Physiological and gene expression analysis of heterosis in maize hybrid B73×Mo17 under low nitrogen availability

Maize is a globally important food crop and an industrial raw material for which nitrogen is an essential nutrient. Owing to heterosis, hybrid maize is advantageous in terms of development and stress resistance. However, the mechanisms underlying heterosis under low nitrogen availability remain poorly understood. This study aimed to investigate whether maize hybrid B73×Mo17 and its reciprocal cross show heterosis at the early seedling stage under different nitrogen availability (0.5, 1, 2, and 5 mM NO₃⁻) and to explore the corresponding physiological and gene-expression foundations. Under low nitrogen stress, the maize hybrids Mo17×B73 and B73×Mo17 exhibited superior performance in biomass production, lateral root length, root number, and root surface area compared to their parental lines, B73 and Mo17. In contrast, no significant differences were observed among these lines under normal conditions. Moreover, both hybrids showed significantly higher total nitrogen, chlorophyll, and soluble protein contents, and nitrate reductase, glutamine synthetase, and glutamate dehydrogenase activities than B73 or Mo17 under low nitrogen availability. Furthermore, the primary nitrate-response genes showed higher expression levels in both F1 hybrids than in their respective parents under low nitrate conditions; additionally, in the hybrids Mo17×B73 and B73×Mo17, the transcripts of ZmNRT3.1A and ZmNIR1 derived from the B73 allele were more abundant than those from the Mo17 allele, indicating an allelic expression bias toward the B73 parent. This study elucidates the physiological and molecular mechanisms underlying heterosis in maize under low nitrogen availability, providing a mechanistic basis for breeding maize varieties with enhanced adaptation to low-nitrogen soils.