An Overview of Mapping Quantitative Trait Loci in Peanut (Arachis hypogaea L.).

Quantitative Trait Loci (QTL) mapping has been thoroughly used in peanut genetics and breeding in spite of the narrow genetic diversity and the segmental tetraploid nature of the cultivated species. QTL mapping is helpful for identifying the genomic regions that contribute to traits, for estimating the extent of variation and the genetic action (i.e., additive, dominant, or epistatic) underlying this variation, and for pinpointing genetic correlations between traits. The aim of this paper is to review the recently published studies on QTL mapping with a particular emphasis on mapping populations used as well as traits related to kernel quality. We found that several populations have been used for QTL mapping including interspecific populations developed from crosses between synthetic tetraploids and elite varieties. Those populations allowed the broadening of the genetic base of cultivated peanut and helped with the mapping of QTL and identifying beneficial wild alleles for economically important traits. Furthermore, only a few studies reported QTL related to kernel quality. The main quality traits for which QTL have been mapped include oil and protein content as well as fatty acid compositions. QTL for other agronomic traits have also been reported. Among the 1261 QTL reported in this review, and extracted from the most relevant studies on QTL mapping in peanut, 413 (~33%) were related to kernel quality showing the importance of quality in peanut genetics and breeding. Exploiting the QTL information could accelerate breeding to develop highly nutritious superior cultivars in the face of climate change.

Constitutive overexpression of rice metallothionein-like gene OsMT-3a enhances growth and tolerance of Arabidopsis plants to a combination of various abiotic stresses.

Metallothioneins (MT) are primarily involved in metal chelation. Recent studies have shown that MT proteins are also involved in the responses of plants to various environmental stresses. The rice metallothionein-like gene OsMT-3a is upregulated by salinity and various abiotic stressors. A DNA construct containing the complete OsMT-3a coding sequence cloned downstream to the CaMV35S promoter was transformed into Arabidopsis and homozygous single-copy transgenic lines were produced. Compared to wild-type plants, transgenic plants showed substantially increased salinity tolerance (NaCl), drought tolerance (PEG), and heavy metal tolerance (CdCl2) as individual stresses, as well as different combinations of these stresses. Relevantly, under unstressed control conditions, vegetative growth of transgenic plants was also improved. The shoot Na+ concentration and hydrogen peroxide in transgenic plants were lower than those in wild-type plants. OsMT-3a-overexpressing Arabidopsis lines accumulated higher levels of Cd2+ in both shoots and roots following CdCl2 treatment. In the transgenic MT-3a lines, increased activity of two major antioxidant enzymes, catalase and ascorbate peroxidase, was observed. Thus, rice OsMT-3a is a valuable target gene for plant genetic improvement against multiple abiotic stresses.

Differential responses of two Egyptian barley (Hordeum vulgare L.) cultivars to salt stress.

Although barley (Hordeum vulgare L.) is considered a salt tolerant crop species, productivity of barley is affected differently by ionic, osmotic, and oxidative stresses resulting from a salty rhizosphere. The current study was conducted to elucidate the mechanism of salt tolerance in two barley cultivars, Giza128 and Giza126. The two cultivars were exposed to 200 mM NaCl hydroponically for 12 days. Although both cultivars accumulated a large amount of Na+ in their leaves with similar concentrations, the growth of Giza128 was much better than that of Giza126, as measured by maintaining a higher dry weight, relative growth rate, leaf area, and plant height. To ascertain the underlying mechanisms of this differential tolerance, first, the relative expression patterns of the genes encoding Na+/H+ antiporters (NHX) and the associated proton pumps (V-PPase and V-ATPase) as well as the gene encoding the plasma membrane PM H+-ATPase were analyzed in leaf tissues. Salt stress induced higher HvNHX1 expression in Giza128 (3.3-fold) than in Giza126 (1.9-fold), whereas the expression of the other two genes, HvNHX2 and HvNHX3, showed no induction in either cultivar. The expression of HvHVP1 and HvHVA was higher in Giza128 (3.8- and 2.1-fold, respectively) than in Giza126 (1.6- and 1.1-fold, respectively). The expression of the PM H+-ATPase (ha1) gene was induced more in Giza128 (8.8-fold) than in Giza126 (1.8-fold). Second, the capacity for ROS detoxification was assessed using the oxidative stress biomarkers electrolyte leakage ratio (ELR) and the concentrations of malondialdehyde (MDA) and hydrogen peroxide (H2O2), and these parameters sharply increased in Giza126 leaves by 66.5%, 42.8% and 50.0%, respectively, compared with those in Giza128 leaves. The antioxidant enzyme (CAT, APX, sPOD, GR, and SOD) activities were significantly elevated by salt treatment in Giza128 leaves, whereas in Giza126, these activities were not significantly altered. Overall, the results indicate that the superior salt tolerance of Giza128 is primarily the result of the ability to counter Na+-induced oxidative stress by increasing antioxidant enzyme levels and possibly by increasing vacuolar Na+ sequestration and prevention of cellular K+ leakage.

