Spring cold stress at high altitudes in southeastern Xizang activates <i>CsABF2</i> to regulate chlorophyll degradation and phenolic biosynthesis in tea plants

The tea plant (<i>Camellia sinensis</i>), native to warm and humid low-latitude regions of southwestern China, has expanded to higher altitudes, including southeastern Xizang, where cultivation above 2500 m poses challenges due to low accumulated temperatures. However, the impact of high-altitude climatic conditions, particularly temperature, on tea growth remains underexplored. To investigate, weather stations were deployed at three altitudes in southeastern Xizang to monitor spring temperature fluctuations: Medog (MD, 1200 m), Zayü (ZY, 1720 m), and Layue in Bayi District (BY, 2600 m). Field observations and meteorological data indicated that the milder spring temperatures in MD and ZY facilitated normal budburst and growth, whereas the lower temperatures in BY delayed budburst and resulted in leaf yellowing and browning. Comparative experiments revealed that seedlings exposed to fluctuating low temperatures (10°C/4°C) experienced the most severe cold injury and exhibited the lowest germination rates compared to seedlings under constant-temperature treatments. Transcriptome analysis uncovered differential expression of genes involved in chlorophyll degradation, lignin biosynthesis, and flavonoid pathways under cold stress. Functional characterization of the cold-induced transcription factor <i>CsABF2</i> revealed its central role in activating these pathways, as evidenced by antisense oligodeoxynucleotide (AsODN) silencing and promoter activation assays, to activate key downstream genes: <i>CsSGR1</i> (chlorophyll degradation), <i>CsPALa</i> (phenylpropanoid pathway), and <i>CsMYB6c</i> (flavonoid biosynthesis). These results provide mechanistic insights into how spring temperature variability at high altitudes impairs tea plant development and alters quality-related metabolites, offering a molecular basis for improving cold resilience in tea cultivation.