Abstract
Ascorbate peroxidase (APX) plays an important role in detoxifying reactive oxygen species under environmental stress. Although previous work in drought-tolerant wild watermelon has shown an increase in chloroplast APX enzyme activity under drought, molecular entities of APX have remained uncharacterized. In this study, structure and transcriptional regulation of the APX gene family in watermelon were characterized. Five APX genes, designated as CLAPX1 to CLAPX5, were identified from watermelon genome. The mRNA alternative splicing was suggested for CLAPX5, which generated two distinct deduced amino acid sequences at their C-terminus, in resemblance to a reported alternative splicing of chloroplast APXs in pumpkin. This observation suggests that two isoenzymes for stromal and thylakoid-bound APXs may be generated from the CLAPX5 gene. Phylogenetic analysis classified CLAPX isoenzymes into three clades, i.e., chloroplast, microbody, and cytosolic. Physiological analyses of wild watermelon under drought showed a decline in stomatal conductance and CO2 assimilation rate, and a significant increase in the enzyme activities of both chloroplast and cytosolic APXs. Profiles of mRNA abundance during drought were markedly different among CLAPX genes, suggesting distinct transcriptional regulation for the APX isoenzymes. Up-regulation of CLAPX5-I and CLAPX5-II was observed at the early phase of drought stress, which was temporally correlated with the observed increase in chloroplast APX enzyme activity, suggesting that transcriptional up-regulation of the CLAPX5 gene may contribute to the fortification of chloroplast APX activity under drought. Our study has provided an insight into the functional significance of the CLAPX gene family in the drought tolerance mechanism in this plant.
Highlights
Drought-associated water deficit is one of the major factors restricting plant productivity and crop yields worldwide
Using the protein sequences deduced from all ascorbate peroxidase (APX) genes in Arabidopsis thaliana (Panchuk et al, 2002; Table A2) as the queries, we identified five homologous genes in the whole genome sequence of cultivar watermelon (Citrullus lanatus subsp. vulgaris cv. 97103) in the Cucurbit Genomics Database
These five putative APX genes were designated as CLAPX1 to CLAPX5, according to the order of highest sequence similarity with the translated sequence of Arabidopsis AtAPX1 (Table 1)
Summary
Drought-associated water deficit is one of the major factors restricting plant productivity and crop yields worldwide. Some of the key molecules generated in large quantities in plant cells during environmental stress are the reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), superoxide radical (O2-), and hydroxyl radical (·OH) (Gill & Tuteja, 2010; You & Chan, 2015). An array of non-enzymatic antioxidants, such as ascorbate, glutathione, carotenoids, and tocopherols, serve as defense agents for protecting plant cells from oxidative injuries. These enzymes and antioxidants are mutually related in the glutathione-ascorbate cycle, which plays an essential role in resisting water deficit and oxidative stress under drought (Li et al, 2013)
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