Sugar metabolism and accumulation in the fruit of transgenic apple trees with decreased sorbitol synthesis

Both sorbitol and sucrose are synthesized in source leaves and transported to fruit for supporting fruit growth in tree fruit species of the Rosaceae family.In apple(Malus domestica),antisense suppression of aldose-6-phosphate reductase,the key enzyme for sorbitol synthesis,significantly decreased the sorbitol concentration but increased the sucrose concentration in leaves,leading to a lower sorbitol but a higher sucrose supply to fruit in these plants.In response to this altered carbon supply,the transgenic fruit had lower concentration of sorbitol and much higher concentration of glucose but similar levels of fructose,sucrose,and starch throughout fruit development relative to the untransformed control.Activities of sorbitol dehydrogenase,fructokinase,and sucrose phosphate synthase were lower,whereas activities of neutral invertase,sucrose synthase,and hexokinase were higher in the transgenic fruit during fruit development.Transcript levels of MdSOT1,MdSDHs,MdFK2,and MdSPS3/6 were downregulated,whereas transcript levels of MdSUC1/4,MdSUSY1-3,MdNIV1/3,MdHKs,and MdTMT1 were upregulated in the transgenic fruit.These findings suggest that the Sucrose cycle and the sugar transport system are very effective in maintaining the level of fructose and provide insights into the roles of sorbitol and sucrose in regulating sugar metabolism and accumulation in sorbitol-synthesizing species.

Linkage map and QTL mapping of red flesh locus in apple using a R1R1×R6R6 population

The red flesh in apple fruit is a desired trait by consumers and it is associated to the anthocyanin content,which is mainly controlled by MdMYB10 with a R6 promoter.In this study,a high-density linkage group was constructed using the‘Fuji’x‘Red3’population which contained homozygous alleles R1R1 and R6R6,respectively.The linkage group consists of 7630 SNPs along 17 linkage groups,spanning 2270.21 cM,with an average density of 0.30 cM permarker.The cyanidin-3-galactoside concentration was used as the phenotypic data in QTL analysis.Moreover,one QTL peak which was flaked by two markers,marker2187260 to marker2173766,with LOD scores of 4.49 was detected.This QTL ranged from 0 to 40.79 cM on the top of linkage group(LG16).In addition one candidate molecular marker(marker2175442)in this QTL was identified,which was significant correlated with the flesh cyanidin-3-galactoside concentration.These genetic findings enrich the breeding basis of fruit flesh coloration in apple.

Regulation of phenylpropanoid biosynthesis by MdMYB88 and MdMYB124 contributes to pathogen and drought resistance in apple

MdMYB88 and MdMYB124 have been demonstrated to be responsible for lignin accumulation in apple under drought stress.In this study,using a metabolomic approach,we identified differentially accumulated phenylpropanoid and flavonoid metabolites in MdMYB88/124 transgenic RNAi plants under control and long-term drought stress conditions in apple roots.We confirmed the regulation of phenylalanine by MdMYB88 and MdMYB124 via UPLC-MS in apple roots under both control and drought conditions.Using Electrophoretic Mobility Shift Assay(EMSA)and ChIPquantitative PCR(qPCR)analyses,we found that MdMYB88 positively regulates the MdCM2 gene,which is responsible for phenylalanine biosynthesis,through binding to its promoter region.Under long-term drought conditions,MdMYB88/124 RNAi plants consistently accumulated increased amounts of H2O2 and MDA,while MdMYB88 and MdMYB124 overexpression plants accumulated decreased amounts of H2O2 and MDA.We also examined the accumulation of metabolites in the phenylpropanoid biosynthesis pathway in the leaves of MdMYB88 and MdMYB124 transgenic apple plants after long-term drought stress.We found that metabolites responsible for plant defense,including phenylpropanoids and flavonoids,accumulated less in the RNAi plants but more in the overexpression plants under both control and drought conditions.We further demonstrated that MdMYB88/124 RNAi plants were more sensitive to Alternaria alternata f.sp.mali and Valsa mali,two pathogens that currently severely threaten apple production.In contrast,MdMYB88 and MdMYB124 overexpression plants were more tolerant to these pathogens.The cumulative results of this study provided evidence for secondary metabolite regulation by MdMYB88 and MdMYB124,further explained the molecular roles of MdMYB88 and MdMYB124 in drought resistance,and provided information concerning molecular aspects of their roles in disease resistance.

