The environmental stress is a major area of scientific concern because it constraints plant as well as crop productivity. This situation has been further worsened by anthropogenic activities. Therefore, there is a much scientific saddle on researchers to enhance crop productivity under environmental stress in order to cope with the increasing food demands. The abiotic stresses such as salinity, drought, cold, and heat negatively influence the survival, biomass production and yield of staple food crops. According to an estimate of FAO, over 6 % of the world’s land is affected by salinity. Thus, salinity stress appears to be a major constraint to plant and crop productivity. Here, we review our understanding of salinity impact on various aspects of plant metabolism and its tolerance strategies in plants.

Effect of salinity stress on plants and its tolerance strategies: a review.

Plant proteome changes under abiotic stress--contribution of proteomics studies to understanding plant stress response

Halophytes are defined as plants that are adapted to live in soils containing high concentrations of salt and benefiting from it, and thus represent an ideal model to understand complex physiological and genetic mechanisms of salinity stress tolerance. It is also known that oxidative stress signalling and reactive oxygen species (ROS) detoxification are both essential components of salinity stress tolerance mechanisms. This paper comprehensively reviews the differences in ROS homeostasis between halophytes and glycophytes in an attempt to answer the questions of whether stress-induced ROS production is similar between halophytes and glycophytes; is the superior salinity tolerance in halophytes attributed to higher antioxidant activity; and is there something special about the specific ‘pool’ of enzymatic and non-enzymatic antioxidants in halophytes. We argue that truly salt-tolerant species possessing efficient mechanisms for Na + exclusion from the cytosol may not require a high level of antioxidant activity, as they simply do not allow excessive ROS production in the first instance. We also suggest that H 2O2 ‘signatures’ may operate in plant signalling networks, in addition to well-known cytosolic calcium ‘signatures’. According to the suggested concept, the intrinsically higher superoxide dismutase (SOD) levels in halophytes are required for rapid induction of the H2O2 ‘signature’, and to trigger a cascade of adaptive responses (both genetic and physiological), while the role of other enzymatic antioxidants may be in decreasing the basal levels of H2O2, once the signalling has been processed. Finally, we emphasize the importance of non-enzymatic antioxidants as the only effective means to prevent detrimental effects of hydroxyl radicals on cellular structures.

ROS homeostasis in halophytes in the context of salinity stress tolerance

Soil salinity is a major abiotic stress that limits plant growth and agriculture productivity. To cope with salt stress, plants have evolved complex salt-responsive signaling and metabolic processes at the cellular, organ, and whole-plant levels. Investigation of the physiological and molecular mechanisms underlying plant salinity tolerance will provide valuable information for effective engineering strategies. Current proteomics provides a high-throughput approach to study sophisticated molecular networks in plants. In this review, we describe a salt-responsive protein database by an integrated analysis of proteomics-based studies. The database contains 2171 salt-responsive protein identities representing 561 unique proteins. These proteins have been identified from leaves, roots, shoots, seedlings, unicells, grains, hypocotyls, radicles, and panicles from 34 plant species. The identified proteins provide invaluable information toward understanding the complex and fine-tuned plant salt-tolerance mechanisms in photosynthesis, reactive oxygen species (ROS) scavenging, ion homeostasis, osmotic modulation, signaling transduction, transcription, protein synthesis/turnover, cytoskeleton dynamics, and cross-tolerance to different stress conditions.

Mechanisms of plant salt response: insights from proteomics.

A Proteomic Study of the Response to Salinity and Drought Stress in an Introgression Strain of Bread Wheat

Soil salinity is a major abiotic constraint to agricultural productivity. We successfully bred a new common wheat (Triticum aestivum L.) introgression variety (Shanrong No. 3) with high salt-tolerance via asymmetric somatic hybridization between common wheat cultivar (Jinan 177) and UV-irradiated Agropyron elongatum (Thinopyrum ponticum Podp). We report here a comparative proteomic analysis to investigate variety-specific and salt-responsive proteins between seedling-roots of Shanrong No. 3 and Jinan 177. In total, 114 spots reproducibly presented differential expression patterns on 2-DE maps. Of them, 34 were variety-specific and 49 were salt-responsive. We identified 110 spots by MALDI-TOF MS and partially confirmed by MALDI-TOF-TOF MS, and functionally classified them into signal transduction, transcription and translation, transporting, chaperones, proteolysis and detoxification, etc. Meanwhile, we also found the alteration of protein expression of Shanrong No. 3 through inhibition of old proteins and production of novel ones, change in abundance and sensitivity of some nonsalt-responsive and salt-responsive proteins, as well as PTMs. Furthermore, comparison between proteome and transcripteome using cDNA microarray showed that there were only 20 proteins with abundances correlative to signal densities of corresponding EST probes. This study gives us a global insight into proteomic difference between Shanrong No. 3 and Jinan 177 in constitute and to salt-response.

Proteomic analysis on a high salt tolerance introgression strain of Triticum aestivum/Thinopyrum ponticum

The bread wheat cultivar Shanrong No.3 (SR3) is a salinity tolerant derivative of an asymmetric somatic hybrid between cultivar Jinan 177 (JN177) and tall wheatgrass (Thinopyrum ponticum). To reveal some of the mechanisms underlying its elevated abiotic stress tolerance, both SR3 and JN177 were exposed to iso-osmotic NaCl and PEG stress, and the resulting gene expression was analysed using a customized microarray. Some genes associated with stress response proved to be more highly expressed in SR3 than in JN177 in non-stressed conditions. Its unsaturated fatty acid and flavonoid synthesis ability was also enhanced, and its pentose phosphate metabolism was more active than in JN177. These alterations in part accounted for the observed shift in the homeostasis related to reactive oxygen species (ROS). The specific down-regulation of certain ion transporters after a 0.5 h exposure to 340 mM NaCl demonstrated that Na(+) uptake occurred rapidly, so that the early phase of salinity stress imposes more than simply an osmotic stress. We discussed the possible effect of the introgression of new genetic materials in wheat genome on stress tolerance.

