The Nuclear Cap-Binding Complex as an Emerging Post-Transcriptional Regulator of Cuticular Wax Biosynthesis and Drought Tolerance in Barley

Abstract

Cuticular wax is barley's first line of defense against nonstomatal water loss under drought, and the classical wax biosynthesis pathway from very-long-chain fatty acid elongation through the alkane-forming and β-diketone branches, transcriptionally coordinated by the SHINE-family regulator HvWIN1 was comprehensively reviewed alongside its wheat counterpart in 2022. Since that review, however, a distinct and largely unanticipated regulatory layer has emerged. Barley TILLING mutants in the small subunit of the nuclear cap-binding complex, CBP20, show markedly increased epicuticular wax deposition, faster stomatal closure, and improved water retention under drought apparently through post-transcriptional control of alkane-forming pathway genes rather than through the canonical HvWIN1-mediated transcriptional route. Follow-up studies in 2024 and 2025 have extended this finding to ABA sensitivity during seed germination and to distinct, stage-specific roles for the CBP20 and CBP80 subunits during reproductive development. This review synthesizes the classical barley wax biosynthesis pathway, updates it with this post-2022 cap-binding complex literature, and proposes an integrated model in which the nuclear cap-binding complex acts as a post-transcriptional checkpoint intersecting ABA signaling and the canonical wax biosynthesis transcriptional network. We identify the mechanistic relationship between these two regulatory layers as the central open question for future barley drought-tolerance breeding.