Functional Characterisation of <i>HAIRPLUS</i> ( <i>NtHAP</i> ) Genes Involved in Trichome Development and Specialised Metabolism of Tobacco

Plant glandular trichomes are specialised epidermal structures capable of synthesising, storing and secreting numerous varieties of secondary metabolites in different classes and are central to plant defence and the biosynthesis of high-value metabolites. In this study, we characterised the HAIRPLUS (HAP) gene family and uncovered its role as a conserved regulator of trichome development and metabolism in tobacco. Four homologues, NtHAP1a, NtHAP1b, NtHAP2a and NtHAP2b, were identified and functional studies using RNAi and CRISPR-Cas9 revealed that NtHAPs act as negative regulators of glandular trichome development. Suppression of NtHAPs resulted in increased trichome density and enlarged glandular heads, as well as enhanced accumulation of diterpenoids (e.g., neophytadiene) and increased nicotine levels. Additionally, NtHAP1 appeared to have a stronger effect on trichome density. This study establishes the NtHAP genes as key negative regulators of glandular trichome development in tobacco, expanding their functional scope from trichome morphogenesis to metabolic regulation and highlighting their evolutionary conservation across Solanaceae. These findings pave the way for both fundamental research into trichome biology and practical applications in metabolic engineering and crop improvement, such as pest resistance.