Abstract
In field conditions, especially in arid and semi-arid areas, tea plants are often simultaneously exposed to various abiotic stresses such as cold and drought, which have profound effects on leaf senescence process and tea quality. However, most studies of gene expression in stress responses focus on a single inciting agent, and the confounding effect of multiple stresses on crop quality and leaf senescence remain unearthed. Here, global transcriptome profiles of tea leaves under separately cold and drought stress were compared with their combination using RNA-Seq technology. This revealed that tea plants shared a large overlap in unigenes displayed “similar” (26%) expression pattern and avoid antagonistic responses (lowest level of “prioritized” mode: 0%) to exhibit very congruent responses to co-occurring cold and drought stress; 31.5% differential expressed genes and 38% of the transcriptome changes in response to combined stresses were unpredictable from cold or drought single-case studies. We also identified 319 candidate genes for enhancing plant resistance to combined stress. We then investigated the combined effect of cold and drought on tea quality and leaf senescence. Our results showed that drought-induced leaf senescence were severely delayed by (i) modulation of a number of senescence-associated genes and cold responsive genes, (ii) enhancement of antioxidant capacity, (iii) attenuation of lipid degradation, (iv) maintenance of cell wall and photosynthetic system, (v) alteration of senescence-induced sugar effect/sensitivity, as well as (vi) regulation of secondary metabolism pathways that significantly influence the quality of tea during combined stress. Therefore, care should be taken when utilizing a set of stresses to try and maximize leaf longevity and tea quality.
Highlights
For cold treatment (CT), plants were exposed to decreased temperatures of acclimation (15/10◦C) for 3 days and chilling (6/4◦C for 3 days and 4/2◦C for 9 days) for 12 days For combined stress treatments (CD), we applied cold and drought stress simultaneously for 15 days
At the time of harvest, a significant reduction in relative water content (RWC) and Fv/Fm, and an increase in EL were found for CT and CD, which was emphasized even more on drought treatment (DT) (Figures 1B–D)
Our analysis revealed a substantial number of CD-unique differential expressed genes (DEGs) (3543, 31.5%), which were absent from the list of DEGs under CT and DT
Summary
A rapid recovery of photosynthesis after combined stress was observed to be associated with high APX and SOD activities in drought-resistant plants. These findings support the role of the antioxidant mechanisms to be an essential strategy to guard plants against stress combinations (Pandey et al, 2015). Cold and drought stress have been extensively studied in tea plants, little is known about how their combination impacts plants on a whole-genome basis (Wang et al, 2013, 2016; Zheng et al, 2015; Liu et al, 2016). Little is known about the molecular mechanism of stress-induced leaf senescence in C. sinensis and how leaf senescence is delayed by low temperature has not been investigated at the transcriptome level so far
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