A histone deacetylase, FaSRT1-2, plays multiple roles in regulating fruit ripening, plant growth and stresses resistance of cultivated strawberry.

Sirtuins (SRTs) are a group of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase that target both histone and nonhistone proteins. The biological function of SRT in horticultural plants has been rarely studied. In this study, FaSRT1-2 was identified as a key member of the 8 FaSRTs encoded in cultivated strawberry genome. Transient overexpression of FaSRT1-2 in strawberry fruit accelerated ripening, increased the content of anthocyanins and sugars, enhanced ripening-related gene expression. Moreover, stable transformation of FaSRT1-2 in strawberry plants resulted in enhanced vegetative growth, increased sensitivity to heat stress and increased susceptibility to Botrytis cinerea infection. Interestingly, knocking out the homologous gene in woodland strawberry had the opposite effects. Additionally, we found the content of stress-related hormone abscisic acid (ABA) was decreased, while the growth-related gibberellin (GA) concentration was increased in FaSRT1-2 overexpression lines. Gene expression analysis revealed induction of heat shock proteins, transcription factors, stress-related and antioxidant genes in the FaSRT1-2-overexpressed plants while knocked-out of the gene had the opposite impact. In conclusion, our findings demonstrated that FaSRT1-2 could positively promote strawberry plant vegetative growth and fruit ripening by affecting ABA and GA pathways. However, it negatively regulates the resistance to heat stress and B. cinerea infection by influencing the related gene expression.

Genome-wide analysis and functional validation reveal the role of late embryogenesis abundant genes in strawberry (Fragaria × ananassa) fruit ripening.

BACKGROUND: Late embryogenesis abundant (LEA) proteins play important roles in plant growth and development, as well as stresses responsiveness. Nowadays, it has been found that LEAs also have function in fruit ripening. However, the comprehensive analysis on a genome-wide basis of LEA family remains limited, and the role of LEA in fruit ripening has not been fully explored yet, especially in strawberry, an economic important plant and ideal material for studying fruit ripening. RESULTS: In this study, a total of 266 putative LEA proteins were identified and characterized in strawberry genome. Subcellular localization prediction indicated that they were mostly localized in chloroplast, cytoplasm and nucleus. Duplication events detection revealed that whole genome duplication or segmental was the main driver for the expansion of LEA family in strawberry. The phylogenetic analysis suggested that FaLEAs were classified into eight groups, among which, LEA2 was the largest subgroup with 179 members, followed by LEA3, dehydrin (DHN), LEA4 and SMP (seed maturation protein). The LEA1 and DHN groups were speculated to play dominant roles in strawberry fruit development and ripening, according to their larger proportion of members detected as differentially expressed genes during such process. Notably, the expression of FaLEA167 belonging to LEA1 group was altered by strawberry maturation, and inhibited by overexpression of negative regulators of ripening (a cytosolic/plastid glyceraldehyde-3-phosphate dehydrogenase, FaGAPC2 and a cytosolic pyruvate kinase, FaPKc2.2). Subsequently, overexpression of FaLEA167 significantly increased the percentage of fruit at green stage, while reduced the full red fruit proportion. In consistent, the anthocyanins content and the fruit skin color variable reflecting a range from greenness to redness (a* value) were significantly reduced. Whereas, FaLEA167 overexpression apparently up-regulated citric acid, soluble protein and malondialdehyde content, but had no obvious effects on total soluble solids, sugar, flavonoids, phenolics content and antioxidant capacity. CONCLUSIONS: These findings not only provided basic information of FaLEA family for further functional research, but also revealed the involvement of FaLEA167 in negatively regulating strawberry fruit ripening, giving new insights into understanding of FaLEA functions.

Genome-Wide Identification and Comparative Transcriptome Methods Reveal FaMDHAR50 Regulating Ascorbic Acid Regeneration and Quality Formation of Strawberry Fruits.

Ascorbic acid (AsA) is a crucial water-soluble antioxidant in strawberry fruit, but limited research is currently available on the identification and functional validation of key genes involved in AsA metabolism in strawberries. This study analyzed the FaMDHAR gene family identification, which includes 168 genes. Most of the products of these genes are predicted to exist in the chloroplast and cytoplasm. The promoter region is rich in cis-acting elements related to plant growth and development, stress and light response. Meanwhile, the key gene FaMDHAR50 that positively regulates AsA regeneration was identified through comparative transcriptome analysis of 'Benihoppe' strawberry (WT) and its natural mutant (MT) with high AsA content (83 mg/100 g FW). The transient overexpression experiment further showed that overexpression of FaMDHAR50 significantly enhanced the AsA content by 38% in strawberry fruit, with the upregulated expression of structural genes involved in AsA biosynthesis (FaGalUR and FaGalLDH) and recycling and degradation (FaAPX, FaAO and FaDHAR) compared with that of the control. Moreover, increased sugar (sucrose, glucose and fructose) contents and decreased firmness and citric acid contents were observed in the overexpressed fruit, which were accompanied by the upregulation of FaSNS, FaSPS, FaCEL1 and FaACL, as well as the downregulation of FaCS. Additionally, the content of pelargonidin 3-glucoside markedly decreased, while cyanidin chloride increased significantly. In summary, FaMDHAR50 is a key positive regulatory gene involved in AsA regeneration in strawberry fruit, which also plays an important role in the formation of fruit flavor, apperance and texture during strawberry fruit ripening.

FaGAPC2/FaPKc2.2 and FaPEPCK reveal differential citric acid metabolism regulation in late development of strawberry fruit.

Citric acid is the primary organic acid that affects the taste of strawberry fruit. Glycolysis supplies key substrates for the tricarboxylic acid cycle (TCA cycle). However, little is known about the regulatory mechanisms of glycolytic genes on citric acid metabolism in strawberry fruits. In this study, the citric acid content of strawberry fruit displayed a trend of rising and decreasing from the initial red stage to the full red stage and then dark red stage. Thus, a difference in citric acid metabolic regulation was suspected during strawberry fruit development. In addition, overexpression of either cytoplasm glyceraldehyde-3-phosphate dehydrogenase (FxaC_14g13400, namely FaGAPC2) or pyruvate kinase (FxaC_15g00080, namely FaPKc2.2) inhibited strawberry fruit ripening and the accumulation of citric acid, leading to a range of maturity stages from partial red to full red stage. The combined transcriptome and metabolome analysis revealed that overexpression of FaGAPC2 and FaPKc2.2 significantly suppressed the expression of phosphoenolpyruvate carboxykinase (FxaC_1g21491, namely FaPEPCK) but enhanced the content of glutamine and aspartic acid. Meanwhile, the activities of PEPCK and glutamate decarboxylase (GAD) were inhibited, but the activities of glutamine synthase (GS) were increased in FaGAPC2/FaPKc2.2-overexpressed fruit. Further, functional verification demonstrated that overexpression of FaPEPCK can promote strawberry fruit ripening, resulting in a range of maturity stage from full red to dark red stage, while the citric acid synthase (CS) activities and citric acid content were significantly decreased. Overall, this study revealed that FaGAPC2/FaPKc2.2 and FaPEPCK perform an important role in reducing citric acid content in strawberry fruit, and FaGAPC2/FaPKc2.2 mainly by promoting the GS degradation pathway and FaPEPCK mainly by inhibiting the CS synthesis pathway.