Overexpression of the ascorbate peroxidase gene from eggplant and sponge gourd enhances flood tolerance in transgenic Arabidopsis

Mechanisms of salinity tolerance and their possible application in the breeding of vegetables

BACKGROUND

In dry and semi-arid areas, salinity is the most serious hazard to agriculture, which can affect plant growth and development adversely. Over-accumulation of Na⁺ in plant organs can cause an osmotic effect and an imbalance in nutrient uptake. However, its harmful impact can vary depending on genotype, period of exposure to stress, plant development stage, and concentration and content of salt. To overcome the unfavorable effect of salinity, plants have developed two kinds of tolerance strategies based on either minimizing the entrance of salts by the roots or administering their concentration and diffusion.

RESULTS

Having sufficient knowledge of Na⁺ accumulation mechanisms and an understanding of the function of genes involved in transport activity will present a new option to enhance the salinity tolerance of vegetables related to food security in arid regions. Considerable improvements in tolerance mechanisms can be employed for breeding vegetables with boosted yield performance under salt stress. A conventional breeding method demands exhaustive research work in crops, while new techniques of molecular breeding, such as cutting-edge molecular tools and CRISPR technology are now available in economically important vegetables and give a fair chance for the development of genetically modified organisms.

CONCLUSIONS

Therefore, this review highlights the molecular mechanisms of salinity tolerance, various molecular methods of breeding, and many sources of genetic variation for inducing tolerance to salinity stress.

Identification and charactering of APX genes provide new insights in abiotic stresses response in Brassica napus

Ascorbate peroxidase (APX) plays an important role in scavenging H₂O₂ and balancing ROS content in plant cells, which is of great significance for the growth and development of life and resistance to external stress. However, up to now, APXs in Brassica napus (B. napus) have not been systematically characterized. In this study, a total of 26 BnaAPX genes were identified, which were distributed on 13 chromosomes and divided into five phylogenetic branches. Gene structure analysis showed that they had a wide varied number of exons while BnaAPXs proteins contained more similar motifs in the same phylogenetic branches. qRT-PCR analysis of 26 BnaAPX gene expression patterns showed that three putative cytosol BnaAPX genes BnaAPX1, BnaAPX2, BnaAPX9, two putatice microsomal genes BnaAPX18 and BnaAPX25 were up-regulated rapidly and robustly under high salt, water shortage and high temperature stresses. In addition, the above three abiotic stresses led to a significant increase in APX activity. The results provide basic and comprehensive information for further functional characterization of APX gene family in B. napus.

OsAPX1 Positively Contributes to Rice Blast Resistance

Ascorbate peroxidases (APXs) maintain cellular reactive oxygen species (ROS) homeostasis through their peroxidase activity. Here, we report that OsAPX1 also promotes ROS production such that a delicate cellular ROS homeostasis is achieved temporally after Magnaporthe oryzae infection. OsAPX1 specifically induces ROS production through increasing respiratory burst oxidase homologs (OsRBOHs) expression and can be inhibited by DPI, a ROS inhibitor. The time-course experiment data show that the simultaneous induction of OsAPX1 and OsRBOHs leads to ROS accumulation at an early stage; whereas a more durable expression of OsAPX1 leads to ROS scavenging at a later stage. By the temporal switching between ROS inducer and eliminator, OsAPX1 triggers an instant ROS burst upon M. oryzae infection and then a timely elimination of ROS toxicity. We find that OsAPX1 is under the control of the miR172a-OsIDS1 regulatory module. OsAPX1 also affects salicylic acid (SA) synthesis and signaling, which contribute to blast resistance. In conclusion, we show that OsAPX1 is a key factor that connects the upstream gene silencing and transcription regulatory routes with the downstream phytohormone and redox pathway, which provides an insight into the sophisticated regulatory network of rice innate immunity.

