Data from: Multiscale investigation of mealiness in apple: an atypical role for a pectin methylesterase during fruit maturation

Apple fruit mealiness is one of the most important textural problems that results from an undesirable ripening process during storage. This phenotype is characterized by textural deterioration described as soft, grainy and dry fruit. Despite several studies, little is known about mealiness development and the associated molecular events. In this study, we integrated phenotypic, microscopic, transcriptomic and biochemical analyses to gain insights into the molecular basis of mealiness development.ResultsInstrumental texture characterization allowed the refinement of the definition of apple mealiness. In parallel, a new and simple quantitative test to assess this phenotype was developed.Six individuals with contrasting mealiness were selected among a progeny and used to perform a global transcriptome analysis during fruit development and cold storage. Potential candidate genes associated with the initiation of mealiness were identified. Amongst these, the expression profile of an early down-regulated transcript similar to an Arabidopsis thaliana pectin methylesterase gene (AtPME2) matched with mealiness development. In silico analyses of this Malus x domestica PME gene (MdPME2) confirmed its specific pattern compared with all other identified MdPME genes. Protein fusion experiments showed that MdPME2 is secreted into the apoplast in accordance with a possible activity on pectin structure. Further microscopic analysis indicated a progressive loss of cell to cell adhesion in mealy apple fruits. Biochemical analysis revealed specific modifications of pectin residues associated with mealiness, without global changes in the degree of methylesterification of pectins.ConclusionsThese data support the role of PME in cell wall remodelling during apple fruit development and ripening and suggest a local action of these enzymes. Mealiness may partially result from qualitative and spatial variations of pectin microarchitecture rather than quantitative pectin differences, and these changes may occur early in fruit development. The specific MdPME2 gene highlighted in this study could be a good early marker of texture unfavourable trait in apple.苹果果实发绵（apple fruit mealiness）是贮藏期间由异常成熟过程引发的最主要的质地劣变问题之一。该性状的特征为质地劣变，表现为果实绵软、粉质且干涩。尽管已有多项相关研究，但目前对发绵性状的发生过程及其关联分子事件的认知仍十分有限。本研究整合了表型分析、显微观察、转录组学分析与生化分析手段，以解析苹果果实发绵性状发生的分子基础。 结果 通过仪器化质地表征，研究完善了苹果发绵性状的定义。与此同时，本研究开发了一种全新且简便的量化检测方法，用于评估该性状。研究从一个杂交后代群体中筛选出6个发绵程度差异显著的单株，在果实发育与冷藏过程中对其开展了全转录组分析。研究鉴定出了与发绵性状起始相关的潜在候选基因。其中，一个早期下调的转录本，其序列与拟南芥果胶甲酯酶基因（AtPME2）高度相似，该转录本的表达模式与发绵性状的发生过程相吻合。对该苹果（Malus × domestica）果胶甲酯酶基因MdPME2的生物信息学分析证实，相较于所有已鉴定的其他苹果果胶甲酯酶基因，MdPME2具有独特的表达模式。蛋白融合实验结果显示，MdPME2可被分泌至质外体（apoplast），这与其可能参与果胶结构修饰的功能相符。进一步的显微观察表明，发绵苹果果实中细胞间的黏附性呈进行性丧失。生化分析结果显示，与发绵性状相关的果胶残基发生了特异性修饰，但果胶的甲酯化程度并未出现整体变化。 结论 本研究数据证实了果胶甲酯酶（pectin methylesterase, PME）在苹果果实发育与成熟过程中参与细胞壁重构的作用，并表明这类酶的作用具有局部性。苹果发绵性状可能部分源于果胶微观结构的定性与空间分布变化，而非果胶含量的数量差异，且这类变化可能在果实发育早期就已发生。本研究中重点关注的MdPME2基因，可作为评估苹果质地劣变不良性状的良好早期标志物。