Blue LED Light and Stress-induced Modulation of Carotenoid Biosynthesis and Gene Expression in Juice Sacs of Pumelo (Citrus grandis) Cultured in Vitro

Carotenoid biosynthesis in citrus is intricately regulated by developmental cues and environmental stimuli; however, elucidating these regulatory networks in planta remains challenging due to environmental variability. This study employed an in vitro culture system using juice sacs from ‘Siam Red Ruby’ and ‘Benimadoka’ pumelo cultivars to examine the influence of environmental factors on carotenoid metabolism under controlled conditions. Throughout a four-week culture period at 25 °C, the juice sacs exhibited cellular enlargement without callus formation, accompanied by significant changes in carotenoid content and gene expression. Treatment with blue LED light (470 nm peak), 5% mannitol, and 5% sodium chloride significantly enhanced the accumulation of key carotenoids, such as phytoene, ζ-carotene, lycopene, α-carotene, β-carotene, β-cryptoxanthin, all- trans -violaxanthin, and 9- cis -violaxanthin. These metabolic accompanied by significant changes in carotenoid content and gene expression, particularly the upregulation of key biosynthetic genes such as CitPSY , CitPDS , CitZDS , CitZISO , and CitCRTISO. Differential expression patterns under mannitol and NaCl treatments suggested that distinct signaling pathways may be involved in stress-specific modulation of carotenoid metabolism. The developed in vitro system provides a tractable model for dissecting the molecular regulation of carotenoid biosynthesis in pumelo and offers valuable insights into potential strategies for enhancing carotenoid content and nutritional quality through environmental manipulation.