Abiotic stresses collectively are responsible for crop losses worldwide. Among various abiotic stresses, drought and salinity are the most destructive. Different strategies have been adopted for the management of these stresses. Being complex traits, conventional breeding approaches have shown less success in improving salinity and drought stress tolerance. Roles of compatible solutes in salinity and drought stress tolerance have been studied extensively. At physiological level, osmotic adjustment is an adaptive mechanism involved in drought and/or salinity tolerance and permits the maintenance of turgor pressure under stress conditions. Increasing evidences from series of in vivo and in vitro studies involving physiological, biochemical, genetic, and molecular approaches strongly suggest that osmolytes such as ammonium compounds (polyamines, glycinebetaine, b-alanine betaine, dimethyl-sulfonio propionate and choline-O-sulfate), sugars and sugar alcohols (fructan, trehalose, mannitol, d-ononitol and sorbitol) and amino acids (proline and ectoine) perform important function in adjustment of plants against salinity and drought stresses. Thus, aim of this review is to expose how to osmoprotectants detoxify adverse effect of reactive oxygen species and alleviate drought and salinity stresses. An understanding of the relationship between these two sets of parameters is needed to develop measures for mitigating the damaging impacts of salinity and drought stresses.

Roles of osmoprotectants in improving salinity and drought tolerance in plants: a review

Phytohormones and polyamines regulate plant stress responses by altering GABA pathway.

Drought stress, which causes a decline in quality and quantity of crop yields, has become more accentuated these days due to climatic change. Serious measures need to be taken to increase the tolerance of crop plants to acute drought conditions likely to occur due to global warming. Drought stress causes many physiological and biochemical changes in plants, rendering the maintenance of osmotic adjustment highly crucial. The degree of plant resistance to drought varies with plant species and cultivars, phenological stages of the plant, and the duration of plant exposure to the stress. Osmoregulation in plants under low water potential relies on synthesis and accumulation of osmoprotectants or osmolytes such as soluble proteins, sugars, and sugar alcohols, quaternary ammonium compounds, and amino acids, like proline. This review highlights the role of osmolytes in water-stressed plants and of enzymes entailed in their metabolism. It will be useful, especially for researchers working on the development of drought-resistant crops by using the metabolic-engineering techniques.

Osmoregulation and its actions during the drought stress in plants

Abiotic stresses such as extremes of temperature and pH, high salinity and drought, comprise some of the major factors causing extensive losses to crop production worldwide. Understanding how plants respond and adapt at cellular and molecular levels to continuous environmental changes is a pre-requisite for the generation of resistant or tolerant plants to abiotic stresses. In this review we aimed to present the recent advances on mechanisms of downstream plant responses to abiotic stresses and the use of stress-related genes in the development of genetically engineered crops.

https://www.mdpi.com/1422-0067/13/7/8628/pdf

Recent Molecular Advances on Downstream Plant Responses to Abiotic Stress

In plants, sugar transporters are involved not only in long-distance transport, but also in sugar accumulations in sink cells. To identify members of sugar transporter gene families and to analyze their function in fruit sugar accumulation, we conducted a phylogenetic analysis of the Malus domestica genome. Expression profiling was performed with shoot tips, mature leaves, and developed fruit of "Gala" apple. Genes for sugar alcohol [including 17 sorbitol transporters (SOTs)], sucrose, and monosaccharide transporters, plus SWEET genes, were selected as candidates in 31, 9, 50, and 27 loci, respectively, of the genome. The monosaccharide transporter family appears to include five subfamilies (30 MdHTs, 8 MdEDR6s, 5 MdTMTs, 3 MdvGTs, and 4 MdpGLTs). Phylogenetic analysis of the protein sequences indicated that orthologs exist among Malus, Vitis, and Arabidopsis. Investigations of transcripts revealed that 68 candidate transporters are expressed in apple, albeit to different extents. Here, we discuss their possible roles based on the relationship between their levels of expression and sugar concentrations. The high accumulation of fructose in apple fruit is possibly linked to the coordination and cooperation between MdTMT1/2 and MdEDR6. By contrast, these fruits show low MdSWEET4.1 expression and a high flux of fructose produced from sorbitol. Our study provides an exhaustive survey of sugar transporter genes and demonstrates that sugar transporter gene families in M. domestica are comparable to those in other species. Expression profiling of these transporters will likely contribute to improving our understanding of their physiological functions in fruit formation and the development of sweetness properties.

/pdf/the-malus-domestica-sugar-transporter-gene-family-4pb7d6hhlf.pdf

The Malus domestica sugar transporter gene family: identifications based on genome and expression profiling related to the accumulation of fruit sugars.

Auxin plays important roles in hormone crosstalk and the plant’s stress response. The auxin-responsive Gretchen Hagen3 (GH3) gene family maintains hormonal homeostasis by conjugating excess indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acids (JAs) to amino acids during hormone- and stress-related signaling pathways. With the sequencing of the apple (Malus × domestica) genome completed, it is possible to carry out genomic studies on GH3 genes to indentify candidates with roles in abiotic/biotic stress responses. Malus sieversii Roem., an apple rootstock with strong drought tolerance and the ancestral species of cultivated apple species, was used as the experimental material. Following genome-wide computational and experimental identification of MdGH3 genes, we showed that MdGH3s were differentially expressed in the leaves and roots of M. sieversii and that some of these genes were significantly induced after various phytohormone and abiotic stress treatments. Given the role of GH3 in the negative feedback regulation of free IAA concentration, we examined whether phytohormones and abiotic stresses could alter the endogenous auxin level. By analyzing the GUS activity of DR5::GUS-transformed Arabidopsis seedlings, we showed that ABA, SA, salt, and cold treatments suppressed the auxin response. These findings suggest that other phytohormones and abiotic stress factors might alter endogenous auxin levels. Previous studies showed that GH3 genes regulate hormonal homeostasis. Our study indicated that some GH3 genes were significantly induced in M. sieversii after various phytohormone and abiotic stress treatments, and that ABA, SA, salt, and cold treatments reduce the endogenous level of axuin. Taken together, this study provides evidence that GH3 genes play important roles in the crosstalk between auxin, other phytohormones, and the abiotic stress response by maintaining auxin homeostasis.

