Ionomic, metabolomic and proteomic analyses reveal molecular mechanisms of root adaption to salt stress in Tibetan wild barley

Highlights

• XZ26 had better root growth than XZ169 under salt stress in both soil and hydroponic culture.

• A total of 574 salt-regulated proteins and 153 salt-regulated metabolites were identified in the roots of XZ26 and XZ169.

• XZ26 consumes less energy against salt stress for producing biomass.

Abstract

In our previous study, Tibetan wild barley (Hordeum spontaneum L.) has been found to be rich in the elite accessions with strong abiotic stress tolerance, including salt stress tolerance. However, the molecular mechanism of salt tolerance underlying the wild barley remains to be elucidated. In this study, two Tibetan wild barley accessions, XZ26 (salt-tolerant) and XZ169 (salt-sensitive), were used to investigate ionomic, metabolomic and proteomic responses in roots when exposed to 0, 200 (moderate) and 400 mM (high) salinity. XZ26 showed stronger root growth and maintained higher K concentrations when compared with XZ169 under moderate salinity, while no significant difference was found between the two accessions under high salinity. A total of 574 salt-regulated proteins and 153 salt-regulated metabolites were identified in the roots of both accessions based on quantitative proteomic (iTRAQ methods) and metabolomic (GC-TOF/MS) analysis. XZ26 developed its root adaptive strategies mainly by accumulating more compatible solutes such as proline and inositol, acquiring greater antioxidant ability to cope with ROS, and consuming less energy under salt stress for producing biomass. These findings provide a better understanding of molecular responses of root adaptive strategies to salt stress in the wild barley.

Introduction

At present, over 6% of world's total land area is adversely affected by salinity, including approximately 20% of cultivated land and nearly half of irrigated land (Chinnusamy et al., 2005, FAO, 2015). Moreover, soil salinization is still expanding and becoming more serious in the world, mainly due to excessive irrigation and industrial pollution, posing a great threat to agricultural sustainability. Although a few plants, called halophytes, can adapt to high salinity, most cereal crops, including rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays) and barley (Hordeum vulgare) are glycophytes, which show reduced growth and yield when exposed to salt stress (Glenn et al., 1999, Horie et al., 2012). Therefore, development of crop cultivars with high salt tolerance is extremely important for increasing crop productivity in saline soils.

Among cereals, barley is considered as the most salt tolerant crop, which can grow normally under 200 mM and even higher salt concentration (Munns and Tester, 2008). Consequently, barley is frequently used as a model crop for physiological and molecular studies on salt tolerance. However, narrower genetic diversity in the cultivated barley has become a limitation for identifying alleles which can be used in breeding program for salt tolerant improvement. On the other hand, wild barley, has a much wider genetic diversity, enriched in elite alleles that allow these accessions to perform better especially under abiotic stress (Nevo et al., 1979, Pakniyat et al., 1997, Dai et al., 2012, Dai et al., 2014). In our previous studies, we confirmed the wider genetic variation of Tibetan wild barley in salt tolerance and identified some accessions, including XZ26, XZ16 and XZ113 which showed even higher salt tolerance than CM72, a well-known salt tolerant cultivar (Qiu et al., 2011, Wu et al., 2011). Physiological and molecular changes in different tissues in responses to stress have been compared between wild and cultivated barley (Wu et al., 2013a, Shen et al., 2016, Shen et al., 2017), but the mechanisms of tissue tolerance to salt stress underlying these wild barley are still little known.

Plant roots are directly exposed to various environmental conditions, and will make the response to abiotic stress firstly. Thus, nutrition and water uptakes by roots are apparently inhibited when plants are subjected to salt stress, resulting in a significant reduction in cell elongation, especially at the root tips (Munns and Tester, 2008). After a long-term salt stress, ion toxicity and metabolic disorders occur in roots (Munns and Tester, 2008, Deinlein et al., 2014). On the other hand, adaptive or tolerant responses occur under salt stress, which maintain nutritional status and turgor pressure (Tester and Davenport, 2003, Shelden et al., 2016). Therefore, many strategies are developed in roots for adaptation to salt stress. For instance, Na sequestration in vacuoles, Na excretion, and elevated K uptake (i.e. Na/K homeostasis) are frequently found in the tolerant genotypes, which alleviate ion toxicity in roots (Tester and Davenport, 2003, Shabala and Cuin, 2008). However, little research has been done to understand comprehensively the whole root adaption to salt stress.

At present, ‘omic’ methodologies are widely used to investigate the abiotic stress tolerance in plants (Nawrot et al., 2016, Meena et al., 2017). Multi-omic methods may give insights into different levels for a better understanding of complex molecular networks underlying the mechanism of abiotic tolerance in plants (Nawrot et al., 2016, Wang et al., 2016). For instance, mass spectrometer (MS)-based proteomic and metabolomic profiling analysis could reflect protein and metabolite changes in different tissues of plants in response to environmental stimulus. Zhang et al. (2012) summarized 2171 salt-regulated proteins among 34 plant species, providing a framework of molecular networks for salt responsible proteins. In barley, proteomic comparison was performed between salt tolerant and sensitive cultivars (Witzel et al., 2014, Rasoulnia et al., 2011), and successfully identified some proteins involved in salt tolerance. Previously we compared proteomic and metabolomic differences in shoots of wild and cultivated barleys in response to salt stress (Wu et al., 2013a, Wu et al., 2013b, Shen et al., 2016). However, no research has been done to clarify the genotypic difference of both the proteomic and metabolomic profiles in the roots responding to salt stress.

In this study, two Tibetan wild barley accessions contrasting in salt tolerance (XZ26, tolerant; XZ169, sensitive) (Shen et al., 2016), were used to compare root ionome, metabolome and proteome profiles in response to 200 mM and 400 mM NaCl at seedling stage. The objective of the present study is to understand the adaptive and integrated strategies in roots of wild barleys to salt stress.