Changes in free polyamine levels, expression of polyamine biosynthesis genes, and performance of rice cultivars under salt stress: a comparison with responses to drought.

Phuc T Do,Oliver Drechsel,Dirk K Hincha,Ellen Zuther,Arnd G Heyer

doi:10.3389/fpls.2014.00182

Abstract

Soil salinity affects a large proportion of rural area and limits agricultural productivity. To investigate differential adaptation to soil salinity, we studied salt tolerance of 18 varieties of Oryza sativa using a hydroponic culture system. Based on visual inspection and photosynthetic parameters, cultivars were classified according to their tolerance level. Additionally, biomass parameters were correlated with salt tolerance. Polyamines have frequently been demonstrated to be involved in plant stress responses and therefore soluble leaf polyamines were measured. Under salinity, putrescine (Put) content was unchanged or increased in tolerant, while dropped in sensitive cultivars. Spermidine (Spd) content was unchanged at lower NaCl concentrations in all, while reduced at 100 mM NaCl in sensitive cultivars. Spermine (Spm) content was increased in all cultivars. A comparison with data from 21 cultivars under long-term, moderate drought stress revealed an increase of Spm under both stress conditions. While Spm became the most prominent polyamine under drought, levels of all three polyamines were relatively similar under salt stress. Put levels were reduced under both, drought and salt stress, while changes in Spd were different under drought (decrease) or salt (unchanged) conditions. Regulation of polyamine metabolism at the transcript level during exposure to salinity was studied for genes encoding enzymes involved in the biosynthesis of polyamines and compared to expression under drought stress. Based on expression profiles, investigated genes were divided into generally stress-induced genes (ADC2, SPD/SPM2, SPD/SPM3), one generally stress-repressed gene (ADC1), constitutively expressed genes (CPA1, CPA2, CPA4, SAMDC1, SPD/SPM1), specifically drought-induced genes (SAMDC2, AIH), one specifically drought-repressed gene (CPA3) and one specifically salt-stress repressed gene (SAMDC4), revealing both overlapping and specific stress responses under these conditions.

Highlights

Cultivation of rice (Oryza sativa L.) is limited by environmental stresses, of which salinity and drought represent some of the most devastating ones
For the present investigation of the polyamine response in 18 cultivars, a detailed physiological characterization at two different salt concentrations in an early vegetative stage was performed and revealed a large variation of salt tolerance among the cultivars based on a rank of scoring data that indicated a higher salt sensitivity of japonica as compared to indica cultivars
It was previously reported that salt tolerance of indica cultivars was higher than that of japonica cultivars, denoted by a lower reduction of growth and a better Na+ exclusion (Lee et al, 2003)

Summary

Introduction

Cultivation of rice (Oryza sativa L.) is limited by environmental stresses, of which salinity and drought represent some of the most devastating ones. Soil degradation and salinization affect approximately 3.6 billion of the world’s 5.2 billion ha of dryland used for agriculture (Riadh et al, 2010). Rice (Oryza sativa L.) is considered to be moderately sensitive to salinity (Akita and Cabuslay, 1990) with a clear distinction between initial effects of salinity and long-term effects that result from the accumulation of salt in expanded leaves (Yeo et al, 1991). Salt stress causes reduction in leaf expansion, relative growth rate (Akita and Cabuslay, 1990) and photosynthesis (Nakamura et al, 2002; Cha-Um et al, 2006), as well as enhanced senescence (Lutts et al, 1996). Three major processes have been considered to participate in protection against high cytosolic Na+: (1) the minimization of Na+ entry into cells; (2) the compartmentation of Na+ into the vacuole; and (3) the increased efflux of Na+ out of the cell driven by specific ion transporters (Chinnusamy et al, 2005)

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Conclusion