Abstract
Plants are known to respond to Ultraviolet-B radiation (UV-B: 280–320 nm) by generating phenolic metabolites which absorbs UV-B light. Phenolics are extraordinarily abundant in Camellia sinensis leaves and are considered, together with pleasant volatile terpenoids, as primary flavor determinants in tea beverages. In this study, we focused on the effects of UV-B exposure (at 35 μW cm–2 for 0, 0.5, 2, and 8 h) on tea transcriptional and metabolic alterations, specifically related to tea flavor metabolite production. Out of 34,737 unigenes, a total of 18,081 differentially expressed genes (DEGs) due to UV-B treatments were identified. Additionally, the phenylpropanoid pathway was found as one of the most significantly UV-B affected top 20 KEGG pathways while flavonoid and monoterpenoid pathway-related genes were enhanced at 0.5 h. In the UVR8-signal transduction pathway, UVR8 was suppressed at both short and long exposure of UV-B with genes downstream differentially expressed. Divergent expression of MYB4 at different treatments could have differentially altered structural and regulatory genes upstream of flavonoid biosynthesis pathways. Suppression of MYB4-1&3 at 0.5 h could have led to the up-regulation of structural CCOAOMT-1&2, HST-1&2, DFR-4, ANR-2, and LAR-1&3 genes resulting in accumulation of specialized metabolites at a shorter duration of UV-B exposure. Specialized metabolite profiling revealed the correlated alterations in the abundances of catechins and some volatile terpenoids in all the treatments with significant accumulation of specialized metabolites at 0.5 h treatment. A significant increase in specialized metabolites at 0.5 h treatment and no significant alteration observed at longer UVB treatment suggested that shorter exposure to UV-B led to different display in gene expression and accumulation of specialized metabolites in tea shoots in response to UV-B stress. Taken together, our results indicated that the UV-B treatment applied in this study differentially altered the UVR8-signal transduction, flavonoid and terpenoid pathways at transcriptional and metabolic levels in tea plants. Our results show strong potential for UV-B application in flavor improvement in tea at the industrial level.
Highlights
Tea is a widely consumed non-alcoholic beverage, made from tender tea leaves (Camellia sinensis) (Banerjee, 1992), which contain substantial volatiles and extraordinarily high amount of polyphenols (16–30%, dry weight) (Graham, 1992) such as epigallocatechin-3-gallate (EGCG) and epicatechin-3-gallate (ECG)
C. sinensis is native to southwest Asia and is grown at high altitudes (Meegahakumbura et al, 2018), receiving high fluence rates of Ultraviolet-B radiation (UV-B) radiation (280–320 nm) compared to plant species grown in low altitude regions (Madronich et al, 1995; Casati et al, 2006)
Among the affected Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways categorized into cellular processes, environmental information processes, genetic information processes and metabolism, the majority of differentially expressed genes (DEGs) were associated with metabolism processes
Summary
Tea is a widely consumed non-alcoholic beverage, made from tender tea leaves (Camellia sinensis) (Banerjee, 1992), which contain substantial volatiles and extraordinarily high amount of polyphenols (16–30%, dry weight) (Graham, 1992) such as epigallocatechin-3-gallate (EGCG) and epicatechin-3-gallate (ECG). Ambient UV-B at low fluence rates has regulatory effects on plant growth and development, biochemical composition (Boccalandro, 2001; Suesslin and Frohnmeyer, 2003) and affect the expression of a wide range of genes (Casati and Walbot, 2003; Casati and Walbot, 2004; Jenkins, 2009) These UV-B specific alterations in plants are mediated by the signaling transduction module of UV RESISTANCE LOCUS8 (UVR8) and COP1 (Rizzini et al, 2011; Jenkins, 2017), which contains multiple transcription factors such as ELONGATED HYPOCOTYL5 (HY5) (Binkert et al, 2014), and MYB family members (Stracke et al, 2009). The genes (UVR-8, COP1, HY5) in the UV-B signaling transduction module are not consistently UV-B up-regulated (Liu L. et al, 2018) as found in many other plant species (Rizzini et al, 2011; Jenkins, 2017)
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