Characterization of Salinity Tolerance of Transgenic Rice Lines Harboring HsCBL8 of Wild Barley (Hordeum spontanum) Line from Qinghai-Tibet Plateau

Progress and prospects in harnessing wild relatives for genetic enhancement of salt tolerance in rice

Salt stress is the second most devastating abiotic stress after drought and limits rice production globally. Genetic enhancement of salinity tolerance is a promising and cost-effective approach to achieve yield gains in salt-affected areas. Breeding for salinity tolerance is challenging because of the genetic complexity of the response of rice plants to salt stress, as it is governed by minor genes with low heritability and high G × E interactions. The involvement of numerous physiological and biochemical factors further complicates this complexity. The intensive selection and breeding efforts targeted towards the improvement of yield in the green-revolution era inadvertently resulted in the gradual disappearance of the loci governing salinity tolerance and a significant reduction in genetic variability among cultivars. The limited utilization of genetic resources and narrow genetic base of improved cultivars have resulted in a plateau in response to salinity tolerance in modern cultivars. Wild species are an excellent genetic resource for broadening the genetic base of domesticated rice. Exploiting novel genes of underutilized wild rice relatives to restore salinity tolerance loci eliminated during domestication can result in significant genetic gain in rice cultivars. Wild species of rice, Oryza rufipogon and Oryza nivara, have been harnessed in the development of a few improved rice varieties like Jarava and Chinsura Nona 2. Furthermore, increased access to sequence information and enhanced knowledge about the genomics of salinity tolerance in wild relatives has provided an opportunity for the deployment of wild rice accessions in breeding programs, while overcoming the cross-incompatibility and linkage drag barriers witnessed in wild hybridization. Pre-breeding is another avenue for building material that are ready for utilization in breeding programs. Efforts should be directed towards systematic collection, evaluation, characterization, and deciphering salt tolerance mechanisms in wild rice introgression lines and deploying untapped novel loci to improve salinity tolerance in rice cultivars. This review highlights the potential of wild relatives of Oryza to enhance tolerance to salinity, track the progress of work, and provide a perspective for future research.

The NAC-type transcription factor GmNAC20 improves cold, salinity tolerance, and lateral root formation in transgenic rice plants

NAC-type transcription factors are crucial players in the abiotic stress responses of plants. Soybean NAC-type transcription factor GmNAC20 was transformed into rice genome via Agrobacterium method of transformation to improve abiotic stress tolerance. Integration and expression of GmNAC20 were verified by the DNA blot hybridization, immunoblotting, RT-PCR, and quantitative RT-PCR in T3 generation of transgenic rice plants. Significant expression of GmNAC20 was found in transgenic plants under salinity, cold, and IAA treatments. The transgenic rice plants expressing GmNAC20 displayed enhanced salinity and cold stress tolerance via upregulating the abiotic stress-responsive genes. Furthermore, T3 transgenic plants retained relative water content, chlorophyll content with enhanced accumulation of proline content than wild-type plants under salinity, and cold stress environments. The decrease in MDA content and electrolyte leakage with a significant increase in antioxidant enzyme activities were noticed in transgenic rice plants under either salinity or cold stress conditions, compared to wild-type plants. Overexpression of GmNAC20 in rice plants also induced the lateral root formation, associated with upregulation of auxin signaling-related genes. Taken together, our results indicated that GmNAC20 acts as a positive regulator for conferring salinity and cold tolerance in rice plants and appropriate candidate for improving salinity and cold stress in other important food crops.

Creation of male-sterile lines that can be restored to fertility by exogenous methyl jasmonate for the establishment of a two-line system for the hybrid production of rice (Oryza sativa L.)

Male sterility is widely used in the production of hybrid seeds in rice, but the use of genic male sterility is limited because of the high labour cost for maintaining male-sterile lines. Previous studies using T-DNA insertional mutagenesis demonstrated that disrupting the expression of oxophytodienoic acid reductase 3 (OPR3), which is involved in the jasmonate biosynthesis pathway, results in a kind of male sterility that can be restored to fertility by exogenous jasmonate in Arabidopsis. Here, we created male-sterile mutations by editing the second and fourth exons of OsOPR7 in rice through clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system 9. The induced mutagenesis at these exons resulted in 31.8% and 23.9% male-sterile plants in the T0 generation, respectively. We screened male-sterile lines that can be restored to fertility by exogenous methyl jasmonate in the T0, T1 and T2 rice populations and characterized the anther and agronomic traits of the transgenic plants. Results showed the successful generation of male-sterile lines through the silencing of OsOPR7, the orthologous gene of Arabidopsis OPR3, in a field crop, paving the way for the establishment of a two-line system for rice hybrid production. The system consists of a male-sterile line that can be maintained by spraying methyl jasmonate and a restoring line that confers pollen.

