Physiological And Molecular Mechanisms In Response To Saline-alkaline Stress In Rice (Oryza Sativa L.)

Saline-alkaline (SA) stress is a complex effect of three main factors:high salinity, alkalinity and high osmotic pressure. This study dissected thegrowth, physiological and molecular responses of rice (Oryza sativa L.) toabove-mentioned SA stress factors simulated by using NaCl, Na2CO3andPEG6000, respectively.Seed germination was most strongly affected by osmotic stress,followed by alkaline and salt stresses, indicating that water availability isthe determining factor for rice seed germination under SA stress.Meanwhile, the seedling growth was most reduced under alkaline stress.The alkaline treatment significantly reduced total biomass, total root length(TRL), root surface area (RSA), roots numbers (RN), and root volume(RV), while it increased root diameter (RD) and Na+/K+ratio. Further, itcaused severe damages to the root system as shown by a remarkableincrease in degree of cell injury and expression of the cell death-relatedgene OsNAC4. On the other hand, salt stress reduced the total biomass,RSA and RV, and increased the Na+/K+ratio, while to less extent than thoseunder alkaline stress. These results suggest that the deleterious effect ofalkaline stress on rice seedlings is due mainly to high alkalinity that inducescell injury in the root system. In contrast, the response to osmotic stressexhibited the typical adaptive responses to dehydration: reduced relativewater content (RWC), RD, RV, and shoot biomass, whereas increased TRL,RSA, RN, and root biomass.Proline accumulation is a common physiological response to various abiotic stresses in plants. This study found that rice seedlings accumulateproline to different levels in response to different SA stress factors.Expression analysis of genes for proline biosynthesis (OsP5CS1, OsP5CS2and OsP5CR) and catabolism (OsPDH1and OsP5CDH) revealed up-regulation of the proline synthesis genes under osmotic and alkalinestresses. In comparison, under the salt stress, a decrease in catabolicenzyme gene expressions was observed. These results suggest that riceseedlings employ different gene regulation in proline accumulation inresponse to different SA stress factors.The physiological importance of proline accumulation in stresstolerance remains controversial. Many studies have suggested that prolineplays cellular protective roles in stress responses; further, prolineaccumulation has frequently been recognized as a physiological parameterof stress adaptation responses. On the other hand, some studies haveindicated that proline accumulation is a consequence of stress responses.This study examined whether proline accumulation in rice (Oryza sativaL.) seedlings is correlated with stress tolerance by using25japonicavarieties with differing levels of SA stress tolerances. The results indicatedthat the proline levels in all the japonica varieties increased significantlyin response to the investigated SA stress factors, namely, salt, alkaline, andosmotic stresses. However, no significant correlations between the basal orelevated proline levels and tolerance to any of the investigated stressfactors were determined. These findings suggest that proline accumulationis a symptomatic result of cellular injury under various stresses and thatproline accumulation levels do not serve as a reliable parameter forassessing SA stress tolerance in rice. On the other hand, several root growthindices of rice seedlings showed good correlations with the investigated stress tolerances, and therefore, represent a useful set of growth parametersfor assessing SA stress tolerance in rice.