Synergistic photoregulation of abscisic acid and gibberellin enhances embryogenic callus induction and proliferation in Indica rice

Efficient embryogenic callus induction remains a major constraint in the genetic transformation of Indica rice. Although light is known to promote callus formation, its underlying phytohormonal regulation is not well understood. In this study, mature embryos of Indica rice ‘T98B’ were cultured under a 12 h light–dark cycle (light) or continuous darkness (dark).

Callus induction occurred more rapidly under light. The ABA levels under light were 92.1% and 59.7% higher on 5 and 7 days of induction, and 62.2% higher on day 7 of subculture compared with dark. GA₃ content was 20.6% lower under light on day 5 but increased by 43.5% and 44.4% on days 7 of induction and subculture, respectively. Transcriptome analysis showed significant enrichment of differentially expressed genes in hormone signaling pathways. qPCR validation revealed early upregulation of the ABA receptor gene OsPYL6 and the GA-degrading gene OsGA3ox8 under light, followed by downregulation of OsPYL4/OsPYL6 and strong induction of the GA biosynthesis gene OsGA3ox2 at 7 days of induction. These results indicate that light sequentially activates ABA signaling, which may contribute to the initiation of embryogenic callus formation, and subsequently enhances GA biosynthesis, which may regulate proliferation.

Overall, light sequentially activates ABA signaling, which may be associated with embryogenic callus initiation, and then enhances GA biosynthesis, which may regulate sustained proliferation, providing a hormone-based strategy to boost Indica rice transformation efficiency.