Effect of fruit maturity on UV-B-induced post-harvest anthocyanin accumulation in red Chinese sand pear

The effect of fruit maturity on UV-B-induced post-harvest anthocyanin accumulation in red Chinese sand pear (Pyrus pyrifolia Nakai) cultivar ‘Mantianhong’ was evaluated. During the irradiation, compared with the fruit harvested at 20 days before harvest (DBH) and 10 DBH, the mature fruit (harvested at commercial harvest date) had higher soluble solids content, soluble sugars concentration but lower firmness and starch content. In addition, higher content of anthocyanin has been detected in mature fruits than in immature fruits due to the significant increase in the expression of genes related to anthocyanin biosynthesis, especially PpCHS, PpF3H, PpANS, PpUFGT, PyMYB10 and PpbHLH in red Chinese sand pears. Hierarchical clustering analysis suggested that most genes related to anthocyanin biosynthesis showed a coordinate expression pattern. These findings are helpful in understanding the molecular mechanism of anthocyanin biosynthesis and regulation, which could lead to the development of new technologies for improving fruit color in Chinese sand pears and other fruits.     

Abscisic acid plays an important role in the regulation of strawberry fruit ripening

The plant hormone abscisic acid (ABA) has been suggested to play a role in fruit development, but supporting genetic evidence has been lacking. Here, we report that ABA promotes strawberry (Fragaria ananassa) fruit ripening. Using a newly established Tobacco rattle virus-induced gene silencing technique in strawberry fruit, the expression of a 9-cis-epoxycarotenoid dioxygenase gene (FaNCED1), which is key to ABA biosynthesis, was down-regulated, resulting in a significant decrease in ABA levels and uncolored fruits. Interestingly, a similar uncolored phenotype was observed in the transgenic RNA interference (RNAi) fruits, in which the expression of a putative ABA receptor gene encoding the magnesium chelatase H subunit (FaCHLH/ABAR) was down-regulated by virus-induced gene silencing. More importantly, the uncolored phenotype of the FaNCED1-down-regulated RNAi fruits could be rescued by exogenous ABA, but the ABA treatment could not reverse the uncolored phenotype of the FaCHLH/ABAR-down-regulated RNAi fruits. We observed that down-regulation of the FaCHLH/ABAR gene in the RNAi fruit altered both ABA levels and sugar content as well as a set of ABA- and/or sugar-responsive genes. Additionally, we showed that exogenous sugars, particularly sucrose, can significantly promote ripening while stimulating ABA accumulation. These data provide evidence that ABA is a signal molecule that promotes strawberry ripening and that the putative ABA receptor, FaCHLH/ABAR, is a positive regulator of ripening in response to ABA.     

Effect of simultaneous down-regulation of pectate lyase and endo-β-1,4-glucanase genes on strawberry fruit softening

Strawberry is a soft fruit with a short postharvest shelf-life. The loss of fruit firmness during ripening is mainly due to the disassembly of parenchyma cell walls mediated by the expression of genes encoding enzymes acting on pectins, such as pectate lyase, or hemicellulose, e.g. endo-β-1,4-glucanase. To determine if the simultaneous down-regulation of FaplC and FaEG3 genes, encoding a pectate lyase and a endo-β-1,4-glucanase, respectively, exerted an additive effect on strawberry softening, transgenic plants expressing tandem antisense sequences of both genes under the control of the constitutive promoter CaMV35S were generated. Fifteen independent transgenic lines were obtained and fruit yields and several quality parameters of transgenic ripe fruit were recorded during two consecutive years. Fruit yield was reduced in most of the lines, especially in the first evaluation period, and five out of 15 lines (33 %) did not set fruit. The expression of FaplC and FaEG3 genes was measured in ripe fruits from six selected lines showing the highest fruit yields. All selected lines showed a high level of FaplC gene silencing, ranging from 97 to 71 %; however, FaEG3 gene expression was only significantly down-regulated in two lines. Fruit colour and soluble solids contents were similar in control and transgenic ripe fruits, while fruit weight was slightly lower than control in some of the lines. In all lines, transgenic fruits were significantly firmer than control, with an increase in firmness ranging from 19 to 32 %. The reduction of fruit softening in transgenic fruits was not correlated with the suppression of FaEG3 gene expression, and lines with the highest simultaneous down-regulation of FaplC and FaEG3 showed similar fruit firmness to lines where only FaplC was suppressed. These results indicate that pectate lyase and endo-β-1,4-glucanase do not act in an additive or synergistic way during strawberry softening, and question the role of glucanases in this process.     