Pyramiding ascorbate-glutathione pathway in Lycopersicum esculentum confers tolerance to drought and salinity stress

Stacking Glutathione-Ascorbate pathway genes (PgSOD, PgAPX, PgGR, PgDHAR and PgMDHAR) under stress inducible promoter RD29A imparts significant tolerance to drought and salinity stress in Solanum lycopersicum. Although the exposure of plants to different environmental stresses results in overproduction of reactive oxygen species (ROS), many plants have developed some unique systems to alleviate the ROS production and mitigate its deleterious effect. One of the key pathways that gets activated in plants is ascorbate glutathione (AsA-GSH) pathway. To demonstrate the effect of this pathway in tomato, we developed the AsA-GSH overexpression lines by stacking the genes of the AsA-GSH pathway genes isolated from Pennisetum glaucoma (Pg) including PgSOD, PgAPX, PgGR, PgDHAR and PgMDHAR under stress inducible promoter RD29A. The overexpression lines have an improved germination and seedling growth with concomitant elevation in the survival rate. The exposure of transgenic seedlings to varying stress regiments exhibited escalation in the antioxidant enzyme activity and lesser membrane damage as reflected by decreased electrolytic leakage and little accumulation of malondialdehyde and H₂O₂. Furthermore, the transgenic lines accumulated high levels of osmoprotectants with increase in the relative water content. The increased photosynthetic activity and enhanced gaseous exchange parameters further confirmed the enhanced tolerance of AsA-GSH overexpression lines. We concluded that pyramiding of AsA-GSH pathway genes is an effective strategy for developing stress resistant crops.

Genetics and breeding for climate change in Orphan crops

Climate change is rapidly changing how we live, what we eat and produce, the crops we breed and the target traits. Previously underutilized orphan crops that are climate resilient are receiving much attention from the crops research community, as they are often the only crops left in the field after periods of extreme weather conditions. There are several orphan crops with incredible resilience to biotic and abiotic stresses. Some are nutritious, while others provide good sources of biofuel, medicine and other industrial raw materials. Despite these benefits, orphan crops are still lacking in important genetic and genomic resources that could be used to fast track their improvement and make their production profitable. Progress has been made in generating draft genomes of at least 28 orphan crops over the last decade, thanks to the reducing cost of sequencing. The implementation of a structured breeding program that takes advantage of additional modern crop improvement tools such as genomic selection, speed breeding, genome editing, high throughput phenotyping and breeding digitization would make rapid improvement of these orphan crops possible, but would require coordinated research investment. Other production challenges such as lack of adequate germplasm conservation, poor/non-existent seed systems and agricultural extension services, as well as poor marketing channels will also need to be improved if orphan crops were to be profitable. We review the importance of breeding orphan crops under the increasing effects of climate change, highlight existing gaps that need to be addressed and share some lessons to be learned from major crops.

Use of Non-Destructive Measurements to Identify Cucurbit Species (Cucurbita maxima and Cucurbita moschata) Tolerant to Waterlogged Conditions

Limited information is available regarding the physiology of squash plants grown under waterlogging stress. The objectives of this study were to investigate the growth and physiological performances of three cucurbit species, Cucurbita maxima cultivar (cv.) OK-101 (OK) and Cucurbita moschata cv. Early Price (EP) and Strong Man (SM), in response to waterlogging conditions, and to develop a precise, integrated, and quantitative non-destructive measurement of squash genotypes under stress. All tested plants were grown in a growth chamber under optimal irrigation and growth conditions for a month, and the pot plants were then subjected to non-waterlogging (control) and waterlogging treatments for periods of 1, 3, 7, and 13 days (d), followed by a 3-d post-waterlogging recovery period after water drainage. Plants with phenotypes, such as fresh weight (FW), dry weight (DW), and dry matter (DM) of shoots and roots, and various physiological systems, including relative water content (RWC), soil and plant analysis development (SPAD) chlorophyll meter, ratio of variable/maximal fluorescence (Fv/Fm), quantum photosynthetic yield (YII), normalized difference vegetation index (NDVI), and photochemical reflectance index (PRI) values, responded differently to waterlogging stress in accordance with the duration of the stress period and subsequent recovery period. When plants were treated with stress for 13 d, all plants exhibited harmful effects to their leaves compared with the control, but EP squash grew better than SM and OK squashes and exhibited stronger tolerance to waterlogging and showed less injury. Changes in the fresh weight, dry weight, and dry matter of shoots and roots indicated that OK plants suffered more severely than EP plants at the 3-d drainage period. The values of RWC, SPAD, Fv/Fm, YII, NDVI, and PRI in both SM and OK plants remarkably decreased at waterlogging at the 13-d time point compared with controls under identical time periods. However, the increased levels of SPAD, Fv/Fm, YII, NDVI, and PRI observed on 7 d or 13 d of waterlogging afforded the EP plant leaf with improved waterlogged tolerance. Significant and positive correlations were observed among NDVI and PRI with SPAD, Fv/Fm, and YII, indicating that these photosynthetic indices can be useful for developing non-destructive estimations of chlorophyll content in squashes when screening for waterlogging-tolerant plants, for establishing development practices for their cultivation in fields, and for enhanced cultivation during waterlogging in frequently flooded areas.

