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.

苹果果实绵化（apple fruit mealiness）是贮藏期间成熟过程异常所引发的最关键质地问题之一。该表型以质地劣变为特征，表现为果实发软、呈颗粒状且质地干涩。尽管已有多项相关研究，但学界对绵化的发生机制及其关联分子事件仍知之甚少。本研究整合表型分析、显微观察、转录组学与生化分析手段，以解析苹果果实绵化发生的分子基础。

结果 仪器化质地表征工作细化了苹果果实绵化的定义。与此同时，本研究开发出一种全新且简便的定量检测方法，用于评估该表型。从某杂交后代群体中筛选出6个绵化程度差异显著的个体，用于开展果实发育与冷藏过程中的全局转录组分析，最终鉴定出与绵化起始相关的潜在候选基因。其中，一条早期下调的转录本（与拟南芥果胶甲酯酶基因AtPME2同源）的表达谱与绵化发生过程高度匹配。对栽培苹果（Malus × domestica）果胶甲酯酶基因MdPME2的生物信息学分析证实，相较于其他已鉴定的MdPME家族基因，该基因的表达模式具有特异性。蛋白融合实验显示，MdPME2可分泌至质外体（apoplast），这与其可能作用于果胶结构的功能相符。进一步显微分析表明，绵化苹果果实的细胞间黏附性逐渐丧失。生化分析揭示，绵化果实的果胶残基发生特异性修饰，但果胶的甲酯化程度并未出现全局变化。

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塑的作用，并提示此类酶的作用具有局部性。苹果果实绵化可能并非源于果胶含量的整体差异，而是由果胶微观结构的定性与空间分布变化所介导，且这类变化可能在果实发育早期即已发生。本研究中鉴定得到的特异性MdPME2基因，有望成为评估苹果果实质地不良性状的早期分子标志物。