Decoding the circular RNA-mediated drought response in horsegram (Macrotyloma uniflorum)

The present study explores the drought stress responses in two genetically distinct horsegram genotypes (IC-94637 and IC-426521), with a focus on the regulation of circular RNAs (circRNAs) under controlled conditions. Experimental plants were grown in pots in a polyhouse and subjected to drought stress by withholding water. Later, physiological and molecular changes were assessed in normal and drought-stressed plants of these genotypes. Physiological parameters, including relative water content, stomatal closure, electrolyte leakage, and chlorophyll concentration, were measured. Further, a transcriptome-wide analysis was conducted to assess changes in circRNA expression under control and drought conditions. Genotype IC-94637 exhibited greater drought tolerance, as indicated by higher relative water content (79.05%), moderate stomatal closure (46%), reduced electrolyte leakage (32.57%), and increased chlorophyll levels (24.65 mg/ml). In contrast, IC-426521 showed lower chlorophyll concentration (20.01 mg/ml), increased electrolyte leakage (39.81%), reduced water content (74.53%), and higher stomatal closure (49.5%). Transcriptome analysis revealed that in IC-94637, four circRNA-related genes were expressed, of which two genes encoding ABC transporter proteins were upregulated, while the other two genes related to extensin (Ext) proteins were downregulated during drought stress. On the other hand, genotype IC-426521 exhibited expression of eight circRNA-related genes, but none of them was upregulated during drought stress. Notably, during physiological evaluations, genotype IC-94637 exhibited a rapid transition to the reproductive phase under controlled drought conditions, prioritizing seed production over extended vegetative growth, which led to early permanent wilting. This drought avoidance strategy resulted in a high seed yield despite a shortened growth period. In contrast, genotype IC-426521 showed resistance under prolonged stress but exhibited quicker wilting under acute water deficiency, indicating a distinct adaptation mechanism. Transcriptomic analysis revealed genotype-specific regulation of circular RNAs (circRNAs), suggesting differential molecular pathways contributing to drought tolerance. While differential gene expression in horsegram under drought stress has been extensively studied, this is the first report to explore the role of circular RNAs (circRNAs) under drought stress in this crop. Our findings provide deeper insights into how circRNAs contribute to stress adaptation and suggest potential functional links between circRNAs and their host genes in modulating drought tolerance in horsegram.