Melatonin is a multifunctional signaling molecule, ubiquitously distributed in different parts of plants and responsible for stimulating several physiological responses to adverse environmental conditions. In the current review, we showed that the biosynthesis of melatonin occurred in plants by themselves, and accumulation of melatonin fluctuated sharply by modulating its biosynthesis and metabolic pathways under stress conditions. Melatonin, with its precursors and derivatives, acted as a powerful growth regulator, bio-stimulator, and antioxidant, which delayed leaf senescence, lessened photosynthesis inhibition, and improved redox homeostasis and the antioxidant system through a direct scavenging of reactive oxygen species (ROS) and reactive nitrogen species (RNS) under abiotic and biotic stress conditions. In addition, exogenous melatonin boosted the growth, photosynthetic, and antioxidant activities in plants, confirming their tolerances against drought, unfavorable temperatures, salinity, heavy metals, acid rain, and pathogens. However, future research, together with recent advancements, would support emerging new approaches to adopt strategies in overcoming the effect of hazardous environments on crops and may have potential implications in expanding crop cultivation against harsh conditions. Thus, farming communities and consumers will benefit from elucidating food safety concerns.

Anthocyanin production in bicolored sweet cherry (Prunus avium cv. Rainier) fruit is induced by light exposure, leading to red coloration. The phytohormone abscisic acid (ABA) is essential for this process, but the regulatory relationships that link light and ABA with anthocyanin-associated coloration are currently unclear. In this study, we determined that light treatment of bicolored sweet cherry fruit increased anthocyanin accumulation and induced ABA production and that ABA participates in light-modulated anthocyanin accumulation in bicolored sweet cherry. Two B-box (BBX) genes, PavBBX6/9, were highly induced by light and ABA treatments, as was anthocyanin accumulation. The ectopic expression of PavBBX6 or PavBBX9 in Arabidopsis (Arabidopsis thaliana) increased anthocyanin biosynthesis and ABA accumulation. Overexpressing PavBBX6 or PavBBX9 in sweet cherry calli also enhanced light-induced anthocyanin biosynthesis and ABA accumulation. Additionally, transient overexpression of PavBBX6 or PavBBX9 in sweet cherry peel increased anthocyanin and ABA contents, whereas silencing either gene had the opposite effects. PavBBX6 and PavBBX9 directly bound to the G-box elements in the promoter of UDP glucose-flavonoid-3-O-glycosyltransferase (PavUFGT), a key gene for anthocyanin biosynthesis, and 9-cis-epoxycarotenoid dioxygenase 1 (PavNCED1), a key gene for ABA biosynthesis, and enhanced their activities. These results suggest that PavBBX6 and PavBBX9 positively regulate light-induced anthocyanin and ABA biosynthesis by promoting PavUFGT and PavNCED1 expression, respectively. Our study provides insights into the relationship between the light-induced ABA biosynthetic pathway and anthocyanin accumulation in bicolored sweet cherry fruit.

Drought stress substantially reduces the productivity of apple plants and severely restricts the development of apple industry. Malus sieversii, wild apples with excellent drought resistance, is a valuable wild resource for a rootstock improvement of cultivated apple (Malus domestica). miRNAs and their targets play essential roles in plant growth and stress responses, but their roles in drought stress responses in apple are unknown. Here, we demonstrate that microRNA156ab is upregulated in M. sieversii in response to drought stress. Overexpressing msi-miR156ab promoted auxin accumulation, maintained the growth of apple plants, and increased plant resistance to osmotic stress. Antioxidant enzyme activities and proline contents were also increased in miR156ab-OE transgenic apple lines, which improved drought resistance. The squamosa promoter binding protein-like transcription factor MsSPL13 is the target of msi-miR156ab, as demonstrated by 5′-RACE and dual luciferase assays. Heterologous expression of MsSPL13 decreased auxin contents and inhibited growth in Arabidopsis (Arabidopsis thaliana) under normal and stress conditions. The activities of antioxidant enzymes were also suppressed in MsSPL13-OE transgenic Arabidopsis, reducing drought resistance. We showed that MsSPL13 regulates the expression of the auxin-related genes MsYUCCA5, PIN-FORMED7 (MsPIN7), and Gretchen Hagen3-5 (MsGH3-5) by binding to the GTAC cis-elements in their promoters, thereby regulating auxin metabolism. Finally, we demonstrated that the miR156ab-SPL13 module is involved in mediating the difference in auxin metabolism and stress responses between M. sieversii and M26 (M. domestica) rootstocks. Overall, these findings reveal that the miR156ab-SPL13 module enhances drought stress tolerance in apples by regulating auxin metabolism and antioxidant enzyme activities.

Softening is a key step during fruit ripening that is modulated by the interplay between multiple phytohormones. The antagonistic action of abscisic acid (ABA) and auxin determines the rate of fruit ripening and softening. However, the transcription factors that integrate ABA and auxin signals to regulate fruit softening remain to be determined. In this study, we identified several DNA-binding with One Finger (Dof) transcription factors essential for ABA-promoted fruit softening, based on transcriptome analysis of two sweet cherry (Prunus avium L.) varieties with different fruit firmness. We show that PavDof6 directly binds to the promoters of genes encoding cell wall-modifying enzymes to activate their transcription, while PavDof2/15 directly repress their transcription. Transient overexpression of PavDof6 and PavDof2/15 in sweet cherry fruits resulted in precocious and delayed softening, respectively. In addition, we show that the auxin response factor PavARF8, the expression of whose encoding gene is repressed by ABA, activates PavDof2/15 transcription. Furthermore, PavDof2/6/15 and PavARF8 directly bind to the 9-cis-epoxycarotenoid dioxygenase 1 (PavNCED1) promoter and regulate its expression, forming a feedback mechanism for ABA-mediated fruit softening. These findings unveil the physiological framework of fruit softening and establish a direct functional link between the ABA–PavARF8–PavDofs module and cell-wall-modifying genes in mediating fruit softening.

Deep sequencing identified small non-coding RNAs involved in drought stress tolerance and a microRNA-target module that enhances drought stress tolerance in Mal

In this study, the fruit quality, anthocyanin content and antioxidant enzyme activities of skin and pulp of southern blueberries (Vaccinium corymbosum) from China y at five developmental stages (green, pink, red, purple and blue) were investigated and anthocyanins were characterized and quantified by HPLC during the considered developmental stages. The results indicatated that the contents of phenolic, flavonoids and anthocyanin as well as antioxidant enzyme activities varied depending on the developmental stages. The correlation values between total phenolic content (TPC), total flavonoids content (TFC) and total anthocyanin content (TAC) were significant. The highest activity of peroxidase (POD) and catalase (CAT) was found in red fruit, and the variety of monomeric anthocyanin increased gradually, skin from blue fruit possessed higher TAC. However, the highest activity of polyphenol oxidase (PPO) and superoxide dismutase (SOD) were detected in green and blue fruit, respectively. In the present work, the differences regarding phytochemical profiles and antioxidant enzyme activities were mainly correlated with developmental stages of fruit.