PbGA2ox8 induces vascular-related anthocyanin accumulation and contributes to red stripe formation on pear fruit

Fruit with stripes,which are generally longitudinal,can occur naturally,but the bioprocesses underlying this phenomenon are unclear.Previously,we observed an atypical anthocyanin distribution that caused red-striped fruit on the spontaneous pear bud sport“Red Zaosu”(Pyrus bretschneideri Rehd.).In this study,comparative transcriptome analysis of the sport and wild-type“Zaosu”revealed that this atypical anthocyanin accumulation was tightly correlated with abnormal overexpression of the gene-encoding gibberellin(GA)2-beta-dioxygenase 8,PbGA2ox8.Consistently,decreased methylation was also observed in the promoter region of PbGA2ox8 from“Red Zaosu”compared with“Zaosu”.Moreover,the GA levels in“Red Zaosu”seedlings were lower than those in“Zaosu”seedlings,and the application of exogenous GA4 reduced abnormal anthocyanin accumulation in“Red Zaosu”.Transient overexpression of PbGA2ox8 reduced the GA4 level and caused anthocyanin accumulation in pear fruit skin.Moreover,the presence of red stripes indicated anthocyanin accumulation in the hypanthial epidermal layer near vascular branches(VBs)in“Red Zaosu”.Transient overexpression of PbGA2ox8 resulting from vacuum infiltration induced anthocyanin accumulation preferentially in calcium-enriched areas near the vascular bundles in pear leaves.We propose a fruit-striping mechanism,in which the abnormal overexpression of PbGA2ox8 in“Red Zaosu”induces the formation of a longitudinal array of anthocyanin stripes near vascular bundles in fruit.

Competition between anthocyanin and kaempferol glycosides biosynthesis affects pollen tube growth and seed set of Malus

Flavonoids play important roles in regulating plant growth and development.In this study,three kaempferol 3-O-glycosides were identi fi ed and mainly accumulated in fl owers but not in leaves or fruits of Malus.In Malus,fl ower petal color is normally white,but some genotypes have red fl owers containing anthocyanin.Anthocyanin biosynthesis appears to be in competition with kaempferol 3-O-glycosides production and controlled by the biosynthetic genes.The white fl ower Malus genotypes had better-developed seeds than the red fl ower genotypes.In fl owers,the overexpression of MYB10 in Malus domestica enhanced the accumulation of anthocyanin,but decreased that of kaempferol 3-O-glycosides.After pollination the transgenic plants showed slower pollen tube growth and fewer developed seeds.Exogenous application ofdifferent fl avonoid compounds suggested that kaempferol 3-O-glycosides,especially kaempferol 3-O-rhamnoside,regulated pollen tube growth and seed set rather than cyanidin or quercetin 3-O-glycosides.It was found that kaempferol 3-O-rhamnoside might regulate pollen tube growth through effects on auxin,the Rho of plants(ROP)GTPases,calcium and the phosphoinositides signaling pathway.With the inhibition of auxin transport,the transcription levels of Heat Shock Proteins(HSPs)and ROP GTPases were downregulated while the levels were not changed or even enhanced when blocking calcium signaling,suggesting that HSPs and ROP GTPases were downstream of auxin signaling,but upstream of calcium signaling.In summary,kaempferol glycoside concentrations in pistils correlated with auxin transport,the transcription of HSPs and ROP GTPases,and calcium signaling in pollen tubes,culminating in changes to pollen tube growth and seed set.