Detachment-accelerated ripening and senescence of strawberry (Fragaria × ananassa Duch. cv. Akihime) fruit and the regulation role of multiple phytohormones

To study the detachment stress on the ripeness of strawberry fruit, physiological characteristics of strawberry fruit on and off plant during ripeness and senescence processes were investigated. The results indicated that the ripeness of strawberry fruit upon detachment was accelerated, in terms of firmness, soluble solid content and especially color development. The color of fruit off plant changed rapidly from white to full red in 1–2 days. The respiratory rate in fruit off plant was strengthened, higher than that on plant. Abscisic acid level and ethylene production in fruit off plant were also higher than those on plant and auxin degradation was exacerbated by detachment. Expression levels of FaMYB1, FabHLH3 and FaTTG1 were generally reduced with phenotypes of redder color and more anthocyanin accumulation in fruit off plant. Results also suggested that the detachment initially stimulated ethylene and abscisic acid production and auxin degradation, which modulated ripening-related gene expression and at last enhanced fruit pigmentation.     

5-Aminolevulinic acid promotes anthocyanin accumulation in Fuji apples

5-Aminolevulinic acid (ALA) is an essential precursor of all tetrapyrrole compounds such as chlorophylls and heme in plants. It has also been suggested widely for applications to crops to enhance growth and production as a plant growth regulator. However, how successful ALA can be used in fruit production was rarely reported. We conducted a field experiment at eight locations in four provinces across eastern China; and the results showed that application of ALA solutions to ‘Fuji’ apple (Malus × domestica Borkh.) fruits 20 days prior to harvest significantly increased the anthocyanin content in the fruit skin. Also, ALA treatment increased the anthocyanin content of the detached apple skin in a growth chamber. Results from the semi-quantitive RT-PCR analysis showed that ALA induced gene expressions related to anthocyanin biosynthesis, including the structural genes Pal, Chs and Ufgt, and regulatory genes Myb, bHLH and Wd40. When levulinic acid (LA), an inhibitor of ALA dehydrase, was added, ALA promotion of anthocyanin accumulation and up-regulation of gene expressions were inhibited. Taken together, these results suggest that ALA promotion of anthocyanin accumulation in apples was facilitated by the up-regulation of gene expression, which might be related to the conversion of ALA to porphyrins.     

A UDP‐glucosyltransferase functions in both acylphloroglucinol glucoside and anthocyanin biosynthesis in strawberry (Fragaria × ananassa)

Physiologically active acylphloroglucinol (APG) glucosides were recently found in strawberry (Fragaria sp.) fruit. Although the formation of the APG aglycones has been clarified, little is known about APG glycosylation in plants. In this study we functionally characterized ripening‐related glucosyltransferase genes in Fragaria by comprehensive biochemical analyses of the encoded proteins and by a RNA interference (RNAi) approach in vivo. The allelic proteins UGT71K3a/b catalyzed the glucosylation of diverse hydroxycoumarins, naphthols and flavonoids as well as phloroglucinols, enzymatically synthesized APG aglycones and pelargonidin. Total enzymatic synthesis of APG glucosides was achieved by co‐incubation of recombinant dual functional chalcone/valerophenone synthase and UGT71K3 proteins with essential coenzyme A esters and UDP‐glucose. An APG glucoside was identified in strawberry fruit which has not yet been reported in other plants. Suppression of UGT71K3 activity in transient RNAi‐silenced fruits led to a loss of pigmentation and a substantial decrease of the levels of various APG glucosides and an anthocyanin. Metabolite analyses of transgenic fruits confirmed UGT71K3 as a UDP‐glucose:APG glucosyltransferase in planta. These results provide the foundation for the breeding of fruits with improved health benefits and for the biotechnological production of bioactive natural products.