Nod factor‐independent ‘crack‐entry’ symbiosis in dalbergoid legume Arachis hypogaea

Abstract

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.

Nodule INception‐independent epidermal events lead to bacterial entry during nodule development in peanut (Arachis hypogaea)

“…Notably, an accumulation/path of entry of Bradyrhizobium sp. SEMIA 6144 was never visualized in a previous study with four different peanut cultivars (Guha et al ., 2022), suggesting Bradyrhizobium crack entry in peanut does not involve enough crowding of bacteria to yield a visible signal under confocal microscopy. By contrast, bacteria accumulated outside the root epidermis at the base of root hairs (compare Fig.…”

“…Unlike other plants, peanut develops multicellular root hairs only from the base of the lateral roots (Boogerd & van Rossum, 1997). These root hairs show typical ‘Shepherd's crook’ structures after inoculation, indicating root hair responsiveness to rhizobia (Chandler, 1978; Guha et al ., 2022; Raul et al ., 2022). In Aeschynomene afraspera , curling of the axillary root hairs has been reported (Bonaldi et al ., 2011).…”

Molecular Mechanisms of Intercellular Rhizobial Infection: Novel Findings of an Ancient Process

“…The Nod factors are not required for colonization of the peanut root surface but are required for the induction of cortical cell division of the first infected cell that will form the nodule primordia, when interacting with its natural symbionts ( Ibañez and Fabra, 2011 ). However, very recent occurrences of Nod-independent nodulation have been reported in Arachis when inoculated with non-cognate symbionts, but the underlying mechanisms are not yet documented ( Guha et al, 2022 ). Within the Aeschynomene genus, some species nodulate in a NF-dependent fashion and their root infection by bradyrhizobia occurs in a way that is comparable to A. hypogaea ( Giraud et al, 2007 ).…”

The Rhizobial Type 3 Secretion System: The Dr. Jekyll and Mr. Hyde in the Rhizobium–Legume Symbiosis

“…In any case, intercellular infection can develop via a crack-entry mechanism, typically through epidermal fissures of emerging lateral roots, such as in Sesbania , Aeschynomene or Arachis , or in root zones where massive root hair curling and twisting takes place, such as in the interaction of strain IRBG74 with L. japonicus [ 19 ]. Several studies indicate NF are required in some but not all the intercellular infection processes described so far [ 15 , 19 , 22 , 23 , 24 ].…”