MdDREB2A in apple is involved in the regulation of multiple abiotic stress responses

Abiotic stress has a serious effect on plant growth.The transcription factor DREB2A is a member of the AP2/ERF family,which is widely involved in abiotic stress response.However,the function of apple MdDREB2A has not been systematically investigated.In this study,MdDREB2A was isolated from the cultivar‘Royal Gala’.The open reading frame of MdDREB2A was 1197 bp in length and it encoded a protein of 398 amino acidswithmolecularweight of 43.8 kD.As a transcription factor,MdDREB2Awas located in the nucleus.qRT-PCR analysis showed that MdDREB2A was involved in responses to drought,salt,and ABA stresses.Under these stress treatments,the relative electrical conductivity,superoxide anion and malondialdehyde(MDA)in transgenic materials significantly decreased,and the content of proline increased in MdDREB2A transgenic plants,compared to the controls,indicating that MdDREB2A transgenic apple calli and transgenic Arabidopsis exhibited improved resistance to abiotic stress.This study introduces a candidate gene for the genetic improvement of crop resistance and reveals important function of MdDREB2A in the regulation of abiotic stress response.

The Growth-Promoting Mechanism of Brevibacillus laterosporus AMCC100017 on Apple Rootstock Malus robusta

Plant growth-promoting rhizobacteria(PGPR) located in rhizobacteria soil are beneficial to plant growth and development. A PGPR strain AMCC100017 of Brevibacillus laterosporus synthesizes the plant hormone IAA in a tryptophan-dependent manner. In this study, the AMCC100017 strain was used to treat Malus robusta, an excellent natural rootstock for apple production, and assess its ability to promote growth. The fresh weight, dry weight, plant height, and lateral root growth of M. robusta were significantly increased with treatment. The presence of the AMCC100017 strain increased IAA content in M. robusta and promoted root secretion of tryptophan. Colonization of the strain in the roots allowed continuous synthesis of IAA and promoted plant growth. In addition, the photosynthetic efficiency in leaves increased after microbial treatment, and the utilization of nitrogen, zinc, iron, copper and magnesium in leaves was increased, which was conducive to photosynthesis.Interestingly, the activities of CAT and SOD, as well as the contents of ROS in plants colonized by AMCC100017 were increased compared to control plants, but the activities of POD and MDA contents were decreased. AMCC100017 strain affected the antioxidant capacity and stress resistance of plants. AMCC100017 strain increased the content of soluble protein and soluble sugar in plants, improved plant metabolic activity and stress resistance. Therefore, AMCC100017 not only increased IAA content and photosynthetic efficiency to promote M. robusta growth, but also affected plant through multiple metabolic pathways.

Genome-wide identification and expression analyses of homeodomain-leucine zipper family genes reveal their involvement in stress response in apple(Malus×domestica)

The homeodomain-leucine zipper(HD-Zip)family has been shown to perform amultitude of functions during plant development and stress responses;however,the familymembers and functions have not been identified in apple(Malus×domestica).In this study,83 HD-Zips(MdHDZs)were identified in the apple genome.They were assembled into four subgroups according to the classification in Arabidopsis,where MdHDZs in the same subgroup had similar gene structures and conserved protein motifs.Putative cis-element analysis of MdHDZs promoter regions uncovered numerous elements related to the response of stress and plant hormones.In addition,twelve transcripts of the MdHDZs showed different expression patterns under salt,drought,low temperature and ABA stresses by quantitative reverse transcription-PCR(qRT-PCR)assay.To further explore the function of MdHDZs in apple,MdHDZ3 was selected to verify its function under salt,low temperature and ABA stresses;and genetic transformation was used to obtain MdHDZ3 transgenic apple calli.The results demonstrated that MdHDZ3 increased sensitivity to salt,low temperature and abscisic acid in apple calli,suggesting that MdHDZ3 plays an important role in response to stresses.Subcellular localization and three-dimensional structural analysis revealed that MdHDZ3 was a nuclear-localized protein.Taken together,these findings provide potential information for further identification of HD-Zip proteins in apple.

Genome-wide identification and functional characterization of the MdCLE peptide family in apple(Malus×domestica)

The CLAVATA3/EMBRYO SURROUNDING REGION-related(CLE)peptides are critical for stem cell homeostasis in plant shoot and root apical meristem.Although CLE genes have been reported in numerous plants,there is limited information for apple.Here,twenty-five MdCLE genes were identified fromapple genome(Apple Genome V1.0 predicted peptides).Analysis of chromosomal location showed that the 25 MdCLE genes were located on 12 of 17 apple chromosomes.Genetic structure analysis showed that 21 of 25 the MdCLE genes were intron-free.Expression patterns showed thatmost of the MdCLE genes showed higher expression levels in leaves and root compared with the other tissues.In addition,expression analysis demonstrated that MdCLE genes had different gene expression patterns for abiotic stress treatment,suggesting their potential roles in acclimation of apple to adverse environments.The 25 MdCLE genes encoded 19 different CLE peptides,and they were divided into two groups depending on their effects on the inhibition of Arabidopsis root growth.Our results suggest that MdCLE genesmay have an important but redundant function in regulating plant growth and development,and this work provides valuable references for further investigation of the biological functions of MdCLE genes.

