•The spontaneous, gain-of-function alleles rin and nor function additively in repressing tomato fruit ripening.•The spontaneous Cnr mutant allele is epistatic to rin and nor mutations.•The function ...of the genes and their spontaneous mutant alleles need to be evaluated separately, as the latter produced gain-of-function alleles.
Tomato fruit ripening is regulated by transcription factors (TFs), their downstream effector genes, and the ethylene biosynthesis and signalling pathway. Spontaneous non-ripening mutants ripening inhibitor (rin), non-ripening (nor) and Colorless non-ripening (Cnr) correspond with mutations in or near the TF-encoding genes MADS-RIN, NAC-NOR and SPL-CNR, respectively. Here, we produced heterozygous single and double mutants of rin, nor and Cnr and evaluated their functions and genetic interactions in the same genetic background. We showed how these mutations interact at the level of phenotype, individual effector gene expression, and sensory and quality aspects, in a dose-dependent manner. Rin and nor have broadly similar quantitative effects on all aspects, demonstrating their additivity in fruit ripening regulation. We also found that the Cnr allele is epistatic to rin and nor and that its pleiotropic effects on fruit size and volatile production, in contrast to the well-known dominant effect on ripening, are incompletely dominant, or recessive.
Kiwifruit are classified as climacteric since exogenous ethylene (or its analogue propylene) induces rapid ripening accompanied by ethylene production under positive feedback regulation. However, ...most of the ripening-associated changes (Phase 1 ripening) in kiwifruit during storage and on-vine occur largely in the absence of any detectable ethylene. This ripening behavior is often attributed to basal levels of system I ethylene, although it is suggested to be modulated by low temperature.
To elucidate the mechanisms regulating Phase 1 ripening in kiwifruit, a comparative transcriptome analysis using fruit continuously exposed to propylene (at 20 °C), and during storage at 5 °C and 20 °C was conducted. Propylene exposure induced kiwifruit softening, reduction of titratable acidity (TA), increase in soluble solids content (SSC) and ethylene production within 5 days. During storage, softening and reduction of TA occurred faster in fruit at 5 °C compared to 20 °C although no endogenous ethylene production was detected. Transcriptome analysis revealed 3761 ripening-related differentially expressed genes (DEGs), of which 2742 were up-regulated by propylene while 1058 were up-regulated by low temperature. Propylene exclusively up-regulated 2112 DEGs including those associated with ethylene biosynthesis and ripening such as AcACS1, AcACO2, AcPL1, AcXET1, Acβ-GAL, AcAAT, AcERF6 and AcNAC7. Similarly, low temperature exclusively up-regulated 467 DEGS including AcACO3, AcPL2, AcPMEi, AcADH, Acβ-AMY2, AcGA2ox2, AcNAC5 and AcbZIP2 among others. A considerable number of DEGs such as AcPG, AcEXP1, AcXET2, Acβ-AMY1, AcGA2ox1, AcNAC6, AcMADS1 and AcbZIP1 were up-regulated by either propylene or low temperature. Frequent 1-MCP treatments failed to inhibit the accelerated ripening and up-regulation of associated DEGs by low temperature indicating that the changes were independent of ethylene. On-vine kiwifruit ripening proceeded in the absence of any detectable endogenous ethylene production, and coincided with increased expression of low temperature-responsive DEGs as well as the decrease in environmental temperature.
These results indicate that kiwifruit possess both ethylene-dependent and low temperature-modulated ripening mechanisms that are distinct and independent of each other. The current work provides a foundation for elaborating the control of these two ripening mechanisms in kiwifruit.
Recently, DNA methylation was proposed to regulate fleshy fruit ripening. Fleshy fruits can be distinguished by their ripening process as climacteric fruits, such as tomatoes, or non-climacteric ...fruits, such as strawberries. Tomatoes undergo a global decrease in DNA methylation during ripening, due to increased expression of a DNA demethylase gene. The dynamics and biological relevance of DNA methylation during the ripening of non-climacteric fruits are unknown.
Here, we generate single-base resolution maps of the DNA methylome in immature and ripe strawberry. We observe an overall loss of DNA methylation during strawberry fruit ripening. Thus, ripening-induced DNA hypomethylation occurs not only in climacteric fruit, but also in non-climacteric fruit. Application of a DNA methylation inhibitor causes an early ripening phenotype, suggesting that DNA hypomethylation is important for strawberry fruit ripening. The mechanisms underlying DNA hypomethylation during the ripening of tomato and strawberry are distinct. Unlike in tomatoes, DNA demethylase genes are not upregulated during the ripening of strawberries. Instead, genes involved in RNA-directed DNA methylation are downregulated during strawberry ripening. Further, ripening-induced DNA hypomethylation is associated with decreased siRNA levels, consistent with reduced RdDM activity. Therefore, we propose that a downregulation of RdDM contributes to DNA hypomethylation during strawberry ripening.
Our findings provide new insight into the DNA methylation dynamics during the ripening of non-climacteric fruit and suggest a novel function of RdDM in regulating an important process in plant development.
Background and Aims
The grape ripening disorder called berry shrivel is characterised by cessation of sugar accumulation, low pH, impaired anthocyanin biosynthesis in dark‐skinned cultivars and water ...loss. We tested whether damage to the bunch peduncle vasculature can induce berry shrivel and whether cell death in the rachis precedes cell death in the berries.
Methods and Results
Peduncle phloem removal, but not xylem removal, resulted in symptoms resembling berry shrivel. Fluorescein diacetate staining, however, and microscopic observations of non‐manipulated bunches showed that cell death in the berries often preceded that in the rachis. Mesocarp cell death started around the central vasculature proximal to the seeds and spread outward as symptoms worsened. Berries near the bunch tip were the earliest to be affected and developed the most severe symptoms, but ripening of all berries on symptomatic bunches was impaired.
Conclusions
Loss of functionality of the rachis phloem may not be the primary cause of berry shrivel. The disorder may originate in the distal berries of a bunch and spread towards the proximal berries as they soften at the onset of ripening.
Significance of the Study
This study contributes towards an eventual understanding of the causes and consequences of an important grape ripening disorder.
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