Abstract
Split-root systems (SRS) constitute an appropriate methodology for studying the relevance of both local and systemic mechanisms that participate in the control of rhizobia-legume symbioses. In fact, this kind of approach allowed to demonstrate the autoregulation of nodulation (AON) systemic response in soybean in the 1980s. In SRS, the plant main root is cut and two lateral roots that emerge from the seedlings after root-tip removal are confined into separate compartments. After several days of growth, these plants have two separate roots that can be inoculated with the same or with different bacteria, at the same or at different times. In this work, we have used a non-destructive SRS to study two different competitiveness relations between rhizobial strains in soybean roots. One of them is the competition for nodulation between two soybean-nodulating rhizobia: the slow-grower Bradyrhizobium japonicum USDA110 and the fast-grower Sinorhizobium fredii HH103. The second is the competitive blocking of S. fredii 257DH4 nodulation in the American soybean Osumi by Sinorhizobium fredii USDA257, which is unable to nodulate American soybeans. Our results showed that the competitiveness relationships studied in this work are mitigated or even avoided when the competitive strains are spatially separated in different compartments containing half-roots from the same plant, suggesting that competitive relations are more related to local plant responses. In our opinion, split-root systems are an appropriate approach to further study competitive relations among rhizobial strains.
Highlights
Rhizobia are soil proteobacteria that establish specific symbiotic relationships with leguminous plants resulting in the formation of nitrogen-fixing nodules in plant roots [1]
Split-root systems (SRS) constitute an appropriate methodology for studying the relevance of both local and systemic mechanisms that participate in the control of rhizobia-legume symbioses
Our results showed that the competitiveness relationships studied in this work are mitigated or even avoided when the competitive strains are spatially separated in different compartments containing half-roots from the same plant, suggesting that competitive relations are more related to local plant responses
Summary
Rhizobia are soil proteobacteria that establish specific symbiotic relationships with leguminous plants resulting in the formation of nitrogen-fixing nodules in plant roots [1]. In response to flavonoids exuded by the roots, rhizobia secrete specific signal molecules called Nod factors or LCOs (for lipochitooligosaccharides) that, after their perception by plant receptor-kinases, promote root hair curling and induce the development of root nodules in the plant [2,3]. Rhizobia penetrate into the root moving along tubular structures formed by invagination of the root hair cell membrane. These tubular structures, called infection threads, reach the nodule symbiotic cells in which bacteria are released and differentiate into nitrogen-fixing bacteroids. In addition to Nod factors, other bacterial molecules, surface polysaccharides and proteins secreted by bacterial secretion systems, play important roles in the establishment of an efficient symbiosis with the host plant [4]
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