Overexpression of the Arabidopsis and winter squash superoxide dismutase genes enhances chilling tolerance via ABA-sensitive transcriptional regulation in transgenic Arabidopsis

The winter squash (Cucurbita moschata, Cm) superoxide dismutase (SOD) CmSOD gene and Arabidopsis thaliana (At)SOD gene were transferred under a ubiquitin promoter into Arabidopsis via Agrobacterium tumefaciens. The expression and amount of SOD and the SOD activities in the AtSOD and CmSOD transgenic lines were significantly higher than those of non-transgenic (NT) plants exposed to 23 or 4°C treatment for 6 ~ 192-h periods. Furthermore, expressions of the cold-inducible gene (AtCBF2) and desiccation-responsible transcription factors (AtRD29A/B) were also activated in all transgenic lines compared to NT plants after chilling treatments. Compared to NT plants under chilling stress, superoxide (•O2^-) accumulation was significantly lower, and chlorophyll (Chl) contents were significantly higher in all transgenic lines with higher SOD activity. Moreover, Arabidopsis seedlings overexpressing AtSOD and CmSOD also displayed greater resistance to chilling and less oxidative injury than NT plants under chilled conditions, indicating that the overexpression of AtSOD and CmSOD in Arabidopsis enhanced chilling tolerance by eliminating •O2^-. The expression of AtRD29A was strongly up-regulated only in AtSOD transgenic plants treated with abscisic acid (ABA), while it was repressed in other transgenic plants, indicating ABA-sensitive AtCBF2 and AtRD29A/B transcriptional regulation signaling pathways in transgenic Arabidopsis under chilling conditions.

Functional characterization of 1-aminocyclopropane-1-carboxylic acid oxidase gene in Arabidopsis thaliana and its potential in providing flood tolerance

Ethylene is a phytohormone that has gained importance through its role in stress tolerance and fruit ripening. In our study we evaluated the functional potential of the enzyme involved in ethylene biosynthesis of plants called ACC (aminocyclopropane-1-carboxylic acid) oxidase which converts precursor ACC to ethylene. Studies on ethylene have proven that it is effective in improving the flood tolerance in plants. Thus our goal was to understand the potential of ACC oxidase gene overexpression in providing flood tolerance in transgenic plants. ACC oxidase gene was PCR amplified and inserted into the pBINmgfp5-er vector, under the control of a constitutive Cauliflower Mosaic Virus promoter. GV101 strain of Agrobacterium tumefaciens containing recombinant pBINmgfp5-er vector (referred herein as pBIN-ACC) was used for plant transformation by the 'floral dip' method. The transformants were identified through kanamycin selection and grown till T3 (third transgenic) generation. The flood tolerance was assessed by placing both control and transgenic plants on deep plastic trays filled with tap water that covered the soil surface. Our result shows that wild-type Arabidopsis could not survive more than 20 days under flooding while the transgenic lines survived 35 days, suggesting development of flood tolerance with overexpression of ACC oxidase. Further molecular studies should be done to elucidate the role and pathways of ACC oxidase and other phytohormones involved in the development of flood adaptation.

Oxidative Stress Associated with Chilling Injury in Immature Fruit: Postharvest Technological and Biotechnological Solutions

Immature, vegetable-like fruits are produced by crops of great economic importance, including cucumbers, zucchini, eggplants and bell peppers, among others. Because of their high respiration rates, associated with high rates of dehydration and metabolism, and their susceptibility to chilling injury (CI), vegetable fruits are highly perishable commodities, requiring particular storage conditions to avoid postharvest losses. This review focuses on the oxidative stress that affects the postharvest quality of vegetable fruits under chilling storage. We define the physiological and biochemical factors that are associated with the oxidative stress and the development of CI symptoms in these commodities, and discuss the different physical, chemical and biotechnological approaches that have been proposed to reduce oxidative stress while enhancing the chilling tolerance of vegetable fruits.