Overexpression of the apple expansin-like gene MdEXLB1 accelerates the softening of fruit texture in tomato

Fruit firmness is an important quality trait of apple fruit texture,and the pre-harvest ripening period is the key period for the formation of apple fruit texture.Expansin is a cell wall loosing protein family that has four subfamilies:α-expansin(EXPA),β-expansin(EXPB),expansin-like A(EXLA),and expansin-like B(EXLB).In this study,we investigated the key period of pre-harvest texture formation in‘Golden Delicious’apples based on fruit longitudinal diameter,transverse diameter,firmness,tissue structure,respiration intensity,ethylene release rate,and expansin activity.Within the 10 days before harvest,the fruit was found to reach maturity.Semi-quantitative RT-PCR revealed that most of the expansins were expressed at the ripening stage before harvest.The biological function of the EXLB subfamily gene,MdEXLB1,was further identified,and its subcellular localization on the cell wall was confirmed by transient transformation experiments.Compared with the wild type(WT),the transgenic tomato lines overexpressing MdEXLB1 had lower plant height,earlier fruiting period,fewer days for fruit ripening,higher fruit maturity,lower fruit firmness,higher fruit expansin activity,more discrete flesh cell structure,and accelerated fruit ripening process.Overall,this is the first study to propose that the apple EXLB subfamily gene,MdEXLB1,has biological functions and plays an important role in promoting fruit ripening and softening.

Efficiency of potassium-solubilizing Paenibacillus mucilaginosus for the growth of apple seedling

Chemical potassium(K)fertilizer is commonly used in apple(Malus domestica L.Borkh)production but K is easily fixed by soil,resulting in reduced K ferilizer utilization and wasted resources.K-solubilizing bacteria(KSB)can cost-effectively increase the soluble K content in rhizosphere soil.Therefore,the objectives were to select high-efficiency KSB from apple orchards under various soil management models and evaluate their effects on apple seedling growth.Maize(Zea maysL.)straw mulching(MSM)increased the total carbon(TC),total nitrogen(TN)and available potassium(AK)in the rhizosphere and improved fruit quality.The number of KSB in the rhizosphere soil of MSM was 9.5×10×CFU g1 soil,which was considerably higher than that in the other mulching models.Fourteen KSB strains were isolated with relative K solubilizing ability ranging from 17 to 30%,and five strains increased the dry weight per apple seedling.The most eficient strain was identified as Paenibacillus mucilaginosus through morphological observation and sequence analysis of 16S rDNA,named JGK.After inoculation,the colonization of JGK in soil decreased from 4.0 to 1.5×10×CFU g^-1 soil within 28 d.The growth of the apple seedlings and the K accumulation in apple plants were promoted by irigation with 50 mL JGK bacterial solution(1×10^9 CFU mL^-1),but there was no significant increase in the AK content of rhizosphere soil.High-performance liquid phase analysis(HPLC)data showed that the JGK metabolites contained phytohormones and organic acids.Hence,the JGK strain promoted the growth of two-month-old apple seedlings by stimulating function of the produced phytohormones and enhanced K solubility by acidification for apple seedling uptake.This study enriches the understanding of KSB and provides an effective means to increase the K utilization efficiency of apple production.