Abstract
The arbuscular mycorrhizal (AM) symbiosis has been shown to improve maize tolerance to different drought stress scenarios by regulating a wide range of host plants aquaporins. The objective of this study was to highlight the differences in aquaporin regulation by comparing the effects of the AM symbiosis on root aquaporin gene expression and plant physiology in two maize cultivars with contrasting drought sensitivity. This information would help to identify key aquaporin genes involved in the enhanced drought tolerance by the AM symbiosis. Results showed that when plants were subjected to drought stress the AM symbiosis induced a higher improvement of physiological parameters in drought-sensitive plants than in drought-tolerant plants. These include efficiency of photosystem II, membrane stability, accumulation of soluble sugars and plant biomass production. Thus, drought-sensitive plants obtained higher physiological benefit from the AM symbiosis. In addition, the genes ZmPIP1;1, ZmPIP1;3, ZmPIP1;4, ZmPIP1;6, ZmPIP2;2, ZmPIP2;4, ZmTIP1;1, and ZmTIP2;3 were down-regulated by the AM symbiosis in the drought-sensitive cultivar and only ZmTIP4;1 was up-regulated. In contrast, in the drought-tolerant cultivar only three of the studied aquaporin genes (ZmPIP1;6, ZmPIP2;2, and ZmTIP4;1) were regulated by the AM symbiosis, resulting induced. Results in the drought-sensitive cultivar are in line with the hypothesis that down-regulation of aquaporins under water deprivation could be a way to minimize water loss, and the AM symbiosis could be helping the plant in this regulation. Indeed, during drought stress episodes, water conservation is critical for plant survival and productivity, and is achieved by an efficient uptake and stringently regulated water loss, in which aquaporins participate. Moreover, the broader and contrasting regulation of these aquaporins by the AM symbiosis in the drought-sensitive than the drought-tolerant cultivar suggests a role of these aquaporins in water homeostasis or in the transport of other solutes of physiological importance in both cultivars under drought stress conditions, which may be important for the AM-induced tolerance to drought stress.
Highlights
Crop adaptation to new environments is of crucial importance, especially in a climate change scenario
The experiment consisted of a factorial design with two factors: (1) inoculation treatment, with non-inoculated control plants (C) and plants inoculated with the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis, strain EEZ 58 (Ri); (2) water regime, so that one half of the plants were cultivated under well-watered conditions (WW) throughout the entire experiment and the other half of the plants were subjected to drought stress for 12 days before harvest (DS)
This study highlights the divergent responses to AM symbiosis of two maize genotypes differing in drought tolerance: PR34G13, a drought-tolerant cultivar, and PR34B39, a drought-sensitive cultivar (DuPont Pioneer Corporation)
Summary
Crop adaptation to new environments is of crucial importance, especially in a climate change scenario. In order to secure food production in the future, efforts need to be directed to understand the mechanisms of plant adaptation and tolerance to abiotic stresses like water shortage, as these events are expected to intensify in coming years (Elliott et al, 2014). Maize requires more water at the later vegetative and reproductive stages that at seedlings stage, but at the early crop establishment phase, water stress influences seedlings adaptation and their grain yield potential, because of premature flowering and a longer anthesis-silk interval (Cao and Wj, 2004; Min et al, 2016) Despite the amount of information about crop responses to water deficit, our knowledge about the mechanisms originating drought tolerance in maize seedlings is still restricted (Min et al, 2016). Previous studies of drought tolerance in maize have shown that tolerant cultivars enhanced antioxidant activity, presented lower lipid peroxidation, improved accumulation of osmolytes and turgor adjustment, maintained photosynthetic activity and regulated aquaporin genes (Anjum et al, 2016; Min et al, 2016)
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