Abstract
Temperature is a major environmental signal that governs plant growth and development. A moderately high ambient temperature alters plant metabolism without significant induction of heat–stress responses. Despite ancillary reports on the negative effect of warmer climate on tea quality, information on specific effect of sub high temperature (SHT) on theanine accumulation is scanty. L-Theanine is the most abundant free amino acid in tea (Camellia sinensis L.) leaves that contributes to the unique umami flavor of green tea infusion. Tea harvested in warmer months lacks distinctive umami taste due to low theanine content. In this study, we showed that SHT (35°C) gradually decreased theanine concentration over time, which was closely associated with the SHT-induced suppression in theanine biosynthetic genes. 24-epibrassinolide (BR), a bioactive brassinosteroids, attenuated the SHT-induced reduction in theanine concentration by upregulating the transcript levels of theanine biosynthetic genes, such as ARGININE DECARBOXYLASE (CsADC), GLUTAMINE SYNTHETASE (CsGS), GLUTAMATE SYNTHASE (CsGOGAT) and THEANINE SYNTHASE (CsTS). Furthermore, time-course analysis of the activity of theanine biosynthetic enzyme reveals that BR-induced regulation of GS and GOGAT activity plays essential role in maintaining theanine content in tea leaves under SHT, which is consistent with the central position of GOGAT in theanine biosynthetic pathway. Therefore, it is convincing to propose that exogenous BR treatment can be advocated to improve summer tea quality by enhancing in vivo accumulation of theanine. However, a future challenge is to use this information on the role of BR in theanine biosynthesis and thermotolerance to further understand how BR may be tuned to benefit plant fitness for enhancing tea quality.
Highlights
Tea is the second most popular beverage after water in the world (Tounekti et al, 2013)
How sub high temperature (SHT) subdues theanine concentration in tea leaves, we performed a timecourse analysis of genes that encode key enzymes involved in theanine biosynthesis, such as arginine decarboxylase (ADC), Glutamine synthetase (GS), GOGAT, and TS
The results showed that transcript abundance of GS, GOGAT, and TS consistently decreased soon after imposition of SHT, whereas GS exhibited maximum decreases as compared to the others (Figure 3)
Summary
Tea is the second most popular beverage after water in the world (Tounekti et al, 2013). While effect of extreme high temperatures on plant growth can be recognized by visible sign and yield reduction, sub high temperatures (SHT), which are just above physiological optimum, can cause massive metabolic changes without showing visible symptoms (Ahammed et al, 2016; Lu et al, 2017; Ohama et al, 2017) These responses are cumulative that comprise physiological, metabolic and transcriptional reprogramming (Liu Z.W. et al, 2016; Wu et al, 2016; Ohama et al, 2017). A positive effect of BR on the biosynthesis of tea polyphenols and amino acids has been revealed, its effect on theanine content in tea leaves under high temperature conditions remains largely unclear. The results suggest that regulation of theanine biosynthesis by BR can be exploited to improve both quality and medicinal value of summer tea
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