English abstract
Some genotypes of date palm (Phoenix dactylifera L.) are salt-tolerant;
however, other genotypes are significantly affected by salinity. This study aimed to
understand the contribution of exophytic plant growth-promoting rhizobacteria (PGPR)
in salinity tolerance of two different date palm genotypes. Therefore, rhizospheric
bacterial communities of two date palm cultivars of contrasting salinity tolerance were
investigated in this study. The salt-tolerant 'Umsila' and salt-susceptible 'Zabad' plants
were exposed to normal and 250 mM NaCl stress conditions. The proximal soils of the
roots were collected, DNA was extracted from the soil samples, and a cultureindependent approach using Illumina® MiSeq™ sequence analysis was carried out to
identify the bacterial communities present in the soil samples based on the 16S rRNA
gene sequences. A culture-dependent approach was carried out in a parallel study to
isolate bacterial cells and functionally characterize them. The results revealed 771
operational taxonomic units (OTUs), 62 of which were differentially accumulated in
response to salinity than the control samples. Ordination analysis showed significant (P
= 0.001) changes among the PGPR communities of date palm in response to salinity in
both cultivars compared to the control treatments; however, the genotype did not show
a similar degree of the effect on the microbial communities' structures. The results also
revealed that Proteobacteria was a dominant phylum of a majority of Marinobacter
genus. Culture-dependent approaches revealed the isolation of 206 strains, including
strains that produced NH3, indole acetic acid (IAA), and 1-aminocyclopropane-1-
carboxylic acid (ACC) deaminase and also to solubilize minerals such as Fe3+, PO4
3-
,
Zn2+
, and K+
. Genotyping of the isolated bacteria showed that the number of strains that
contained ACC deaminase (acdS) gene and colonized to the rhizosphere of 'Zabad' was
higher than the number of strains that harboured that gene and colonized to the
rhizosphere of 'Umsila' when plants were grown under salinity. This notion may
suggest that the PGPR is not the unique source of salinity tolerance for 'Umsila'.
Gnotobiotic root elongation and ACC deaminase enzyme activity assays of two strains
isolated in this study (Achromobacter xylosoxidans F2-1-20 and Bacillus pumilus Y24)
showed that these strains could significantly enhance the growth of canola seeds under
50 mM NaCl, as indicated by the root growth, apparently by producing ACC deaminase
enzyme. The results obtained from this study have offered some insights into the
rhizospheric bacterial community distribution among date palm cultivars and showed
that the PGPR might contribute to salinity tolerance. However, the plant's genomic
structure is still the primary source of information required for salinity tolerance in date
palm.
