Genetic Diversity, Salinity Tolerance and Physiological Responses to NaCl of Six Rice (Oryza sativa L.) Cultivars

Transcriptome Revealed GhPP2C43-A Negatively Regulates Salinity Tolerance in an Introgression Line from a Semi-wild Upland Cotton

Salt damage is one of the major threats to sustainable cotton production owing to the limited arable land in China mainly occupied by the production of staple food crops. Salt-stress tolerant cotton varieties are lacking in production and, the mechanisms underpinning salt-stress tolerance in cotton remain enigmatic. Here, DM37, an intraspecific introgression line from G. hirsutum race yucatanense acc TX-1046 into the G. hirsutum acc TM-1 background, was found to be highly tolerant to salt stress. Its seed germination rate and germination potential were significantly higher than the recipient TM-1 under salt stress. Physiological analysis showed DM37 had higher proline content and Peroxidase activity, as well as lower Na+/K+ ratios at the seedling stage, consistent with higher seedling survival rate after durable salt stress. Furthermore, comparative transcriptome analysis revealed that responsive patterns to salt stress in DM37 were different from TM-1. Weighted Correlation Network Analysis (WGCNA) demonstrated that co-expression modules associated with salt stress in DM37 also differed from TM-1. Out of them, GhPP2C43-A, a phosphatase gene, exhibited negative regulation of salt-stress tolerance verified by VIGS and transgenic Arabidopsis. Gene expression showed GhPP2C43-A in TM-1 was induced by durable salt stress but not in DM37 probably attributing to the variation of cis-element in its promoter, thereby being conferred different salt-stress tolerance. Our result would provide new genes/germplasms from semi-wild cotton in salt-stress tolerant cotton breeding. This study would give us new insights into the mechanisms underpinning the salt-stress tolerance in cotton.

Salt tolerance in rice: seedling and reproductive stage QTL mapping come of age

Reproductive stage salinity tolerance is most critical for rice as it determines the yield under stress. Few studies have been undertaken for this trait as phenotyping was cumbersome, but new methodology outlined in this review seeks to redress this deficiency. Sixty-three meta-QTLs, the most important genomic regions to target for enhancing salinity tolerance, are reported. Although rice has been categorized as a salt-sensitive crop, it is not equally affected throughout its growth, being most sensitive at the seedling and reproductive stages. However, a very poor correlation exists between sensitivity at these two stages, which suggests that the effects of salt are determined by different mechanisms and sets of genes (QTLs) in seedlings and during flowering. Although tolerance at the reproductive stage is arguably the more important, as it translates directly into grain yield, more than 90% of publications on the effects of salinity on rice are limited to the seedling stage. Only a few studies have been conducted on tolerance at the reproductive stage, as phenotyping is cumbersome. In this review, we list the varieties of rice released for salinity tolerance traits, those being commercially cultivated in salt-affected soils and summarize phenotyping methodologies. Since further increases in tolerance are needed to maintain future productivity, we highlight work on phenotyping for salinity tolerance at the reproductive stage. We have constructed an exhaustive list of the 935 reported QTLs for salinity tolerance in rice at the seedling and reproductive stages. We illustrate the chromosome locations of 63 meta-QTLs (with 95% confidence interval) that indicate the most important genomic regions for salt tolerance in rice. Further study of these QTLs should enhance our understanding of salt tolerance in rice and, if targeted, will have the highest probability of success for marker-assisted selections.

Phenotypic and physiological responses to salt exposure in Sorghum reveal diversity among domesticated landraces

PREMISE

Soil salinity negatively impacts plant function, development, and yield. To overcome this impediment to agricultural productivity, variation in morphological and physiological response to salinity among genotypes of important crops should be explored. Sorghum bicolor is a staple crop that has adapted to a variety of environmental conditions and contains a significant amount of standing genetic diversity, making it an exemplary species to study variation in salinity tolerance.

METHODS

Twenty-one diverse Sorghum accessions were treated with nonsaline water or 75 mM sodium chloride. Salinity tolerance was assessed via changes in biomass between control and salt-treated individuals. Accessions were first rank-ordered for salinity tolerance, and then individuals spanning a wide range of responses were analyzed for foliar proline and ion accumulation. Tolerance rankings were then overlaid on a neighbor-joining tree.

