Abstract
Alkaline stress (high pH) severely damages root cells, and consequently, inhibits rice (Oryza sativa L.) seedling growth. In this study, we demonstrate the accumulation of reactive oxygen species (ROS) in root cells under alkaline stress. Seedlings of two rice cultivars with different alkaline tolerances, ‘Dongdao-4’ (moderately alkaline-tolerant) and ‘Jiudao-51’ (alkaline-sensitive), were subjected to alkaline stress simulated by 15 mM sodium carbonate (Na2CO3). Alkaline stress greatly reduced seedling survival rate, shoot and root growth, and root vigor. Moreover, severe root cell damage was observed under alkaline stress, as shown by increased membrane injury, malondialdehyde accumulation, and Evan’s Blue staining. The expression of the cell death-related genes OsKOD1, OsHsr203j, OsCP1, and OsNAC4 was consistently upregulated, while that of a cell death-suppressor gene, OsBI1, was downregulated. Analysis of the ROS contents revealed that alkaline stress induced a marked accumulation of superoxide anions () and hydrogen peroxide (H2O2) in rice roots. The application of procyanidins (a potent antioxidant) to rice seedlings 24 h prior to alkaline treatment significantly alleviated alkalinity-induced root damage and promoted seedling growth inhibition, which were concomitant with reduced ROS accumulation. These results suggest that root cell damage, and consequently growth inhibition, of rice seedlings under alkaline stress is closely associated with ROS accumulation. The antioxidant activity of superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase increased under alkaline stress in the roots, probably in response to the cellular damage induced by oxidative stress. However, this response mechanism may be overwhelmed by the excess ROS accumulation observed under stress, resulting in oxidative damage to root cells. Our findings provide physiological insights into the molecular mechanisms of alkalinity-induced damage to root cells, and will contribute to the improvement of alkaline stress tolerance in rice plants.
Highlights
Saline–alkaline (SA) stress due to soil salinization and/or alkalization is a major constraint to crop production worldwide
We previously reported that alkaline treatment markedly reduced rice seedling survival and total biomass, with the most serious adverse impact observed on root growth (Lv et al, 2013; Wei et al, 2015)
We showed that alkaline treatment increased Membrane Injury (MI) (Figure 4A), MDA accumulation (Figure 4B), and Evan’s Blue staining (Figure 5B), and reduced root vigor (Figure 5A)
Summary
Saline–alkaline (SA) stress due to soil salinization and/or alkalization is a major constraint to crop production worldwide. SA stress is characterized by an inordinately high alkalinity (high pH) in addition to high salinity; the pH of SA soil can range from 8.5 to 11 (Abrol et al, 1988; Qadir et al, 2006, 2007; Hossner, 2008; Wang et al, 2009; Amini et al, 2016). Much less attention has been directed toward alkaline stress, which is becoming an increasingly important stress factor that is considerably more damaging to plants than any single neutral saline stress (Qadir et al, 2001; Hartung et al, 2002; Yang et al, 2008; Islam et al, 2011; Chen et al, 2012; Paz et al, 2012; Radi et al, 2012; Shasha, 2012; Lv et al, 2013). It is essential to first understand how plants respond and adapt to alkaline stress, in order to efficiently improve the alkaline tolerance of crop plants
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