A transcriptomic analysis reveals the nature of salinity tolerance of a wheat introgression line

Allopolyploid genome needs wide structural variation to deal with genomic shock. The introgression line, generated via asymmetric somatic hybridization, is introgressed with a minimum of exogenous chromatin, which also leads to genomic shock to induce genetic variation. However, the extent of its genomic variation and its difference from allopolyploidies remains unknown. Here, we explored this issue using the bread wheat cultivar SR3, a derivative of an asymmetric somatic hybrid between the cultivar JN177 and an accession of tall wheatgrass (Thinopyrum elongatum). The ESTs (expressed sequence taqs) were large-scale sequenced using the cDNA library constructed in each of SR3 and JN177. Point mutations and indels (insertions and deletions) of SR3 were calculated, and their difference from the genetic variation of bread wheat and its ancestors were compared, with aim to analyze the extent and pattern of sequence variation induced by somatic hybridization. Both point mutations and indels (insertions and deletions) were frequently induced by somatic hybridization in the coding sequences. While the genomic shock caused by allopolyploidization tends to favor deletion over insertion, there was no evidence for such a preference following asymmetric somatic hybridization. The GC content of sequence adjacent to indel sites was also distinct from what has been observed in allopolyploids. This study demonstrates that asymmetric somatic hybridization induces high frequency of genetic variation in a manner partially different from allopolipoidization. Asymmetric somatic hybridization provides appropriate material to comprehensively explore the nature of the genetic variation induced by genomic shock.

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Asymmetric somatic hybridization induces point mutations and indels in wheat

Asymmetric somatic hybridization is an efficient crop breeding approach by introducing several exogenous chromatin fragments, which leads to genomic shock and therefore induces genome-wide genetic variation. However, the fundamental question concerning the genetic variation such as whether it occurs randomly and suffers from selection pressure remains unknown. Here, we explored this issue by comparing expressed sequence tags of a common wheat cultivar and its asymmetric somatic hybrid line. Both nucleotide substitutions and indels (insertions and deletions) had lower frequencies in coding sequences than in un-translated regions. The frequencies of nucleotide substitutions and indels were both comparable between chromosomes with and without introgressed fragments. Nucleotide substitutions distributed unevenly and were preferential to indel-flanking sequences, and the frequency of nucleotide substitutions at 5′-flanking sequences of indels was obviously higher in chromosomes with introgressed fragments than in those without exogenous fragment. Nucleotide substitutions and indels both had various frequencies among seven groups of allelic chromosomes, and the frequencies of nucleotide substitutions were strongly negatively correlative to those of indels. Among three sets of genomes, the frequencies of nucleotide substitutions and indels were both heterogeneous, and the frequencies of nucleotide substitutions exhibited drastically positive correlation to those of indels. Our work demonstrates that the genetic variation induced by asymmetric somatic hybridization is attributed to both whole genomic shock and local chromosomal shock, which is a predetermined and non-random genetic event being closely associated with selection pressure. Asymmetric somatic hybrids provide a worthwhile model to further investigate the nature of genomic shock induced genetic variation.

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The non-random patterns of genetic variation induced by asymmetric somatic hybridization in wheat.

Asymmetric somatic hybridization is an efficient strategy for crop breeding by introducing exogenous chromatin fragments, which leads to whole genomic shock and local chromosomal shock that induces genome-wide genetic variation including indel (insertion and deletion) and nucleotide substitution. Nucleotide substitution causes synonymous codon usage bias (SCUB), an indicator of genomic mutation and natural selection. However, how asymmetric somatic hybridization affects SCUB has not been addressed. Here, we explored this issue by comparing expressed sequence tags of a common wheat cultivar and its asymmetric somatic hybrid line. Asymmetric somatic hybridization affected SCUB and promoted the bias to A- and T-ending synonymous codon (SCs). SCUB frequencies in chromosomes introgressed with exogenous fragments were comparable to those in chromosomes without exogenous fragments, showing that exogenous fragments had no local chromosomal effect. Asymmetric somatic hybridization affected SCUB frequencies in indel-flanking sequences more strongly than in non-flanking sequences, and this stronger effect was present in both chromosomes with and without exogenous fragments. DNA methylation-driven SCUB shift was more pronounced than other SC pairs. SCUB shift was similar among seven groups of allelic chromosomes as well as three sub-genomes. Our work demonstrates that the SCUB shift induced by asymmetric somatic hybridization is attributed to the whole genomic shock, and DNA methylation is a putative force of SCUB shift during asymmetric somatic hybridization. Asymmetric somatic hybridization provides an available method for deepening the nature of SCUB shift and genetic variation induced by genomic shock.

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Chun Liu

Papers

Proteomic analysis on a high salt tolerance introgression strain of Triticum aestivum/Thinopyrum ponticum

A transcriptomic analysis reveals the nature of salinity tolerance of a wheat introgression line

Asymmetric somatic hybridization induces point mutations and indels in wheat

The non-random patterns of genetic variation induced by asymmetric somatic hybridization in wheat.

Asymmetric Somatic Hybridization Affects Synonymous Codon Usage Bias in Wheat