Functional identification of MdSMXL8.2,the homologous gene of strigolactones pathway repressor protein gene in Malus×domestica

A homologous gene of strigolactones repressor protein gene SMXL7/D53,MdSMXL8.2(GenBank accession No.:MD07G1222400),was cloned from‘Royal Gala’apple(Malus×domestica Borkh.)in this study.The sequence analysis revealed that the length of this gene was 3243 bp,which encoded 1080 amino acids,and had a protein molecular mass of∼110 kD.The phylogenetic tree analysis indicated that the MdSMXL8.2 exhibited the highest sequence similarity with Arabidopsis AtSMXL7.The protein conserved domain analysis revealed that the MdSMXL8.2 contained two ClpA domains.The prediction of the secondary and tertiary structures of the MdSMXL8.2 indicated that it contained 34.54%αhelix,3.43%β-sheet,and 11.76%extended chain.The in-silico analysis suggested that the promoter sequence of MdSMXL8.2 contained several typical cisacting elements,including abscisic acid(ABA),gibberellin(GA),ethylene,auxin,jasmonic acid(JA),salicylic acid(SA),drought,and heat stressresponsive elements.Quantitative real-time(qRT)-PCR analyses revealed that MdSMXL8.2 was expressed in different apple tissues,with the highest transcript level found in the stem.The expression of MdSMXL8.2 was significantly induced by exogenous ABA,PEG and mannitol,while exogenous NaCl significantly inhibited MdSMXL8.2 expression.The growing status of MdSMXL8.2-overexpressed Orin apple callus was worse than the wild type(WT)after NaCl treatment and had a higher malondialdehyde(MDA)content and relative conductance(REC).Additionally,MdSMXL8.2-overexpressed Arabidopsis exhibited shorter root length and a reduction in fresh weight under salt stress,indicating that MdSMXL8.2 negatively regulated salt tolerance in apples.

The Characterization, Authentication, and Gene Expression Pattern of the MdCER Family in Malus domestica

Plant cuticular wax is an important determinant factor of fruit quality. Cuticular wax can protect the fruit from UV radiation, drought, and disease and increase the storage period of fruit. In many species, CER gene is associated with wax and can affect plant response to stress, but no characterization of CER gene and its family has been reported in apples. In this study, we identified 10 MdCER genes in Malus domestica based on the sequences of 10 CER genes in Arabidopsis thaliana. Three-dimensional structures were then defined, and root-mean-square deviation(RMSD)scoring matrixes were used to evaluate the matches. These 10 genes were divided into three classes using phylogenetic methods; namely,class I, II, and III. The predicted MdCER genes were distributed across 7 out of 17 chromosomes with different densities. Furthermore, the gene structures and motif compositions of the MdCER genes were analyzed. The quantitative real-time PCR results indicated that MdCER family genes were mainly expressed in the leaves and stems and rarely in the roots. Most of the MdCER members responded to salicylic acid and polyethylene glycol treatment, indicating that the MdCER family is associated with disease resistance(biotic) and abiotic stress.

THE ANTHOCYANIN BIOSYNTHETIC REGULATOR MDMYB1 POSITIVELY REGULATES ASCORBIC ACID BIOSYNTHESIS IN APPLE

Ascorbic acid(AsA,vitamin C)is involved in the regulation of many aspects of plant growth and development.It is an essential micronutrient for humans and can prevent scurvy,maintain the health of gums and blood vessels,reduce the level of plasma cholesterol and enhance the immune systen.Apple cultivars Orin and Guanghui were crossed to obtain a group of hybrid offspring with and without red flesh in the course of assessing apple germplasm resources.Unexpectedly,the red-flesh apples had higher AsA contents than other apples.Further studies showed that the anthocyanin biosynthetic regulator MdMYBl directly activates the expression of dehydroascorbate reductase gene MdDHAR,thus promoting the activity of the DHAR enzyme and the accumulation of AsA.This finding reveals the mechanism leading to high AsA levels in red-flesh apples and suggests a new idea to cultivate red-flesh apples with high AsA contents and produce AsA efficiently and without pollution.

HIGHLIGHTS OF THE SPECIAL ISSUE ON"HORTICULTURE RESEARCH FOR GREEN AND SUSTAINABLE DEVELOPMENT"

Horticulture is the science and technology of intensively cultivating plants for food,comfort and beautification purposes.Horticulture comprises a wide range of plants and crops including fruit and nut trees,vegetables,edible fungi,and ornamental plants,as well as tea plants also categorized as horticultural crops in some countries including China.Horticultural products are of great value and serve as important dietary sources of antioxidants,vitamins and mineral nutrients for human nutrition and health.Development of a green and sustainable horticulture,producing more and safer fruit and vegetables,is a prerequisite for meeting the ever-increasing demand of the growing human population.