Different regulatory mechanisms of plant hormones in the ripening of climacteric and non-climacteric fruits: a review

This review contains the regulatory mechanisms of plant hormones in the ripening process of climacteric and non-climacteric fruits, interactions between plant hormones and future research directions. The fruit ripening process involves physiological and biochemical changes such as pigment accumulation, softening, aroma and flavor formation. There is a great difference in the ripening process between climacteric fruits and non-climacteric fruits. The ripening of these two types of fruits is affected by endogenous signals and exogenous environments. Endogenous signaling plant hormones play an important regulatory role in fruit ripening. This paper systematically reviews recent progress in the regulation of plant hormones in fruit ripening, including ethylene, abscisic acid, auxin, jasmonic acid (JA), gibberellin, brassinosteroid (BR), salicylic acid (SA) and melatonin. The role of plant hormones in both climacteric and non-climacteric fruits is discussed, with emphasis on the interaction between ethylene and other adjustment factors. Specifically, the research progress and future research directions of JA, SA and BR in fruit ripening are discussed, and the regulatory network between JA and other signaling molecules remains to be further revealed. This study is meant to expand the understanding of the importance of plant hormones, clarify the hormonal regulation network and provide a basis for targeted manipulation of fruit ripening.

The postharvest ripening of strawberry fruits induced by abscisic acid and sucrose differs from their in vivo ripening

In this study, we compared the chemical composition of strawberry (Fragaria × ananassa) fruits that were ripened in vivo (attached to plant) to those ripened during postharvest storage. The effects of the application of abscisic acid (ABA) and sucrose on the postharvest ripening were also evaluated. The results suggested that the postharvest ripening process was dependent on the signal triggered by ABA and differed from in vivo ripening, resulting in fruits with altered chemical composition and firmness. The application of sucrose in unripe strawberries resulted in the induction of ripening, which is dependent on ABA and its derivatives. This induction was more pronounced during the first days of storage and associated with the application of mannitol rather than water, suggesting that mannitol negatively regulated the postharvest strawberry ripening. These results provide further insights into the role of ABA and sucrose in the regulation of postharvest ripening of strawberry.

Analysis of eight phytohormone concentrations, expression levels of ABA biosynthesis genes, and ripening-related transcription factors during fruit development in strawberry

The contents of eight phytohormones and the expression levels of genes encoding enzymes related to abscisic acid (ABA) biosynthesis and deactivation/degradation and transcription factors (TFs) related to fruit ripening were studied in the non-climacteric strawberry fruit (Fragaria × ananassa Duch., cv. 'Seolhyang') at six developmental stages. The hormones tested were ABA, indole-3-acetic acid (IAA), gibberellic acid 4 (GA₄), jasmonic acid (JA), methyljasmonate (MJ), jasmonoyl isoleucine (JA-Ile), salicylic acid (SA), and ethylene (ET). The developmental and ripening stages studied were small green (S1, 11 days post-anthesis, DPA), green (S2, 20 DPA), breaker (S3, 24 DPA), pink (S4, 27 DPA), red (S5, 31 DPA), and fully red (S6, 40 DPA). IAA and GA₄ contents were highest at S1 and gradually decreased after this stage. ABA content was low at S1-S3 and then increased rapidly until peaking at S6. By contrast, MJ content showed no significant changes over time, while SA content gradually increased. JA, JA-Ile, and ET contents were either insufficient for quantification or undetectable. Expression of the ABA biosynthesis genes FaNCED1 and FaABA2 increased during fruit ripening, whereas expression of the ABA deactivation/degradation genes FaUGT75C1 and FaCYP707A1 was high early in development, when ABA content was low, and then decreased. Among four ripening-related TF genes, FaMYB1, FaMYB5, FaMYB10, and FaASR, only the expression of FaMYB10 seemed to be closely related to strawberry fruit ripening. Our study supports the idea that ABA and FaMYB10 appear to be the key hormone and TF regulating strawberry ripening.

