Effects of cold acclimation on sugar metabolism and sugar-related gene expression in tea plant during the winter season

Sugar plays an essential role in plant cold acclimation (CA), but the interaction between CA and sugar remains unclear in tea plants. In this study, during the whole winter season, we investigated the variations of sugar contents and the expression of a large number of sugar-related genes in tea leaves. Results indicated that cold tolerance of tea plant was improved with the development of CA during early winter season. At this stage, starch was dramatically degraded, whereas the content of total sugars and several specific sugars including sucrose, glucose and fructose were constantly elevated. Beyond the CA stage, the content of starch was maintained at a low level during winter hardiness (WH) period and then was elevated during de-acclimation (DC) period. Conversely, the content of sugar reached a peak at WH stage followed by a decrease during DC stage. Moreover, gene expression results showed that, during CA period, sugar metabolism-related genes exhibited different expression pattern, in which beta-amylase gene (CsBAM), invertase gene (CsINV5) and raffinose synthase gene (CsRS2) engaged in starch, sucrose and raffinose metabolism respectively were solidly up-regulated; the expressions of sugar transporters were stimulated in general except the down-regulations of CsSWEET2, 3, 16, CsERD6.7 and CsINT2; interestingly, the sugar-signaling related CsHXK3 and CsHXK2 had opposite expression patterns at the early stage of CA. These provided comprehensive insight into the effects of CA on carbohydrates indicating that sugar accumulation contributes to tea plant cold tolerance during winter season, and a simply model of sugar regulation in response to cold stimuli is proposed.