Tissue-Specific Antioxidant Responses and PAL Gene Expression in Bread Wheat Under Drought Stress.

Background: Drought stress affects crop wheat productivity by inducing biochemical changes in different tissues. Antioxidant enzymes, phenolic compounds and sugars are crucial in the plant's defense against stress. Studying these responses in tolerant and susceptible genotypes can help improve our knowledge about drought tolerance.

Objectives: This study aimed to evaluate tissue-specific (leaf, stem, spike and root) activities of antioxidant enzymes, phenolic content, soluble sugar accumulation, under moderate drought stress. Additionally, the expression of the PAL gene was analyzed in different tissues of drought-tolerant and susceptible wheat genotypes.

Materials and methods: Three wheat genotypes-susceptible (Marvdasht) and tolerants (82, 118)-were grown under drought stress and control conditions. Antioxidant enzyme activities, phenolic compounds, and sugar contents were measured in leaf, stem, and spike tissues. Quantitative Real-time PCR was used to assess PAL gene expression in leaf, stem, spike, and root tissues. Thousand-kernel weight (TKW) was measured as an indicator of performance.

Results: Drought stress led to increased POD, CAT, PPO activities, and phenolic content in all tissues of the susceptible genotype (Marvdasht). However, SOD activity decreased in this genotype but increased in tolerant genotypes. Phenolic content and soluble sugar accumulation increased in all genotypes under drought, except for genotype 82, where soluble sugar decreased in the leaf tissue. PAL gene expression was down-regulated in the susceptible genotype's stem, root, and spike, while up-regulated in the tolerant genotype's stem. As a result of these adaptive responses, yield reduction, measured as TKW, was less severe in the tolerant genotypes compared to the susceptible genotype. Principal component analysis highlighted that drought-tolerant genotypes exhibited the highest levels of antioxidant enzyme activity and soluble sugars under stress.

Conclusions: Enhanced antioxidant activity, phenolic accumulation and tissue-specific activation of the PAL gene are key factors contributing to drought tolerance in wheat. The PAL gene's differential expression suggests distinct responses to drought stress, with the tolerant genotype exhibiting tissue-specific activation. These mechanisms moderate stress-induced damage and reduce yield loss. The study gives emphasis to the importance of integrating biochemical and molecular insights to develop drought-resistance cultivars, offering valuable implications for improving crop production under abiotic stress.