Abstract
The ascorbate-glutathione (ASC-GSH) cycle has an important role in defensive processes against oxidative damage generated by drought stress. In this study, the changes that take place in apoplastic and symplastic ASC-GSH cycle enzymes of the leaf and petiole were investigated under drought stress causing leaf rolling in Ctenanthe setosa (Rose.) Eichler (Marantaceae). Apoplastic and symplastic extractions of leaf and petiole were performed at different visual leaf rolling scores from 1 to 4 (1 is unrolled, 4 is tightly rolled and the others are intermediate forms). Glutathione reductase (GR), a key enzyme in the GSH regeneration cycle, and ascorbate (ASC) were present in apoplastic spaces of the leaf and petiole, whereas dehydroascorbate reductase (DHAR), which uses glutathione as reductant, monodehydroascorbate reductase (MDHAR), which uses NAD(P)H as reductant, and glutathione were absent. GR, DHAR and MDHAR activities increased in the symplastic and apoplastic areas of the leaf. Apoplastic and symplastic ASC and dehydroascorbate (DHA), the oxidized form of ascorbate, rose at all scores except score 4 of symplastic ASC in the leaf. On the other hand, while reduced glutathione (GSH) content was enhanced, oxidized glutathione (GSSG) content decreased in the leaf during rolling. As for the petiole, GR activity increased in the apoplastic area but decreased in the symplastic area. DHAR and MDHAR activities increased throughout all scores, but decreased to the score 1 level at score 4. The ASC content of the apoplast increased during leaf rolling. Conversely, symplastic ASC content increased at score 2, however decreased at the later scores. While the apoplastic DHA content declined, symplastic DHA rose at score 2, but later was down to the level of score 1. While GSH content enhanced during leaf rolling, GSSG content did not change except at score 2. As well, there were good correlations between leaf rolling and ASC-GSH cycle enzyme activities in the leaf (GR and DHAR) and leaf rolling and GSSG. These results showed that in apoplastic and symplastic areas, ASC-GSH cycle enzymes leading ROS detoxification may have a role in controlling leaf rolling.
Highlights
Leaf rolling is one of the most frequent responses to drought stress in some plants (Begg, 1980)
Our results showed that apoplastic washing fluid (AWF) was free from symplastic contamination
G6PDH activity in AWF was below the limits of detection (1 %) in C. setosa
Summary
Leaf rolling is one of the most frequent responses to drought stress in some plants (Begg, 1980). Leaf rolling reduces effective leaf area and transpiration, and is a potentially useful drought avoidance mechanism in arid areas (Clarke, 1986). Removal of ROS is strictly controlled by an array of non-enzymatic and enzymatic antioxidant mechanisms in plants. Ascorbate (ASC) and glutathione (GSH) are the key nonenyzmatic antioxidants and play important roles in protecting cellular components from oxidative stress (Smirnoff and Pallanca, 1996). Both compounds are involved in maintaining cellular redox status. Ascorbate is a substrate for cell wall peroxidases and plays a role in the regulation of cell wall lignification (Takahama and Oniki, 1992; De Cabo et al, 1996; Mehlhorn et al, 1996)
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