Differential responses of CO2 assimilation, carbohydrate allocation and gene expression to NaCl stress in perennial ryegrass with different salt tolerance.

Tao Hu,Longxing Hu,Xunzhong Zhang,Pingping Zhang,Zhuangjun Zhao,Jinmin Fu,Andrew Webber

doi:10.1371/journal.pone.0066090

Abstract

Little is known about the effects of NaCl stress on perennial ryegrass (Lolium perenne L.) photosynthesis and carbohydrate flux. The objective of this study was to understand the carbohydrate metabolism and identify the gene expression affected by salinity stress. Seventy-four days old seedlings of two perennial ryegrass accessions (salt-sensitive ‘PI 538976’ and salt-tolerant ‘Overdrive’) were subjected to three levels of salinity stress for 5 days. Turf quality in all tissues (leaves, stems and roots) of both grass accessions negatively and significantly correlated with GFS (Glu+Fru+Suc) content, except for ‘Overdrive’ stems. Relative growth rate (RGR) in leaves negatively and significantly correlated with GFS content in ‘Overdrive’ (P<0.01) and ‘PI 538976’ (P<0.05) under salt stress. ‘Overdrive’ had higher CO2 assimilation and Fv/Fm than ‘PI 538976’. Intercellular CO2 concentration, however, was higher in ‘PI 538976’ treated with 400 mM NaCl relative to that with 200 mM NaCl. GFS content negatively and significantly correlated with RGR in ‘Overdrive’ and ‘PI 538976’ leaves and in ‘PI 538976’ stems and roots under salt stress. In leaves, carbohydrate allocation negatively and significantly correlated with RGR (r2 = 0.83, P<0.01) and turf quality (r2 = 0.88, P<0.01) in salt-tolerant ‘Overdrive’, however, the opposite trend for salt-sensitive ‘PI 538976’ (r2 = 0.71, P<0.05 for RGR; r2 = 0.62, P>0.05 for turf quality). A greater up-regulation in the expression of SPS, SS, SI, 6-SFT gene was observed in ‘Overdrive’ than ‘PI 538976’. A higher level of SPS and SS expression in leaves was found in ‘PI 538976’ relative to ‘Overdrive’. Accumulation of hexoses in roots, stems and leaves can induce a feedback repression to photosynthesis in salt-stressed perennial ryegrass and the salt tolerance may be changed with the carbohydrate allocation in leaves and stems.

Highlights

Salinity is a severe and increasing threat limiting plant growth and crop yields worldwide [1,2]
The objective of this study were: (1) to investigate the difference in mechanisms by which salt stress lead to a decrease in CO2 assimilation and (2) the differential responses of carbohydrate allocation and gene expression in perennial ryegrass accessions contrasting in salt tolerance
Salt stress resulted in a lower level of normalized relative transpiration (NRT) regardless of NaCl level in two perennial ryegrass accessions during whole experimental period (Fig. 2)

Summary

Introduction

Salinity is a severe and increasing threat limiting plant growth and crop yields worldwide [1,2]. Previous studies have shown that salt stress affects CO2 assimilation in many plants including cotton (Gossypium hirsutum L.) [5,6], bean (Phaseolus vulgaris L.) [7,8], bell pepper (Capsicum annuum L.) [9], celery (Apium graveolens L.) [10], spinach (Spinacia oleracea L.) [11], rice (Oryza sativa L.) [12], sea barleygrass (Hordeum marinum Huds.), cultivated barley (Hordeum vulgare L.) [13] and tomato (Solanum lycopersicum) [14]. Similar results have been obtained for cotton and bean (Phaseolus vulgaris L.) [5], spinach [11], rice [12], and sea barleygrass and cultivated barley [13]. The mechanisms by which salt stress leads to a decrease in CO2 assimilation are still not yet clearly understood

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Conclusion