RESULTS

We found that, while proline is often a good indicator of osmotic adjustment and is historically associated with increased salt tolerance in many species, proline accumulation in sorghum reflects a stress response injury rather than acclimation. When combining ion profiles with stress tolerance indices, the variation observed in tolerance was not a sole result of Na⁺ accumulation, but rather reflected accession-specific mechanisms.

CONCLUSIONS

We identified significant variation in salinity tolerance among Sorghum accessions that may be a result of the domestication history of Sorghum. When we compared our results with known phylogenetic relationships within sorghum, the most parsimonious explanation for our findings is that salinity tolerance was acquired early during domestication and subsequently lost in accessions growing in areas varying in soil salinity.

Stress-driven increase in proline levels, and not proline levels themselves, correlates with the ability to withstand excess salt in a group of 17 Italian rice genotypes

In most plant species, a rapid increase in free proline content occurs following exposure to hyperosmotic stress conditions. However, inconsistent results were reported concerning the role of such an increase on the plant response to water shortage or excess salt. Therefore, the possibility that proline accumulation may help the cell to withstand stress conditions, or that it simply represents a stress marker, is still a matter of debate. A possible relationship between proline accumulation and salt tolerance was investigated in a set of 17 Italian rice varieties. Rice seedlings were exposed to increasing salt concentrations during germination and early growth. The resulting levels of free proline were measured separately in shoots and roots and compared to those in untreated controls. Results were related to the corresponding ability of a given genotype to tolerate stress conditions. Neither absolute proline levels in untreated or in salt-stressed seedlings showed a straightforward relationship to the relative tolerance to salt, estimated as conductivity values able to reduce growth by 10 or 50%. Conversely, a highly significant correlation was found between the increase in proline levels in shoots and the ability to withstand stress. The results strengthen a recent hypothesis suggesting than an increase in proline metabolic rates, more than the resulting proline content, may help the cell to counteract the effects of abiotic stress conditions.

Exogenous proline and trehalose promote recovery of rice seedlings from salt-stress and differentially modulate antioxidant enzymes and expression of related genes

Proline (Pro) and Trehalose (Tre) function as compatible solutes and are upregulated in plants under abiotic stress. They play an osmoprotective role in physiological responses, enabling the plants to better tolerate the adverse effects of abiotic stress. We investigated the effect of exogenous Pro and Tre (10 mM) in seedlings of Thai aromatic rice (cv. KDML105; salt-sensitive) during salt stress and subsequent recovery. Salt stress (S, NaCl) resulted in growth reduction, increase in the Na(+)/K(+) ratio, increase in Pro level and up-regulation of Pro synthesis genes (pyrroline-5-carboxylatesynthetase, P5CS; pyrroline-5-carboxylate reductase, P5CR) as well as accumulation of hydrogen peroxide (H(2)O(2)), increased activity of antioxidative enzymes (superoxide dismutase, SOD; peroxidase, POX; ascorbate peroxidase, APX; catalase, CAT) and transcript up-regulation of genes encoding antioxidant enzymes (Cu/ZnSOD, MnSOD, CytAPX, CatC). Under salt stress, exogenous Pro (PS; Pro+NaCl) reduced the Na(+)/K(+) ratio, further increased endogenous Pro and transcript levels of P5CS and P5CR, but decreased the activity of the four antioxidant enzymes. The transcription of genes encoding several antioxidant enzymes was upregulated. Exogenous Tre (TS; Tre+NaCl) also reduced the Na(+)/K(+) ratio and strongly decreased endogenous Pro. Transcription of P5CS and P5CR was upregulated, the activities of SOD and POX decreased, the activity of APX increased and the transcription of all antioxidant enzyme genes upregulated. Although exogenous osmoprotectants did not alleviate growth inhibition during salt stress, they exhibited a pronounced beneficial effect during recovery period showing higher percentage of growth recovery in PS (162.38%) and TS (98.43%) compared with S (3.68%). During recovery, plants treated with PS showed a much greater reduction in endogenous Pro than NaCl-treated (S) or Tre-treated plants (TS). Increase in CAT activity was most related to significant reduction in H(2)O(2), particularly in the case of PS-treated plants. Advantageous effects of Pro were also associated with increase in APX activity during recovery.