Overexpression of <Emphasis Type="Italic">MeDREB1D</Emphasis> confers tolerance to both drought and cold stresses in transgenic <Emphasis Type="Italic">Arabidopsis</Emphasis>

Cassava (Manihot esculenta) is an important tropical crop with extraordinary tolerance to drought stress but few reports on it. In this study, MeDREB1D was significantly and positively induced by drought stress. Two allelic variants of the gene named MeDREB1D(R-2) and MeDREB1D(Y-3) were identified. Overexpressing MeDREB1D(R-2) and MeDREB1D(Y-3) in Arabidopsis resulted in stronger tolerance to drought and cold stresses. Under drought stress, transgenic plants had more biomass, higher survival rates and less MDA content than wild-type plants. Under cold stress, transgenic plants also had higher survival rates than wild-type plants. To further characterize the molecular function of MeDREB1D, we conducted an RNA-Seq analysis of transgenic and wild-type Arabidopsis plants. The results showed that the Arabidopsis plants overexpressing MeDREB1D led to changes in downstream genes. Several POD genes, which may play a vital role in drought and cold tolerance, were up-regulated in transgenic plants. In brief, these results suggest that MeDREB1D can simultaneously improve plant tolerance to drought and cold stresses.     

<Emphasis Type="Italic">OsNHX2</Emphasis>, an Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporter gene,can enhance salt tolerance in rice plants through more effective accumulation of toxic Na<Superscript>+</Superscript> in leaf mesophyll and bundle sheath cells

The Na⁺/H⁺ antiporters play an important role in salt tolerance in plants. However, the functions of OsNHXs in rice except OsNHX1 have not been well studied. Using the gain- and loss-of-function strategies, we studied the potential role of OsNHX2 in salt tolerance in rice. Overexpression of OsNHX2 (OsNHX2-OE) in rice showed the significant tolerance to salt stress than wild-type plants and OsNHX2 knockdown transgenic plants (OsNHX2-KD). Under salt treatments of 300-mM NaCl for 5 days, the plant fresh weights, relative water percentages, shoot heights, Na⁺ contents, K⁺ contents, and K⁺/Na⁺ ratios in leaves of OsNHX2-OE transgenic plants were higher than those in wild-type plants, while no differences were detected in roots. K⁺/Na⁺ ratios in rice leaf mesophyll cells and bundle sheath cells were higher in OsNHX2-OE transgenic plants than in wild-type plants and OsNHX2-KD transgenic plants. Our data indicate that OsNHX2 plays an important role in salt stress based on leaf mesophyll cells and bundle sheath cells and can be served in genetically engineering crop plants with enhanced salt tolerance.     

A precise and consistent assay for major wall polymer features that distinctively determine biomass saccharification in transgenic rice by near-infrared spectroscopy

Background

The genetic modification of plant cell walls has been considered to reduce lignocellulose recalcitrance in bioenergy crops. As a result, it is important to develop a precise and rapid assay for the major wall polymer features that affect biomass saccharification in a large population of transgenic plants. In this study, we collected a total of 246 transgenic rice plants that, respectively, over-expressed and RNAi silenced 12 genes of the OsGH9 and OsGH10 family that are closely associated with cellulose and hemicellulose modification. We examined the wall polymer features and biomass saccharification among 246 transgenic plants and one wild-type plant. The samples presented a normal distribution applicable for statistical analysis and NIRS modeling.

Results

Among the 246 transgenic rice plants, we determined largely varied wall polymer features and the biomass enzymatic saccharification after alkali pretreatment in rice straws, particularly for the fermentable hexoses, ranging from 52.8 to 95.9%. Correlation analysis indicated that crystalline cellulose and lignin levels negatively affected the hexose and total sugar yields released from pretreatment and enzymatic hydrolysis in the transgenic rice plants, whereas the arabinose levels and arabinose substitution degree (reverse xylose/arabinose ratio) exhibited positive impacts on the hexose and total sugars yields. Notably, near-infrared spectroscopy (NIRS) was applied to obtain ten equations for predicting biomass enzymatic saccharification and seven equations for distinguishing major wall polymer features. Most of the equations exhibited high R ²/R ² _cv/R ² _ev and RPD values for a perfect prediction capacity.

Conclusions

Due to large generated populations of transgenic rice lines, this study has not only examined the key wall polymer features that distinctively affect biomass enzymatic saccharification in rice but has also established optimal NIRS models for a rapid and precise screening of major wall polymer features and lignocellulose saccharification in biomass samples. Importantly, this study has briefly explored the potential roles of a total of 12 OsGH9 and OsGH10 genes in cellulose and hemicellulose modification and cell wall remodeling in transgenic rice lines. Hence, it provides a strategy for genetic modification of plant cell walls by expressing the desired OsGH9 and OsGH10 genes that could greatly improve biomass enzymatic digestibility in rice.

     

Salt stress induced soybean <Emphasis Type="Italic">GmIFS1</Emphasis> expression and isoflavone accumulation and salt tolerance in transgenic soybean cotyledon hairy roots and tobacco

Effects of isoflavones on plant salt tolerance were investigated in soybean (Glycine max L. Merr. cultivar N23674) and tobacco (Nicotiana tabacum L.). Leaf area, fresh weight, net photosynthetic rate (Pn), and transpiration rate (Tr) of soybean N23674 plants treated with 80 mM NaCl were significantly reduced, while a gene (GmIFS1) encoding for 2-hydroxyisoflavone synthase was highly induced, and isoflavone contents significantly increased in leaves and seeds. To test the impact of isoflavones to salt tolerance, transgenic soybean cotyledon hairy roots expressing GmIFS1 (hrGmIFS1) were produced. Salt stress slightly increased isoflavone content in hairy roots of the transgenic control harboring the empty vector but substantially reduced the maximum root length, root fresh weight, and relative water content (RWC). The isoflavone content in hrGmIFS1 roots, however, was significantly higher, and the above-mentioned root growth parameters decreased much less. The GmIFS1 gene was also transformed into tobacco plants; plant height and leaf fresh weight of transgenic GmIFS1 tobacco plants were much greater than control plants after being treated with 85 mM NaCl. Leaf antioxidant capacity of transgenic tobacco was significantly higher than the control plants. Our results suggest that salt stress-induced GmIFS1 expression increased isoflavone accumulation in soybean and improved salt tolerance in transgenic soybean hairy roots and tobacco plants.