Abstract
<em>Braya humilis</em> (Brassicaceae) is a widely distributed plant in arid and semi-arid regions of northern Asia. This plant is well adapted to extremely arid conditions and is a promising candidate species to discover novel drought tolerance strategies. However, not much information about the mechanism(s) mediating drought resistance in this species is currently available. Therefore, the present study aimed to characterize the physiological traits and expression patterns of a polyethylene glycol (PEG)-responsive gene in <em>B. humilis</em> responding to different levels of osmotic stress induced by PEG-6000. Several important physiological parameters were examined, including the levels of relative water content, soluble protein, malondialdehyde, and antioxidant enzyme activity. A tolerance threshold between 20 and 30% PEG-6000 was identified for <em>B. humilis</em>. The water status and oxidative damage below this threshold were maintained at a relatively constant level during the 12 h of treatment. However, once the threshold was exceeded, the water status and oxidative damage were obviously affected after treatment for 4 h. The soluble protein results suggest that <em>B. humilis</em> maintains a vigorous resistance to osmotic stress and that it may play a greater role in osmotic regulation at late stages of stress. Moreover, superoxide dismutase and catalase may be important at preventing oxidative damage in plants at early stages of stress, while peroxidase may be more involved in some biological processes that resist osmotic stress at the late stage, especially in severely damaged plants. Furthermore, a PEG-responsive gene, <em>BhCIPK12</em>, was identified by differential display reverse transcription-polymerase chain reaction (PCR), cloned, and characterized by quantitative real-time PCR. We hypothesized that this gene may play an important role in mediating osmotic stress or drought resistance in plants. Altogether, these results provide valuable insights into the mechanism(s) mediating drought tolerance in <em>B. humilis</em>.
Highlights
Numerous studies have demonstrated that drought affects the normal growth and development of plants by altering their water relation or water balance, inhibiting enzymatic activities, and affecting gene expression [1,2,3]
For the convenience of the discussion that follows, the plants treated with 30% or 40% polyethylene glycol (PEG) for 4 h were characterized as mildly damaged, while the plants treated with the same concentrations of PEG for 12 h were characterized as severely damaged
Loss of water was evident for the plants treated with 30% or 40% PEG
Summary
Numerous studies have demonstrated that drought affects the normal growth and development of plants by altering their water relation or water balance, inhibiting enzymatic activities, and affecting gene expression [1,2,3]. Drought represents an oxidative stress to plants by enhancing the accumulation of reactive oxygen species (ROS) in cells, including superoxide radicals (O2.−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH−) [4] These ROS can oxidize or peroxidize lipids, proteins, enzymes, pigments, and DNA, thereby further damaging the structure and function of cells, and inducing cell death [5]. To alleviate oxidative stress caused by drought, an efficient antioxidant mechanism involving nonenzymatic and enzymatic systems has evolved [7] The former includes flavonoids, alkaloids, and phenols, whereas the enzymatic antioxidant system involves a series of enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) [4,5]. Biochemical, and genetic levels show that antioxidant systems are complex and vary depending on species-specific drought-coping mechanisms and strategies [11,12,13]
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