nodules. Nevertheless, although for indeterminate nodules (with a long-lasting meristem) the organization of microtubules during nodule development has been studied for various species, for determinate ones (with limited meristem activity) such studies are rare. Here, we investigated bacteroid morphology and dynamics of the tubulin cytoskeleton in determinate nodules of four legume species: Glycine max, Glycine soja, Phaseolus vulgaris, and Lotus japonicus. The most pronounced differentiation of bacteroids was observed in G. soja nodules. In meristematic cells in incipient nodules of all analyzed species, the organization of both cortical and endoplasmic microtubules was similar to that described for meristematic cells of indeterminate nodules. In young infected cells in developing nodules of all four species, cortical microtubules formed irregular patterns (microtubules were criss-crossed) and endoplasmic ones were associated with infection threads and infection droplets. Surprisingly, in uninfected cells the patterns of cortical microtubules differed in nodules of G. max and G. soja on the one hand, and P. vulgaris and L. japonicus on the other. The first two species exhibited irregular patterns, while the remaining two exhibited regular ones (microtubules were oriented transversely to the longitudinal axis of cell) that are typical for uninfected cells of indeterminate nodules. In contrast to indeterminate nodules, in mature determinate nodules of all four studied species, cortical microtubules formed a regular pattern in infected cells. Thus, our analysis revealed common patterns of tubulin cytoskeleton in the determinate nodules of four legume species, and species-specific differences were associated with the organization of cortical microtubules in uninfected cells. When compared with indeterminate nodules, the most pronounced differences were associated with the organization of cortical microtubules in nitrogen-fixing infected cells. The revealed differences indicated a possible transition during evolution of infected cells from anisotropic growth in determinate nodules to isodiametric growth in indeterminate nodules. It can be assumed that this transition provided an evolutionary advantage to those legume species with indeterminate nodules, enabling them to host symbiosomes in their infected cells more efficiently.