Targeted anthocyanin biofortification in black rice: a review of spatial metabolic control, growth-defence trade-offs and genotype-environment-management design.

Black rice is a nutrient-dense whole grain in which anthocyanins accumulate predominantly in the outer caryopsis layers, but stronger pigmentation may impose carbon, energy and precursor demands that compromise yield, stress resilience and grain quality. This review synthesizes molecular and physiological controls of rice anthocyanin accumulation. It systematically evaluates whether relationships among anthocyanin content, grain yield, and eating, processing and nutritional quality are synergistic, antagonistic or unresolved. We present a targeted rather than constitutive biofortification framework integrating rice evidence on phenylpropanoid-flavonoid metabolism and the MYB-basic helix-loop-helix-WD40 transcriptional complex with candidate mechanisms involving epigenetic and post-transcriptional control, vacuolar transport and environmental signalling. Rice studies establish the roles of core pigmentation genes and document variation in grain anthocyanin composition. However, quantitative contributions of auxiliary regulators, transport pathways and environmental signalling remain incompletely validated in black-rice caryopses. We distinguish direct rice-grain evidence from other rice-tissue evidence and from cross-species or source-sink hypotheses. Both require grain-specific causal and field validation. We distinguish two breeding scenarios. One improves existing black-rice cultivars with endogenous, spatially organized pigmentation pathways. The other reconstructs grain-specific anthocyanin production in elite white-rice backgrounds and may require coordinated pathway completion and tighter control of tissue specificity and metabolic burden. We propose a Genotype × Environment × Management framework integrating spatial multi-omics, rice-specific validation, targeted editing, inducible regulation and precision agronomy. Restricting anthocyanin synthesis to appropriate tissues, developmental windows and carbon-replete conditions may improve nutritional value while maintaining yield stability, end-use quality and climate resilience.