Ethylene receptors and related proteins in climacteric and non-climacteric fruits

Fruits have been traditionally classified into two categories based on their capacity to produce and respond to ethylene during ripening. Fruits whose ripening is associated to a peak of ethylene production and a respiration burst are referred to as climacteric, while those that are not are referred to as non-climacteric. However, an increasing body of literature supports an important role for ethylene in the ripening of both climacteric and non-climacteric fruits. Genome and transcriptomic data have become available across a variety of fruits and we leverage these data to compare the structure and transcriptional regulation of the ethylene receptors and related proteins. Through the analysis of four economically important fruits, two climacteric (tomato and apple), and two non-climacteric (grape and citrus), this review compares the structure and transcriptional regulation of the ethylene receptors and related proteins in both types of fruit, establishing a basis for the annotation of ethylene-related genes. This analysis reveals two interesting differences between climacteric and non-climacteric fruit: i) a higher number of ETR genes are found in climacteric fruits, and ii) non-climacteric fruits are characterized by an earlier ETR expression peak relative to sugar accumulation.

Ethylene receptors and related proteins in climacteric and non-climacteric fruits

Fruits have been traditionally classified into two categories based on their capacity to produce and respond to ethylene during ripening. Fruits whose ripening is associated to a peak of ethylene production and a respiration burst are referred to as climacteric, while those that are not are referred to as non-climacteric. However, an increasing body of literature supports an important role for ethylene in the ripening of both climacteric and non-climacteric fruits. Genome and transcriptomic data have become available across a variety of fruits and we leverage these data to compare the structure and transcriptional regulation of the ethylene receptors and related proteins. Through the analysis of four economically important fruits, two climacteric (tomato and apple), and two non-climacteric (grape and citrus), this review compares the structure and transcriptional regulation of the ethylene receptors and related proteins in both types of fruit, establishing a basis for the annotation of ethylene-related genes. This analysis reveals two interesting differences between climacteric and non-climacteric fruit: i) a higher number of ETR genes are found in climacteric fruits, and ii) non-climacteric fruits are characterized by an earlier ETR expression peak relative to sugar accumulation.

Abscisic acid stimulates anthocyanin accumulation in 'Jersey' highbush blueberry fruits during ripening

Non-climacteric blueberry (Vaccinium spp.) fruits accumulate high levels of anthocyanins during ripening, which are a good source of dietary antioxidants. This study examined the effects of exogenous abscisic acid (ABA) application on fruit characteristics and anthocyanin accumulation in a northern highbush blueberry (V. corymbosum 'Jersey') during development. Fruits on shrubs were treated with 1gL^-1 ABA before the initiation of fruit colouration. Application of ABA temporarily increased the level of ABA in the fruits during development. Exogenous ABA had no obvious effect on fruit growth, but stimulated fruit colouration by accelerating the accumulation of individual anthocyanins, mainly malvidin, delphinidin and petunidin glycosides. This is the first report to show that ABA promotes the accumulation of anthocyanins in blueberry fruits. However, exogenous ABA also promoted fruit softening, which is undesirable during harvest and shelf life.

The contribution of flowering time and seed content to uneven ripening initiation among fruits within Vitis vinifera L. cv. Pinot noir clusters

Ripening initiation-associated hormonal changes and sugar accumulation for individual fruits differed by seed content and did not depend heavily on flowering time or duration from anthesis to clusters' onset of ripening. For Vitis vinifera, the ripening initiation of individual fruits in a cluster occurs unevenly. This developmental period is called véraison. Why individual fruits initiate ripening at different times is not well studied, though differences in seed content and unequal developmental durations that arise from asynchronous flowering within a cluster have been proposed. This study examined how much both variables contribute to individual fruits' ripening progress by mid-véraison, when half of berries in a cluster have initiated ripening, and whether either or both factors affect the timing of characteristic, ripening-initiation associated changes in abscisic acid and auxin before, at, and after véraison. Overall, developmental duration and flowering time did not sufficiently explain how far berries had progressed into the ripening stage because fruits did not require a fixed amount of time to initiate ripening. Fruits from early and late flowers but of similar seed content were able to initiate ripening at the same time despite differences in chronological age. This suggests either an early developmental enhancement occurred for late-initiated fruits or that flowering time is an inappropriate "day zero". Ultimately, only seed content was linked to the timing and magnitude of ripening-related hormone changes, supporting that seeds have a comparatively larger influence than flowering time on the ripening initiation of individual berries. More specifically, if the fraction of berry weight occupied by seed was high, then the initiation of ripening for that berry and its associated hormone changes were delayed relative to berries with less seed weight versus total berry weight.