Bacteria were isolated from surface disinfected seeds of eight modern corn types and an ancestor of corn, 'teosinte' and identified using 16S rDNA sequences. From each of the modern corn types we obtained Bacillus spp. (including, Bacillus amyloliquefaciens and Bacillus subtilis); while from teosinte we obtained only Pantoea agglomerans and Agrobacterium species. Of these bacteria, only P. agglomerans could actively grow under hypersaline conditions and increase salt tolerance of tropical corn seedlings. In laboratory and greenhouse experiments where plants were watered with a 0.2 mol l(-1) NaCl solution, P. agglomerans was found to enhance the capacity of tropical corn to grow compared to uninoculated controls. The total dry biomass was significantly higher in P. agglomerans-treated plants compared to controls under saline water. Gene expression analysis showed the up-regulation of the aquaporin gene family especially plasma membrane integral protein (ZmPIP) genes in P. agglomerans-treated plants. The plasma membrane integral protein type 2 (PIP2-1) gene in tropical corn seedlings was highly up-regulated by P. agglomerans treatment under salt stress conditions. Microscopic examination of P. agglomerans inoculated seedlings revealed that the bacterium colonized root meristems densely, and as roots developed, the bacterium became sparsely located in cell junctions.
Significance and impact of the study: The enhancement of salt tolerance capacity in tropical corn, an important food crop, has the capacity to increase its cultivation area and yield in saline soils. The application of rhizobacteria to improve salt tolerance of tropical corn is ecofriendly and cost effective. We show that P. agglomerans isolated from teosinte (an ancestor of corn) induces salt tolerance in tropical corn and up-regulation of aquaporin genes. This study shows that microbes that increase salt tolerance may be used to enhance crop growth in saline soils.
Keywords: aquaporin; endophyte; maize; rhizobacterium; salt stress.
© 2014 The Society for Applied Microbiology.