/pdf/genome-wide-analysis-of-the-gh3-family-in-apple-malus-11j4yriaqs.pdf

Genome-wide analysis of the GH3 family in apple (Malus × domestica).

Sorbitol, a major end-product of photosynthesis in many species of the Rosaceae family, accumulates in response to abiotic stressors. However, the relationship that arises between the expression of sorbitol transporters and sorbitol accumulation under abiotic stress remains unclear. In this study, micropropagated ‘Fuji’ apple plants (Malus domestica Borkh. ‘Fuji’) were exposed to two varying degrees of osmotic stress and compared relative to an unstressed control. The osmotic stress was generated by adding PEG 6000 into full-strength Hoagland solution and adjusted the osmotic potential to either −0.75 MPa (mild drought stress [MIS]) or −1.5 MPa (severe drought stress [SES]). Analysis of sorbitol levels via high performance liquid chromatography (HPLC) showed that the sorbitol concentration was elevated in roots, phloem tissues and leaves in both the MIS and SES treatments compared to controls for the entire duration of the experiment. Three cDNA sequences, encoding sorbitol transporters (MdSOT3, MdSOT4 and MdSOT5), were isolated from leaves. Real-time quantitative PCR (RT-qPCR) data suggests that the expression levels of MdSOT3 and MdSOT5 were higher under MIS and SES in roots, phloem tissues and leaves compared to unstressed controls. The average mRNA levels of MdSOT4 in phloem tissues declined under both drought treatments (with the exception being at 2 h of SES). In roots and leaves under SES, mRNA production was increased. These results indicate that the up-regulation of MdSOT3 and MdSOT5 expression is consistent with the accumulation of sorbitol under conditions of osmotic stress in apple plants. They enhanced drought tolerance in vegetative tissues. Increased MdSOT4 mRNA enhanced drought tolerance under SES.

Characterization of Three Sorbitol Transporter Genes in Micropropagated Apple Plants Grown under Drought Stress

A LEAFY/FLORICAULA (LFY/FLO) homolog PpLFL (PrunuspersicaLEAFY/FLORICAULALike) gene was isolated from axillary buds of peach (Prunus persica (L.) Batsch. cv. Bayuecui) during flower induction period. The open reading frame of PpLFL spanned 1,248 bp, encoding a putative protein of 415 amino acid residues, which was with high similarity (50.48 %–84.69 %) to other FLO/LFY inferred proteins from different species. The spatial expression patterns of PpLFL were detected in axillary buds during the periods of flower induction by using immunohistolocalisation. The results showed that PpLFL gene was mainly expressed during flower induction time, and also in leaf and petal promordia at the SAM. For further functional analysis, the PpLFL was constitutively expressed in the Arabidopsis lfy mutant background, and the results showed that overexpression of PpLFL under the control of CaMV 35S promoter can accelerate flowering and give rise to normal flower organs. Our results suggest that PpLFL might play an important role in flower induction, and could act as a functional flower meristem identity gene in peach.

Identification and Characterization of PpLFL, a Homolog of FLORICAULA/LEAFY in Peach ( Prunus persica )

The role of polyol transporters in stress tolerance in plants have been elucidated by many studies. Sorbitol transporter genes MdSOT3, MdSOT4 and MdSOT5 in apple plants, which are important for sorbitol loading and unloading, are regulated by drought stress. To further confirm the role of sorbitol transporters in stress tolerance, the constructs harboring MdSOT3 and MdSOT5 genes were introduced into wild type Arabidopsis plants (Col-0) and the Arabidopsis transformed with MdSOT3 or MdSOT5 performed higher drought stress tolerance compared to WT. In order to further understand how sorbitol transporters are involved in drought tolerance in apple plants, upstream regions of sorbitol transporter genes were isolated from apple plant source leaves by Anchored PCR from genomic DNA obtained, and then were used to drive expression of the GUS reporter in tobacco plants. The results showed that the longest fragments of MdSOT3 and MdSOT5 promoters induced the highest GUS activity under drought stress conditions. Additionally, fragments of these promoters that contain cis-acting elements known to be involved in stress response also induced GUS activity under drought stress. Taken together, our data suggest that increased MdSOT3 and MdSOT5 activity, through cis-acting elements in the promoters of these genes, play important roles in imparting tolerance to drought in micropropagated apple plants.

Isolation and characterization of two sorbitol transporter gene promoters in micropropagated apple plants (Malus × domestica) regulated by drought stress

Hengjiu Lei

Papers

Genome-wide analysis of the GH3 family in apple (Malus × domestica).

Characterization of Three Sorbitol Transporter Genes in Micropropagated Apple Plants Grown under Drought Stress

Identification and Characterization of PpLFL, a Homolog of FLORICAULA/LEAFY in Peach ( Prunus persica )

Isolation and characterization of two sorbitol transporter gene promoters in micropropagated apple plants (Malus × domestica) regulated by drought stress