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes.

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.

Genome-wide expression profiling in leaves and roots of date palm (Phoenix dactylifera L.) exposed to salinity.

BACKGROUND: Date palm, as one of the most important fruit crops in North African and West Asian countries including Oman, is facing serious growth problems due to salinity, arising from persistent use of saline water for irrigation. Although date palm is a relatively salt-tolerant plant species, its adaptive mechanisms to salt stress are largely unknown. RESULTS: In order to get an insight into molecular mechanisms of salt tolerance, RNA was profiled in leaves and roots of date palm seedlings subjected to NaCl for 10 days. Under salt stress, photosynthetic parameters were differentially affected; all gas exchange parameters were decreased but the quantum yield of PSII was unaffected while non-photochemical quenching was increased. Analyses of gene expression profiles revealed 2630 and 4687 genes were differentially expressed in leaves and roots, respectively, under salt stress. Of these, 194 genes were identified as commonly responding in both the tissue sources. Gene ontology (GO) analysis in leaves revealed enrichment of transcripts involved in metabolic pathways including photosynthesis, sucrose and starch metabolism, and oxidative phosphorylation, while in roots genes involved in membrane transport, phenylpropanoid biosynthesis, purine, thiamine, and tryptophan metabolism, and casparian strip development were enriched. Differentially expressed genes (DEGs) common to both tissues included the auxin responsive gene, GH3, a putative potassium transporter 8 and vacuolar membrane proton pump. CONCLUSIONS: Leaf and root tissues respond differentially to salinity stress and this study has revealed genes and pathways that are associated with responses to elevated NaCl levels and thus may play important roles in salt tolerance providing a foundation for functional characterization of salt stress-responsive genes in the date palm.

Salinity-induced expression of HKT may be crucial for Na(+) exclusion in the leaf blade of huckleberry (Solanum scabrum Mill.), but not of eggplant (Solanum melongena L.).

Reduced Na(+) accumulation in the leaf blade is an important aspect of salinity tolerance and high affinity K(+) transporters (HKTs) are known to play a significant role in the process. Huckleberry and eggplant have previously been shown to display 'excluder' and 'includer' characteristics, respectively, under salt stress, but the underlying mechanisms have not been investigated. Here, we isolated the cDNA of the HKT homologs, Solanum scabrum HKT (SsHKT) from huckleberry and Solanum melongena HKT (SmHKT) from eggplant, and analyzed their expressions in different tissues under salt stress. SsHKT expression was markedly induced in the root (28-fold) and stem (7-fold), with a corresponding increase in Na(+) accumulation of 52% and 29%, respectively. Conversely, eggplant accumulated 60% total Na(+) in the leaf blade, with a lower SmHKT expression level in the root (3-fold). Huckleberry also maintained a higher K(+)/Na(+) ratio in the leaf blade compared to eggplant, due to the reduction of its Na(+) concentration and unaltered K(+) concentration. Functional analysis demonstrated that SsHKT-mediated Na(+) influx inhibited yeast growth under Na(+) stress, and that SsHKT did not complement the growth of the K(+) uptake-deficient CY162 strain under K(+)-limiting conditions. These results suggest that the Na(+) accumulation characteristics of both plants are caused by the differential expression of HKT genes, with SsHKT exerting a greater control over the ability of Na(+) to reach the leaf blade in huckleberry, than SmHKT does in eggplant.

Growth, physiological adaptation, and gene expression analysis of two Egyptian rice cultivars under salt stress.

Abiotic stressors, such as high salinity, greatly affect plant growth. In an attempt to explore the mechanisms underlying salinity tolerance, physiological parameters of two local Egyptian rice (Oryza sativa L.) cultivars, Sakha 102 and Egyptian Yasmine, were examined under 50 mM NaCl stress for 14 days. The results indicate that Egyptian Yasmine is relatively salt tolerant compared to Sakha 102, and this was evident in its higher dry mass production, lower leaf Na(+) levels, and enhanced water conservation under salt stress conditions. Moreover, Egyptian Yasmine exhibited lower Na(+)/K(+) ratios in all tissues examined under salinity stress. The ability to maintain such traits seemed to differ in the leaves and roots of Egyptian Yasmine, and the root K(+) content was much higher in Egyptian Yasmine than in Sakha 102. In order to understand the basis for these differences, we studied transcript levels of genes encoding Na(+) and K(+) transport proteins in different tissues. In response to salinity stress, Egyptian Yasmine showed induction of expression of some membrane transporter/channel genes that may contribute to Na(+) exclusion from the shoots (OsHKT1;5), limiting excess Na(+) entry into the roots (OsLti6b), K(+) uptake (OsAKT1), and reduced expression of a Na(+) transporter gene (OsHKT2;1). Therefore, the active regulation of genes related to Na(+) transport at the transcription level may be involved in salt tolerance mechanisms of Egyptian Yasmine, and these mechanisms offer the promise of improved salinity stress tolerance in local Egyptian rice genotypes.