The symbiotic interaction between Azorhizobium caulinodans and Sesbania rostrata molecular cross-talk in a beneficial plant-bacterium interaction

Distinct signaling routes mediate intercellular and intracellular rhizobial infection in Lotus japonicus

Rhizobial infection of legume roots during the development of nitrogen-fixing root nodules can occur intracellularly, through plant-derived infection threads traversing cells, or intercellularly, via bacterial entry between epidermal plant cells. Although it is estimated that around 25% of all legume genera are intercellularly infected, the pathways and mechanisms supporting this process have remained virtually unexplored due to a lack of genetically amenable legumes that exhibit this form of infection. In this study, we report that the model legume Lotus japonicus is infected intercellularly by the IRBG74 strain, recently proposed to belong to the Agrobacterium clade of the Rhizobiaceae. We demonstrate that the resources available for L. japonicus enable insight into the genetic requirements and fine-tuning of the pathway governing intercellular infection in this species. Inoculation of L. japonicus mutants shows that Ethylene-responsive factor required for nodulation 1 (Ern1) and Leu-rich Repeat Receptor-Like Kinase (RinRK1) are dispensable for intercellular infection in contrast to intracellular infection. Other symbiotic genes, including nod factor receptor 5 (NFR5), symbiosis receptor-like kinase (SymRK), Ca2+/calmodulin dependent kinase (CCaMK), exopolysaccharide receptor 3 (Epr3), Cyclops, nodule inception (Nin), nodulation signaling pathway 1 (Nsp1), nodulation signaling pathway 2 (Nsp2), cystathionine-β-synthase (Cbs), and Vapyrin are equally important for both entry modes. Comparative RNAseq analysis of roots inoculated with IRBG74 revealed a distinctive transcriptome response compared with intracellular colonization. In particular, several cytokinin-related genes were differentially regulated. Corroborating this observation, cyp735A and ipt4 cytokinin biosynthesis mutants were significantly affected in their nodulation with IRBG74, whereas lhk1 cytokinin receptor mutants formed no nodules. These results indicate a differential requirement for cytokinin signaling during intercellular rhizobial entry and highlight distinct modalities of inter- and intracellular infection mechanisms in L. japonicus.

Rhizobial infection in Adesmia bicolor (Fabaceae) roots

The native legume Adesmia bicolor shows nitrogen fixation efficiency via symbiosis with soil rhizobia. The infection mechanism by means of which rhizobia infect their roots has not been fully elucidated to date. Therefore, the purpose of the present study was to identify the infection mechanism in Adesmia bicolor roots. To this end, inoculated roots were processed following conventional methods as part of our root anatomy study, and the shape and distribution of root nodules were analyzed as well. Neither root hairs nor infection threads were observed in the root system, whereas infection sites-later forming nodules-were observed in the longitudinal sections. Nodules were found to form between the main root and the lateral roots. It can be concluded that in Adesmia bicolor, a bacterial crack entry infection mechanism prevails and that such mechanism could be an adaptive strategy of this species which is typical of arid environments.

Nodule development on the tropical legume Sesbania virgata under flooded and non-flooded conditions

The interaction between the Brazilian pioneer legume Sesbania virgata and its microsymbiont Azorhizobium doebereinerae leads to the formation of nitrogen-fixing nodules on roots that grow either in well-aerated soils or in wetlands. We studied the initiation and development of nodules under these alternative conditions. To this end, light and fluorescence microscopy were used to follow the bacterial colonisation and invasion into the host and, by means of transmission electron microscopy, we could observe the intracellular entry. Under hydroponic conditions, intercellular invasion took place at lateral root bases and mature nodules were round and determinate. However, on roots grown in vermiculite that allows aerated growth, bacteria also entered via root hair invasion and nodules were both of the determinate and indeterminate type. Such versatility in entry and developmental plasticity, as previously described in Sesbania rostrata, enables efficient nodulation in both dry and wet environments and are an important adaptive feature of this group of semi-tropical plants that grow in temporarily flooded habitats.

Sesbania rostrata: a case study of natural variation in legume nodulation

Legumes acquired the ability to engage in a symbiotic interaction with soil-borne bacteria and establish a nitrogen-fixing symbiosis in a novel root organ, the nodule. Most legume crops and the model legumes Medicago truncatula and Lotus japonicus are infected intracellularly in root hairs via infection threads that lead the bacteria towards a nodule primordium in the root cortex. This infection process, however, does not reflect the great diversity of infection strategies that are used by leguminous plants. An alternative, intercellular invasion occurs in the semiaquatic legume Sesbania rostrata. Bacteria colonize epidermal fissures at lateral root bases and trigger cortical cell death for infection pocket formation and subsequent intercellular and intracellular infection thread progression towards the primordium. This infection mode evolved as an adaptation to waterlogged conditions that inhibit intracellular invasion. In this review, we discuss the molecular basis for this adaptation and how insights into this process contribute to general knowledge of the rhizobial infection process.

SrSymRK, a plant receptor essential for symbiosome formation

The symbiosis between legumes and rhizobia is essential for the nitrogen input into the life cycle on our planet. New root organs, the nodules, are established, which house N2-fixing bacteria internalized into the host cell cytoplasm as horizontally acquired organelles, the symbiosomes. The interaction is initiated by bacterial invasion via epidermal root hair curling and cell division in the cortex, both triggered by bacterial nodulation factors. Of the several genes involved in nodule initiation that have been identified, one encodes the leucine-rich repeat-type receptor kinase SymRK. In SymRK mutants of Lotus japonicus or its orthologs in Medicago sp. and Pisum sativum, nodule initiation is arrested at the level of the root hair interaction. Because of the epidermal block, the role of SymRK at later stages of nodule development remained enigmatic. To analyze the role of SymRK downstream of the epidermis, the water-tolerant legume Sesbania rostrata was used that has developed a nodulation strategy to circumvent root hair responses for bacterial invasion. Evidence is provided that SymRK plays an essential role during endosymbiotic uptake in plant cells.

Switch from intracellular to intercellular invasion during water stress-tolerant legume nodulation

Rhizobia colonize their legume hosts by different modes of entry while initiating symbiotic nitrogen fixation. Most legumes are invaded via growing root hairs by the root hair-curl mechanism, which involves epidermal cell responses. However, invasion of a number of tropical legumes happens through fissures at lateral root bases by cortical, intercellular crack entry. In the semiaquatic Sesbania rostrata, the bacteria entered via root hair curls under nonflooding conditions. Upon flooding, root hair growth was prevented, invasion on accessible root hairs was inhibited, and intercellular invasion was recruited. The plant hormone ethylene was involved in these processes. The occurrence of both invasion pathways on the same host plant enabled a comparison to be made of the structural requirements for the perception of nodulation factors, which were more stringent for the epidermal root hair invasion than for the cortical intercellular invasion at lateral root bases.