Transcriptional Regulation, Epigenetic Memory, and CRISPR-Based Engineering of Combined Abiotic Stress Tolerance in...

Cereal crops including wheat, rice, maize, barley, and sorghum collectively supply most global caloric and protein requirements, yet their productivity is increasingly constrained by combined abiotic stresses that co-occur under field conditions. Simultaneous drought, heat, salinity, and cold impose yield losses that consistently exceed those caused by individual stresses and elicit molecular responses that are qualitatively distinct from single-stress reactions and cannot be inferred from them. Despite this agronomic reality, the molecular mechanisms governing combined stress responses in cereals remain poorly resolved, and no integrated framework connecting the transcriptional, epigenetic, and genome-editing dimensions of combined stress tolerance has previously been articulated for this crop group. This review proposes a three-tier integrated framework for understanding and engineering combined abiotic stress tolerance in major cereals. The first tier encompasses transcription factor networks, including bZIP, WRKY, NAC, AP2/ERF, DREB, MYB, and HSF families, that translate combined stress signals into transcriptional reprogramming through ABA-dependent and ABA-independent pathways, hormonal crosstalk, and osmoprotectant and antioxidant defence systems. The second tier addresses the epigenetic regulatory layer, encompassing DNA methylation, histone modifications, and non-coding RNA pathways that gate TF binding site accessibility and encode stress memory in cereals. The third tier examines CRISPR-based tools, including multiplexed Cas9 editing and dCas9-based epigenome editing, that engineer validated targets from both tiers, while confronting polyploid off-target effects, growth penalties, and a laboratory-to-field validation gap. The three tiers are mechanistically coupled, with TF activity shaping epigenetic landscapes, epigenetic states gating TF access, and both providing precision engineering targets. Critical gaps include the absence of combined-stress epigenomic datasets, limited characterisation in barley and sorghum, and early-stage combined-stress-specific strategies.