Salinity tolerance in barley during germination- homologs and potential genes

Salinity affects more than 6% of the world's total land area, causing massive losses in crop yield. Salinity inhibits plant growth and development through osmotic and ionic stresses; however, some plants exhibit adaptations through osmotic regulation, exclusion, and translocation of accumulated Na+ or Cl-. Currently, there are no practical, economically viable methods for managing salinity, so the best practice is to grow crops with improved tolerance. Germination is the stage in a plant's life cycle most adversely affected by salinity. Barley, the fourth most important cereal crop in the world, has outstanding salinity tolerance, relative to other cereal crops. Here, we review the genetics of salinity tolerance in barley during germination by summarizing reported quantitative trait loci (QTLs) and functional genes. The homologs of candidate genes for salinity tolerance in Arabidopsis, soybean, maize, wheat, and rice have been blasted and mapped on the barley reference genome. The genetic diversity of three reported functional gene families for salt tolerance during barley germination, namely dehydration-responsive element-binding (DREB) protein, somatic embryogenesis receptor-like kinase and aquaporin genes, is discussed. While all three gene families show great diversity in most plant species, the DREB gene family is more diverse in barley than in wheat and rice. Further to this review, a convenient method for screening for salinity tolerance at germination is needed, and the mechanisms of action of the genes involved in salt tolerance need to be identified, validated, and transferred to commercial cultivars for field production in saline soil.

Expression Patterns and Identified Protein-Protein Interactions Suggest That Cassava CBL-CIPK Signal Networks Function in Responses to Abiotic Stresses

Cassava is an energy crop that is tolerant of multiple abiotic stresses. It has been reported that the interaction between Calcineurin B-like (CBL) protein and CBL-interacting protein kinase (CIPK) is implicated in plant development and responses to various stresses. However, little is known about their functions in cassava. Herein, 8 CBL (MeCBL) and 26 CIPK (MeCIPK) genes were isolated from cassava by genome searching and cloning of cDNA sequences of Arabidopsis CBLs and CIPKs. Reverse-transcriptase polymerase chain reaction (RT-PCR) analysis showed that the expression levels of MeCBL and MeCIPK genes were different in different tissues throughout the life cycle. The expression patterns of 7 CBL and 26 CIPK genes in response to NaCl, PEG, heat and cold stresses were analyzed by quantitative real-time PCR (qRT-PCR), and it was found that the expression of each was induced by multiple stimuli. Furthermore, we found that many pairs of CBLs and CIPKs could interact with each other via investigating the interactions between 8 CBL and 25 CIPK proteins using a yeast two-hybrid system. Yeast cells co-transformed with cassava MeCIPK24, MeCBL10, and Na⁺/H⁺ antiporter MeSOS1 genes exhibited higher salt tolerance compared to those with one or two genes. These results suggest that the cassava CBL-CIPK signal network might play key roles in response to abiotic stresses.

OsHAK1, a High-Affinity Potassium Transporter, Positively Regulates Responses to Drought Stress in Rice

Drought is one of the environmental factors that severely restrict plant distribution and crop production. Recently, we reported that the high-affinity potassium transporter OsHAK1 plays important roles in K acquisition and translocation in rice over low and high K concentration ranges, however, knowledge on the regulatory roles of OsHAK1 in osmotic/drought stress is limited. Here, transcript levels of OsHAK1 were found transiently elevated by water deficit in roots and shoots, consistent with the enhanced GUS activity in transgenic plants under stress. Under drought conditions, OsHAK1 knockout mutants (KO) presented lower tolerance to the stress and displayed stunted growth at both the vegetative and reproductive stages. Phenotypic analysis of OsHAK1 overexpression seedlings (Ox) demonstrated that they present better tolerance to drought stress than wild-type (WT). Compared to WT seedlings, OsHAK1 overexpressors had lower level of lipid peroxidation, higher activities of antioxidant enzymes (POX and CAT) and higher proline accumulation. Furthermore, qPCR analysis revealed that OsHAK1 act as a positive regulator of the expression of stress-responsive genes as well as of two well-known rice channel genes (OsTPKb and OsAKT1) involved in K homeostasis and stress responses in transgenic plants under dehydration. Most important, OsHAK1-Ox plants displayed enhanced drought tolerance at the reproductive stage, resulting in 35% more grain yield than WT under drought conditions, and without exhibiting significant differences under normal growth conditions. Consequently, OsHAK1 can be considered to be used in molecular breeding for improvement of drought tolerance in rice.

Poaceae vs. Abiotic Stress: Focus on Drought and Salt Stress, Recent Insights and Perspectives

Poaceae represent the most important group of crops susceptible to abiotic stress. This large family of monocotyledonous plants, commonly known as grasses, counts several important cultivated species, namely wheat (Triticum aestivum), rice (Oryza sativa), maize (Zea mays), and barley (Hordeum vulgare). These crops, notably, show different behaviors under abiotic stress conditions: wheat and rice are considered sensitive, showing serious yield reduction upon water scarcity and soil salinity, while barley presents a natural drought and salt tolerance. During the green revolution (1940-1960), cereal breeding was very successful in developing high-yield crops varieties; however, these cultivars were maximized for highest yield under optimal conditions, and did not present suitable traits for tolerance under unfavorable conditions. The improvement of crop abiotic stress tolerance requires a deep knowledge of the phenomena underlying tolerance, to devise novel approaches and decipher the key components of agricultural production systems. Approaches to improve food production combining both enhanced water use efficiency (WUE) and acceptable yields are critical to create a sustainable agriculture in the future. This paper analyzes the latest results on abiotic stress tolerance in Poaceae. In particular, the focus will be directed toward various aspects of water deprivation and salinity response efficiency in Poaceae. Aspects related to cell wall metabolism will be covered, given the importance of the plant cell wall in sensing environmental constraints and in mediating a response; the role of silicon (Si), an important element for monocots' normal growth and development, will also be discussed, since it activates a broad-spectrum response to different exogenous stresses. Perspectives valorizing studies on landraces conclude the survey, as they help identify key traits for breeding purposes.