Foliar application of Azospirillum brasilense, salicylic acid and zinc on wheat Performance under rain–fed condition

Contribution of plant growth-promoting bacteria to plant gene function, regulation, and modulation may open a new window of hope to improve plant performance in harsh environments. In a preliminary study, Azospirillum brasilense strain Sp7 has been studied for its regulatory role in molecular, biochemical, and physiological processes in wheat exposed to terminal severe drought stress. Moreover, two field experiments were conducted to elucidate the effectiveness of A. brasilense inoculation, salicylic acid, and zinc foliar application on wheat performance under dryland (rain–fed) conditions. In the preliminary experiment, wheat seedlings that were foliarly inoculated with and without A. brasilense were grown under well-watered conditions for four weeks and then exposed to two water regimes: well-watered and drought-stressed conditions. The preliminary experiment aimed to elucidate the changes in proline accumulation, chlorophyll contents, leaf relative water levels, malondialdehyde contents, electrolyte leakage rate, and also quantified the genes associated with proline biosynthesis and the psbA gene (encoding photosystem II protein D1) under drought stress conditions mediated by A. brasilense inoculation. The field experiment was conducted under two contrasting field conditions to elucidate the efficacy of A. brasilense inoculation via foliar application in combination with exogenous application of salicylic acid (SA) and foliar application of Zn on two winter wheat cultivars, Sardari (Triticum aestivum L.) and Saji (Triticum durum L.). Two weeks after drought imposition, foliar Azospirillum-inoculated plants exhibited lower levels of psbA gene expression than uninoculated plants. As the drought progressed, inoculated plants significantly exhibited a higher amount of psbA mRNA accumulation than uninoculated plants. Drought-stressed conditions caused the induction of delta-1-pyrroline-5-carboxylate (P5C) synthase and P5C reductase expression along with proline accumulation. Foliar inoculation with Azospirillum caused a substantial increase in P5C synthase and P5C reductase expression. Results further showed that foliar inoculation alleviated the enhancement of malondialdehyde and electrolyte leakage. The field experiments showed that the application of Zn and/or Azospirillum was more effective on wheat performance than the exogenous application of SA. The best yield was obtained when a combined foliar application of Zn plus Azospirillum was used. Azospirillum inoculation could elevate the adaptability of wheat plants through inducing the transcription level of the related psbA gene, P5CS, and P5CR genes and enhancing proline, thereby ameliorating the adverse effects of drought stress. The present findings provide more insights into the regulatory mechanism of drought stress resistance in wheat by foliar inoculation of Azospirillum and may lay a fundamental basis for future functional studies and open a new era to improve plant tolerance to drought stress.