Sinorhizobium fredii HH103 Invades Lotus burttii by Crack Entry in a Nod Factor–and Surface Polysaccharide–Dependent Manner

Abstract

Sinorhizobium fredii HH103-Rif, a broad host range rhizobial strain, induces nitrogen-fixing nodules in Lotus burttii but ineffective nodules in L. japonicus. Confocal microscopy studies showed that Mesorhizobium loti MAFF303099 and S. fredii HH103-Rif invade L. burttii roots through infection threads or epidermal cracks, respectively. Infection threads in root hairs were not observed in L. burttii plants inoculated with S. fredii HH103-Rif. A S. fredii HH103-Rif nodA mutant failed to nodulate L. burttii, demonstrating that Nod factors are strictly necessary for this crack-entry mode, and a noeL mutant was also severely impaired in L. burttii nodulation, indicating that the presence of fucosyl residues in the Nod factor is symbiotically relevant. However, significant symbiotic impacts due to the absence of methylation or to acetylation of the fucosyl residue were not detected. In contrast S. fredii HH103-Rif mutants showing lipopolysaccharide alterations had reduced symbiotic capacity, while mutants affected in production of either exopolysaccharides, capsular polysaccharides, or both were not impaired in nodulation. Mutants unable to produce cyclic glucans and purine or pyrimidine auxotrophic mutants formed ineffective nodules with L. burttii. Flagellin-dependent bacterial mobility was not required for crack infection, since HH103-Rif fla mutants nodulated L. burttii. None of the S. fredii HH103-Rif surface-polysaccharide mutants gained effective nodulation with L. japonicus.

Sinorhizobium fredii HH103 nolR and nodD2 mutants gain capacity for infection thread invasion of Lotus japonicus Gifu and Lotus burttii

“…S. fredii SVQ318 ( nodD1 ) and SVQ555 ( nodD1 nodD2 ) failed to nodulate L. burttii and L. japonicus Gifu. This result, together with the Nod − phenotype shown by a nodA mutant (Acosta‐Jurado et al, ), further confirmed the strict requirement of Nod‐factors for nodule formation with these two legumes.…”

“…S. fredii HH103 Rif R fails to form nitrogen‐fixing nodules with L. japonicus Gifu but effectively nodulates L. burttii (Sandal et al, ). L. burttii nitrogen‐fixing nodules occupied by HH103 Rif R are the consequence of intercellular bacterial invasion (Acosta‐Jurado et al, ) instead of by the formation of ITs in root hairs. This fact prompted us to investigate how the S. fredii HH103 nodD2 and nolR mutants invade L. japonicus Gifu and L. burttii roots.…”

“…S. fredii HH103 Rif R effectively nodulates L. burttii but it only induces ineffective nodules with L. japonicus (Sandal et al, ). L. japonicus Gifu root responses to inoculation with HH103 can vary in different experimental conditions: from plants without nodules or, rarely, only ineffective nodules in agar‐based nodulation tests (Sandal et al, ) to ineffective nodules in plants grown in jars containing a mixture of vermiculite–perlite (Acosta‐Jurado et al, ). As demonstrated in the present work, those white, ineffective nodules induced by S. fredii HH103 in Lotus japonicus are devoid of bacteria.…”

“…As demonstrated in the present work, those white, ineffective nodules induced by S. fredii HH103 in Lotus japonicus are devoid of bacteria. In a previous work, the symbiotic capacity of a collection of S. fredii HH103 Rif R mutants affected in diverse symbiotic signals (Nod‐factors and surface polysaccharides) or in important metabolic pathways (purine or pyrimidine auxotrophs) was tested with L. burttii and L. japonicus ecotype Gifu (Acosta‐Jurado et al, ). None of the mutants tested in that work became able to induce the formation of effective nitrogen‐fixing nodules with L. japonicus Gifu.…”

Legumes versus rhizobia: a model for ongoing conflict in symbiosis

“…In some instances, mutations introduced into the host receptors or the corresponding symbiont polysaccharides can lead to rhizobia being diverted from their normal route of intracellular infection (Kawaharada et al ., ). However, the rhizobia can nonetheless form nodules via passive crack entry (Acosta‐Jurado et al ., ), or through reentry into plant cells from intercellular space (Kawaharada et al ., ). Moreover, host control over nodulation specificity can also be overwhelmed.…”