No abstract

Bradyrhizobial invasion in dalbergoid legumes like Arachis hypogaea and endophytic bacterial invasions in non-legumes like Oryza sativa occur through epidermal cracks. Here, we show that there is no overlap between the bradyrhizobial consortia that endosymbiotically and endophytically colonise these plants. To minimise contrast due to phylogeographic isolation, strains were collected from Arachis/Oryza intercropped fields and a total of 17 bradyrhizobia from Arachis (WBAH) and 13 from Oryza (WBOS) were investigated. 16SrRNA and concatenated dnaK-glnII-recA phylogeny clustered the nodABC-positive WBAH and nodABC-deficient WBOS strains in two distinct clades. The in-field segregation is reproducible under controlled conditions which limits the factors that influence their competitive exclusion. While WBAH renodulated Arachis successfully, WBOS nodulated in an inefficient manner. Thus, Arachis, like other Aeschynomene legumes support nod-independent symbiosis that was ineffectual in natural fields. In Oryza, WBOS recolonised endophytically and promoted its growth. WBAH however caused severe chlorosis that was completely overcome when coinfected with WBOS. This explains the exclusive recovery of WBOS in Oryza in natural fields and suggests Nod-factors to have a role in counterselection of WBAH. Finally, canonical soxY1 and thiosulphate oxidation could only be detected in WBOS indicating loss of metabolic traits in WBAH with adaptation of symbiotic lifestyle.

Summary Dalbergoids are typified by crack‐entry symbiosis which is evidenced to be Nod Factor (NF)‐independent in several Aeschynomene legumes. Natural symbionts of the dalbergoid legume Arachis hypogaea are always NF‐producing, prompting us to check whether symbiosis in this legume could also be NF‐independent. For this, we followed the symbiosis with two NF‐containing bradyrhizobial strains – SEMIA6144, a natural symbiont of Arachis and ORS285, a versatile nodulator of Aeschynomene legumes, along with their corresponding nodulation (nod) mutants. Additionally, we investigated NF‐deficient bradyrhizobia like BTAi1, a natural symbiont of Aeschynomene indica and the WBOS strains that were natural endophytes of Oryza sativa, collected from an Arachis‐Oryza intercropped field. While SEMIA6144ΔnodC was non‐nodulating, both ORS285 and ORS285ΔnodB could induce functional nodulation, although with lower efficiency than SEMIA6144. On the other hand, all the NF‐deficient strains – BTAi1, WBOS2 and WBOS4 showed comparable nodulation with ORS285 indicating Arachis to harbour an NF‐independent mechanism of symbiosis. Intriguingly, symbiosis in Arachis, irrespective of whether it was NF‐dependent or independent, was always associated with the curling or branching of the rosette root hairs at the lateral root bases. Thus, despite being predominantly described as an NF‐dependent legume, Arachis does retain a vestigial, less‐efficient form of NF‐independent symbiosis.