Symbiotic rhizobia acquisition was mainly in the endosphere of apical root
The invasion process of chickpea symbiotic rhizobia (CSR) was the result of genotype-niche interactions, as we didn’t detect any factors interacting with soil type on CSR abundance (Table 1). The spatial distribution of CSR showed more in the tissue of apical root than in the basal root (Fig. 3c). In the nodulation process, rhizobia in the soil may first colonise the root epidermis at the elongation zone following root-microbe signal communications or may directly bind to the zone of root hairs, while the exact sequence of this process is unknown (Poole et al., 2018). Our root apical zone included the root elongation zone, that was enriched in symbiotic rhizobia and likely facilitated the nodulation observed in the basal root sections.
In addition, CSR enrichment among chickpea genotypes was not related to the flowering phenological trait, as one LF genotype, rather than the EF genotype, harboured a greater proportion of CSR bacteria in the root apex, and the enrichment was stable across soil types. Our results indicated the striking effect of host genotype in controlling nodulation processes, e.g. , producing different host-specific flavonoids and root receptors that recognize symbiotic rhizobia (Sachs et al., 2018). CSR was not related to the phenological trait, e.g. , early maturity, although they might contribute to the amount of N fixed by chickpea (Sadras et al., 2016).