Dual mechanisms regulating glutamate decarboxylases and accumulation of gamma-aminobutyric acid in tea (Camellia sinensis) leaves exposed to multiple stresses.

Xin Mei,Don Grierson,Ying Zhou,Yiyong Chen,Yahui Huang,Ziyin Yang,Lingyun Zhang,Fang Dong,Qing Wei,Xiumin Fu

doi:10.1038/srep23685

Abstract

γ-Aminobutyric acid (GABA) is one of the major inhibitory neurotransmitters in the central nervous system. It has multiple positive effects on mammalian physiology and is an important bioactive component of tea (Camellia sinensis). GABA generally occurs at a very low level in plants but GABA content increases substantially after exposure to a range of stresses, especially oxygen-deficiency. During processing of tea leaves, a combination of anoxic stress and mechanical damage are essential for the high accumulation of GABA. This is believed to be initiated by a change in glutamate decarboxylase activity, but the underlying mechanisms are unclear. In the present study we characterized factors regulating the expression and activity of three tea glutamate decarboxylase genes (CsGAD1, 2, and 3), and their encoded enzymes. The results suggests that, unlike the model plant Arabidopsis thaliana, there are dual mechanisms regulating the accumulation of GABA in tea leaves exposed to multiple stresses, including activation of CsGAD1 enzymatic activity by calmodulin upon the onset of the stress and accumulation of high levels of CsGAD2 mRNA induced by a combination of anoxic stress and mechanical damage.

Highlights

GABA normally occurs in plants at a relative low level but the GABA content increases by up to a dozen-fold after exposure to a range of stresses including mechanical damage, acidosis, salinity, heat, cold, drought, virus infection, and low-oxygen[13,25,26]
(1) What are the potential roles of calmodulin activation and glutamate decarboxylase (GAD) transcript levels in determining the elevated levels of GABA in tea leaves exposed to anoxia and mechanical stress? (2) Which specific GAD isoform is involved in GABA production in tea leaves? (3) Do any other stresses, in addition to anoxic stress, such as mechanical stress associated with picking and handling, play a role in the formation of GABA in plants?
The accumulation of GABA under anoxic stress is observed in many plants including most cultivars of picked tea leaves[38,42,43] but it is important to emphasise the mechanical damage these leaves experience as a result of picking and handling (Figure S1)

Summary

Introduction

GABA normally occurs in plants at a relative low level but the GABA content increases by up to a dozen-fold after exposure to a range of stresses including mechanical damage, acidosis, salinity, heat, cold, drought, virus infection, and low-oxygen[13,25,26]. (1) What are the potential roles of calmodulin activation and GAD transcript levels in determining the elevated levels of GABA in tea leaves exposed to anoxia and mechanical stress?

Results

Conclusion