Journal of Agronomy and Field Innovations

Background: Oryza sativa L. (rice) is an essential food for nearly 3.5 billion people across the world, but drought conditions remain a major threat to its prospects, inflicting significant yield losses of 20–40% in the affected regions. Understanding mechanisms of drought resistance at the molecular level is crucial for breeding climate-adaptable rice varieties by methods of marker-assisted breeding and genetic enhancement. 
Objective: The current research aims to study the expression of eight main drought-responsive genes in rice, using qRT-PCR and establishing the correlation between the molecular changes and indicators of drought stress at the physiological level in order to gain insight into the nature of gene regulation in conditions of low moisture.
Methods: Two distinct rice cultivars ('IR64' as drought-sensitive and 'N22' as drought-resistant) were facing controlled water scarcity (50% of field capacity). Leaves were collected at 0, 24, 48, and 72 hours after imposed drought stress. Extraction of total RNA was performed via TRIzol protocol followed by qRT-PCR for analysis of the eight selected drought-responsive genes (DREB2A, OsNAC6, SNAC1, OsLEA3, OsbZIP23, OsP5CS, OsRAB16A, OsCPK9). Expression was measured relative to 2^-ΔΔCt using reference genes (Actin, UBQ5).  Physiological parameters (relative water content, chlorophyll, proline) were measured and compared with gene expression.
Findings: The drought-resistant cultivar showed higher expression of DREB2A (12.8-fold), OsNAC6 (9.4-fold), and SNAC1 (8.6-fold) compared to drought-sensitive counterpart after 48h of drought stress. The expression of OsP5CS was increased by 7.2-fold in tolerant genotypes indicating correlation (r = 0.92) with proline accumulation. The expression profiles indicated different response patterns between the varieties. Tolerant genotype was up-regulated at all time points while drought-sensitive cultivar presented transient expression profile. Tolerant genotype had higher values of water content (68.2% vs. 48.1%) and chlorophyll.
Conclusion: The utilization of qRT-PCR techniques for determining gene expression proved to have great potential in identifying distinctive characteristics between the drought-resistant and the drought-sensitive strains of rice. In the drought-resistant strains of rice, the upregulated genes are known to be responsible for osmotic adjustment, oxidative stress response, or ABA signaling, thus playing an important role in overcoming drought stress conditions.