Abstract
To uncover mechanism of highly weakened carbon metabolism in chlorotic tea (Camellia sinensis) plants, iTRAQ (isobaric tags for relative and absolute quantification)-based proteomic analyses were employed to study the differences in protein expression profiles in chlorophyll-deficient and normal green leaves in the tea plant cultivar “Huangjinya”. A total of 2110 proteins were identified in “Huangjinya”, and 173 proteins showed differential accumulations between the chlorotic and normal green leaves. Of these, 19 proteins were correlated with RNA expression levels, based on integrated analyses of the transcriptome and proteome. Moreover, the results of our analysis of differentially expressed proteins suggested that primary carbon metabolism (i.e., carbohydrate synthesis and transport) was inhibited in chlorotic tea leaves. The differentially expressed genes and proteins combined with photosynthetic phenotypic data indicated that 4-coumarate-CoA ligase (4CL) showed a major effect on repressing flavonoid metabolism, and abnormal developmental chloroplast inhibited the accumulation of chlorophyll and flavonoids because few carbon skeletons were provided as a result of a weakened primary carbon metabolism. Additionally, a positive feedback mechanism was verified at the protein level (Mg chelatase and chlorophyll b reductase) in the chlorophyll biosynthetic pathway, which might effectively promote the accumulation of chlorophyll b in response to the demand for this pigment in the cells of chlorotic tea leaves in weakened carbon metabolism.
Highlights
Tea (Camellia sinensis) is a perennial evergreen leafy woody plant native to southwest China
Our results showed that levels of PAL and 4-coumarate-CoA ligase (4CL) proteins were reduced in the chlorotic leaves, indicating that flavonoid metabolism was inhibited
A previous study showed that most of flavonoids were accumulated on the leaf surface to protect structure and organization in plants from UV-B radiation damage [33], while in this study, our results indicated that abnormally developed chloroplast inhibited the accumulation of chlorophyll and flavonoids because few carbon skeletons were provided as a result of weakening of carbon metabolism, and the reduction of flavonoids contents was unfavorable for the chlorotic mutant to protect against UV-B radiation damage
Summary
Tea (Camellia sinensis) is a perennial evergreen leafy woody plant native to southwest China. The natural mutant of tea, “Huangjinya”, exhibits chlorotic leaves and lower carbon metabolism than non-chlorotic varieties under sunlight [1,2]. In our previous studies [2,3], metabolomics and transcriptomics analyses were performed on green and chlorotic shoots of “Huangjinya” to gain an overview of the amino acid, flavonoid, and carbohydrate metabolism. These analyses revealed that the weakening of carbon metabolism is accompanied by nitrogen accumulation, suggesting that the metabolism of carbon and nitrogen are unbalanced [3]. The correlation of protein expression with weakened carbon metabolism in chlorotic tea leaves remains to be elucidated
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