Biodiversity of Diazotrophs in Rhizosphere of Potato (Solanum tuberosum L.)

Abstract

Potato is the third most important food crop which requires high fertilizer that leads to environmental pollution. The use of biofertilizer is environment friendly and cost effective hence is suitable for sustainable agriculture. In this study, 44 bacterial isolates including mostly Azospirillum, Bacillus, Brevundimonas, Enterobacter, Pseudomonas and Rhizobium spp. were isolated from potato rhizosphere of which 32 were able to fix atmospheric N2 and 29 showed indole acetic acid (IAA) production. Strains showed high metabolic and genetic diversity based on BioLog, ERIC and BOX-PCR analysis. Out of potential plant growth promoting rhizobacteria (PGPR) tested for inoculation response in potato, it was observed that the N2 fixation (%Ndfa) decreases with the increase in applied N for Rhizobium sp. TN04 and Azospirillum sp. TN09 whereas no effect was observed for Azospirillum sp. TN03. Azospirillum sp. TN09 showed the highest %Ndfa among three tested isolates. Under field conditions, Azospirillum spp. TN03, TN09 and Rhizobium sp. TN04, Rhizobium sp. TN04 increased plant height (18%), dry weight (18.5%) tuber dry weight (14%) and tuber yield (19%). Azospirillum, Brevundimonas and Rhizobium spp. also maintain substantial population with potato roots. Metagenomics were employed to study total bacterial diversity in the rhizosphere of potato from major potato growing areas of Punjab, Pakistan showed that structural community (based on 16S rRNA) is almost similar across all the regions whereas functional diversity (based on nifH) of diazotrophs is mostly variable. Abundance of diazotrophs (108-1010 copies of nifH g-1 of soil) changed among regions. Pyrosequencing of nifH validated maximum diversity in Sheikhupura (446 species) whereas minimum in Gujranwala area (291 species). Overall in the potato rhizosphere, most abundant class was α-proteobacteria followed by β-proteobacteria. The study concludes that abundance of different genera varies across different regions due to many biotic and abiotic factors. Azospirillum spp. TN03, TN09 and Rhizobium sp. TN04 omnipresent and have potential to improve potato yield up to 15-19% subsequently saving 50% nitrogenous fertilizer, are recommended as potential candidates for biofertilizer production for potato crop. The study also shows that potato xvi harbor a diverse range of novel diazotrophs (not known earlier) which opens new horizons for future prospects.