Abstract
Maize (Zea mays L.) is highly sensitive to drought stress, resulting in large losses in yield; therefore, strategies aimed at enhancing drought tolerance are essential. Melatonin improves stress tolerance in plants; however, its mechanism in maize seedlings under drought stress remains unknown. Therefore, we investigated the effects of foliar-sprayed melatonin (100 umol L−1) on the antioxidant system, photosynthetic gas exchange parameters, stomatal behavior, endogenous melatonin and abscisic acid (ABA)-related gene expression in maize seedling leaves under 20% polyethylene glycol (PEG)-induced drought stress. PEG treatment resulted in oxidative stress and stomatal closure, resulting in chlorophyll degradation and inhibition of photosynthesis; thereby, reducing seedling biomass. Melatonin pretreatment significantly improved the relative water content, photosynthetic gas exchange parameters and stomatal behavior; thereby, maintaining chlorophyll contents and photosynthesis. Melatonin also stimulated the antioxidant system, enhancing the clearance of reactive-oxygen species, preventing severe damage under PEG-induced drought. Pre-treatment also increased endogenous melatonin and inhibited up-regulation of NCED1, an ABA synthesis-related gene, as well as selectively up-regulating ABA catabolic genes ABA8ox1 and ABA8ox3, reducing ABA accumulation and inducing stomatal reopening. Overall, these findings suggest that melatonin pre-treatment alleviated the inhibitory effects of drought stress on photosynthesis, enhancing tolerance in maize seedlings.
Highlights
As sessile organisms, plants are inevitably exposed to various environmental stresses (Gururani et al 2015), of which drought is one of the most common, occurring annually in many regions around the world and subsequently inhibitingZhuo Li and Xiaoyu Su have contributed to this work.Stomata regulate the flow of gases and water in and out of plants, which plays an important role in overall survival (Peterson et al 2010)
Pre-treatment with melatonin under polyethylene glycol (PEG)-induced drought (MT + D) effectively improved the relative water content (RWC) of the leaves, with the most significant effect on day 3 (12.17% increase compared to D treatment)
These findings suggest that melatonin pre-treatment could improve the RWC of the leaves under drought conditions; thereby, helping maintain cell pressure
Summary
Plants are inevitably exposed to various environmental stresses (Gururani et al 2015), of which drought is one of the most common, occurring annually in many regions around the world and subsequently inhibitingZhuo Li and Xiaoyu Su have contributed to this work.Stomata regulate the flow of gases and water in and out of plants, which plays an important role in overall survival (Peterson et al 2010). Redox imbalance due to drought causes over production and accumulation of ROS, which induce membrane lipid peroxidation and impair membrane function, leading to chlorophyll degradation and loss of photosynthetic activity (Ippolito et al 2011; Li et al 2011; Anjum et al 2017). In this case, plants activate their antioxidant system to cope with ROS-induced oxidative stress, employing enzymatic [superoxide dismutase [SOD), peroxidase (POD) and catalase (CAT)] and non-enzymatic antioxidants (glutathione (GSH)] (Anjum et al 2011; Ashraf et al 2015). The accumulation of ABA in plant cells is closely associated with ROS production, with drought stress stimulating the activation of C a2+ channels in the plasma membrane, inducing ABA accumulation to regulate stomatal closure (Hu et al 2008; Liu et al 2010; Zhou et al 2014)
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