It has long been recognised that ethylene plays a major role in the ripening
process of climacteric fruit. A more thorough analysis, however, has revealed that a
number of biochemical and molecular processes associated with climacteric fruit
ripening are ethylene-independent. One of the crucial steps of the onset of ripening
is the induction of autocatalytic ethylene production. In ethylene-suppressed melons,
ACC synthase activity is induced at the same time as in control melons, indicating
that ACC biosynthesis during the early stages of ripening seems to be a
developmentally-regulated (ethylene-independent) process. The various ripening
events exhibit differential sensitivity to ethylene. For instance, the threshold level for
degreening of the rind is 1ppm, while 2.5 ppm are required to trigger some
components of the softening process. The saturating level of ethylene producing
maximum effects is less than 5 ppm, which is by far lower than the internal ethylene
concentrations found in the fruit at the climacteric peak (over 100 ppm). In many
fruit chilling temperatures hasten ethylene production and ripening and in some late
season pear varieties, exposure to chilling temperatures is even absolutely required
for the attainment of the capacity to synthesize autocatalytic ethylene. This is
correlated with the stimulation of expression of ACC oxidase and of members of the
ACC synthase gene family. Ethylene operates via a perception and transduction
pathway to induce the expression of genes responsible for the biochemical and
physiological changes observed during ripening. However, only a few genes induced
via the ethylene transduction pathway have been described so far. We have used a
differential display method to isolate novel ethylene-reponsive (ER) cDNA clones of
tomato that potentially play a role in propagating the ethylene response and in
regulating fruit ripening. Collectively, these data permit a general scheme of the
molecular mechanisms of fruit ripening to be proposed.
Summary Soluble sugars, organic acids and volatiles are important components that determine unique fruit flavor and consumer preferences. However, the metabolic dynamics and underlying regulatory networks that modulate overall flavor formation during fruit development and ripening remain largely unknown for most fruit species. In this study, by integrating flavor‐associated metabolism and transcriptome data from 12 fruit developmental and ripening stages of Actinidia chinensis cv Hongyang, we generated a global map of changes in the flavor‐related metabolites throughout development and ripening of kiwifruit. Using this dataset, we constructed complex regulatory networks allowing to identify key structural genes and transcription factors that regulate the metabolism of soluble sugars, organic acids and important volatiles in kiwifruit. Moreover, our study revealed the regulatory mechanism involving key transcription factors regulating flavor metabolism. The modulation of flavor metabolism by the identified key transcription factors was confirmed in different kiwifruit species providing the proof of concept that our dataset provides a suitable tool for clarification of the regulatory factors controlling flavor biosynthetic pathways that have not been previously illuminated. Overall, in addition to providing new insight into the metabolic regulation of flavor during fruit development and ripening, the outcome of our study establishes a foundation for flavor improvement in kiwifruit.
While grapes have been classified as a non-climacteric fruit, we show here that endogenous ethylene production just before veraison is required for an increase in berry size and possibly for anthocyanin accumulation in the ripening berry. Our data also show that the peak of ethylene production just prior to veraison is associated with increased accumulation of ACC oxidase mRNAs, enhanced ACC oxidase activity and higher concentrations of malonyl-ACC. Exposure of clusters to 1-MCP at various times before and after veraison inhibited ripening only in fruit treated at the time of the ethylene peak. Lastly, we observed some feed-back at the ethylene perception level and this response is discussed in relationship to the behaviour of non-climacteric plant tissues.
Summary Tomato ( Solanum lycopersicum L.) plants are cold‐sensitive, and the fruit are susceptible to postharvest chilling injury when stored at low temperature. However, the mechanisms underlying cold stress responses in tomato are poorly understood. We demonstrate that SlGRAS4 , encoding a transcription factor induced by low temperature, promotes chilling tolerance in tomato leaves and fruit. Combined genome‐wide ChIP‐seq and RNA‐seq approaches identified among cold stress‐associated genes those being direct targets of SlGRAS4 and protein studies revealed that SlGRAS4 forms a homodimer to self‐activate its own promoter. SlGRAS4 can also directly bind tomato SlCBF promoters to activate their transcription without inducing any growth retardation. The study identifies the SlGRAS4‐regulon as a new cold response pathway conferring cold stress tolerance in tomato independently of the ICE1‐CBF pathway. This provides new track for breeding strategies aiming to improve chilling tolerance of cultivated tomatoes and to preserve sensory qualities of tomato fruit often deteriorated by storage at low temperatures.
The hormone ethylene regulates a wide range of plant developmental processes and EBF (EIN3-binding F-box) proteins were shown to negatively regulate the ethylene signalling pathway via mediating the degradation of EIN3/EIL proteins. The present study reports on the identification of two tomato F-box genes, Sl-EBF1 and Sl-EBF2 from the EBF subfamily. The two genes display contrasting expression patterns in reproductive and vegetative tissues and in response to ethylene and auxin treatment. Sl-EBF1 and Sl-EBF2 genes are actively regulated at crucial stages in the development of the reproductive organs. Their dynamic expression in flowers during bud-to-anthesis and anthesis-to-post-anthesis transitions, and at the onset of fruit ripening, suggests their role in situations where ethylene is required for stimulating flower opening and triggering fruit ripening. VIGS-mediated silencing of a single tomato EBF gene uncovered a compensation mechanism that tends to maintain a threshold level of Sl-EBF expression via enhancing the expression of the second Sl-EBF gene. In line with this compensation, tomato plants silenced for either of the Sl-EBF genes were indistinguishable from control plants, indicating functional redundancy among Sl-EBF genes. By contrast, co-silencing of both Sl-EBFs resulted in ethylene-associated phenotypes. While reports on EBF genes to date have focused on their role in modulating ethylene responses in Arabidopsis, the present study uncovered their role in regulating crucial stages of flower and fruit development in tomato. The data support the hypothesis that protein degradation via the ubiquitin/26S proteasome pathway is a control point of fruit ripening and open new leads for engineering fruit quality.
Immunocytological studies have previously shown that 1-aminocyclopropane-l-carboxylate oxidase (ACO), the enzyme which catalyses the last step of ethylene biosynthesis, is located in the cell wall of apple and tomato fruit cells. In the present study, a combination of cell fractionation and immunocytological methods have been used in order to determine a precise location within this space. Western blotting assays indicated that more than 70% of ACO antigens of the whole cell are recovered in freshly prepared protoplasts and that these ACO antigens are completely removed upon treatment of protoplasts with proteinase K. Immunocytolabelling showed a periplasmic ACO-antigen signal in protoplasts which is completely absent in proteinase K-treated protoplasts. Taken together, these data demonstrate that, in apple fruit, ACO is located at the external face of the plasma membrane. Possible interactions between the plasma membrane and ACO activity are discussed.
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