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

Abstract

BackgroundApple 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0375-3) contains supplementary material, which is available to authorized users.

Identification of QTLs for Flesh Mealiness in Apple (<i>Malus</i> × <i>domestica</i> Borkh.)

“…It has been suggested that MdAF3 is involved in mealiness (Nobile et al, 2011). However, Segonne et al (2014) did not observe consistent MdAF3 expression profiles using three mealy/non-mealy pairs for comparison. The MdPG1 expression level increased in mealy fruit only after four months of cold storage, suggesting that it is involved in the process during the later storage stages (Segonne et al, 2014).…”

“…The second most significant SNPs, SNP_FB_0832810 and SNP_FB_0832819, were located on the contig flanking the MdPG1. MdPG1 weakens the cellular adhesion in fruit (Atkinson et al, 2012) and could be involved in mealiness development during prolonged storage (Segonne et al, 2014). As our storage conditions were sufficient for expression of MdPG1 (20°C, 4 weeks), the MdPG1 may be the causal gene underlying QTL on chr 10, detected both in the QTL analysis and GWAS.…”

“…However, Segonne et al (2014) did not observe consistent MdAF3 expression profiles using three mealy/non-mealy pairs for comparison. The MdPG1 expression level increased in mealy fruit only after four months of cold storage, suggesting that it is involved in the process during the later storage stages (Segonne et al, 2014). The transgenic approach has clarified the involvement of MdPG1 in the evolution of mealiness.…”

“…A global transcriptomic approach has revealed a role of MdPME2 in the development of this feature (Segonne et al, 2014). The expression level of MdPME2 in fruit of mealy genotypes are lower than those of non-mealy genotypes before and at harvest in addition to after 2 months of storage, suggesting that this gene has a role in stiffening the cell wall.…”

“…Simple cell separation data can be obtained by measuring the loss of weight after shaking fruit discs in a sucrose solution (Iwanami et al, 2005;Motomura et al, 2000) and by counting cells isolated from the flesh after vortexing (Segonne et al, 2014). The reliability of these two methods has been confirmed by comparisons with sensory analyses (Motomura et al, 2000;Segonne et al, 2014). Both methods have advantages when dealing with hundreds of samples and examining the evolution of mealiness after the softening reaches a plateau.…”