Hormonal and Molecular Determinants of in vitro Propagation in Rice

Abstract

In vitro plant regeneration remains a critical bottleneck in rice biotechnology, particularly for indica varieties which exhibit significantly lower regeneration efficiencies compared to japonica subspecies. This review comprehensively examines the hormonal and molecular determinants governing callus induction, development, and regeneration in rice tissue culture systems. The intricate balance between auxin and cytokinin plays a pivotal role in determining developmental outcomes, with high auxin-to-cytokinin ratios favor callus induction, while lower ratios promote shoot regeneration. However, optimal hormone concentrations exhibit strong genotype-dependent variation, with indica cultivars often requiring distinct protocols compared to japonica varieties. Callus browning represents a major physiological constraint that severely reduces regeneration efficiency in indica rice. This phenomenon is initiated by tissue wounding during explant excision and polyphenol oxidase-mediated oxidation of phenolic compounds and subsequent lignification. At the molecular level, key regulatory genes including OsSDG715, OsBBM1, WUSCHEL-related homeobox (WOX) genes, and BOC1 have been identified as critical determinants of callus formation and regenerative competence. Quantitative trait loci (QTLs) associated with callus induction rate, browning tendency, and regeneration capacity have been mapped across multiple chromosomes, with favorable alleles identified in wild rice (Oryza rufipogon) populations. Epigenetic modifications, including histone methylation patterns (H3K27me3 and H3K4me3), DNA methylation status, and microRNA-mediated regulation, further modulate regenerative potential through chromatin remodeling and transcriptional reprogramming. Beyond conventional auxin-cytokinin regulation, other phytohormones including strigolactones, abscisic acid, salicylic acid, and brassinosteroids contribute to somatic embryogenesis and stress responses during in vitro culture. Exogenous additives such as biosynthesized silver nanoparticles, iron supplementation, melatonin, polyamines, and activated charcoal have demonstrated potential to enhance callus quality and regeneration efficiency by modulating oxidative stress responses and secondary metabolite accumulation. Understanding these complex hormonal interactions, genetic determinants, and epigenetic regulatory mechanisms is essential for developing genotype-independent tissue culture protocols and improving transformation efficiency in recalcitrant indica rice varieties, thereby facilitating the broader application of advanced genetic engineering approaches for rice improvement.