The Regulation Of Elevated CO₂ On Anthocyanin Synthesis In Postharvest Strawberry Fruit

Strawberry fruit are easily perishable during storage due to their soft texture,thin epidermis,mechanical injury,as well as high sensitivity to fungal attack.Elevated CO₂as an effective strategy can prolong the shelf life of postharvest strawberry fruit.To date,the effect of elevated CO₂ on color quality is still unclear,and the regulation mechanism of elevated CO₂ on anthocyanin synthesis has not been reported.In this research,the mechanism of anthocyanin synthesis in postharvest strawberry fruit under elevated CO₂was clarified from three aspects:anthocyanin synthesis substrate,anthocyanin synthesis pathway and hormone-mediated anthocyanin synthesis regulation.The obtained results were as follows:1.10% and 20% CO₂ treament significantly delayed the change of fruit firmness and color,and decreased the decay rate as well.While 30%CO₂ treatment caused CO₂injury and color deterioration.Elevated CO₂ treatment decreased NADH content and the ratio of NADH/NAD⁺,and thus accelerated the decrease of energy charge.After 12d of storage,the energy charge in 10%,20%and 30%CO₂-treated fruit was only92.41%,76.07%and 46.67%of that in control group,respectively.In addition,elevated CO₂ inhibited GABA-T activity and Fa GABA-T1 expression at later storage period,leading to the accumulation of GABA and the decrease of NADH as a result.2.Transcriptome analysis revealed that a total of 10368 genes were differentially expressed between the elevated CO₂ group and the control group.Metabolic process enriched the largest number of differentially expressed genes.At the 4^th and 8^th day of storage,2102 and 2663 differential genes were enriched,respectively.This result indicated that elevated CO₂ was widely involved in the regulation of fruit metabolism at the transcriptional level.Based on the analysis of metabolic pathways related to fruit color,we found that elevated CO₂ induced the expression of genes involved in chlorophyll synthesis and inhibited the expression of genes involved in chlorophyll degradation.Meanwhile,the phenylalanine,tyrosine and tryptophan biosynthesis,phenylpropanoid pathway and flavonoid biosynthesis pathways,which are closely related to anthocyanin synthesis,were also significantly regulated by elevated CO₂.In addition,multiple family transcription factors,including MYB transcription factor,leucine zipper protein and ethylene response factor,jointly responded to elevated CO₂and co-participated in the regulation of color formation of postharvest strawberry fruit.3.Elevated CO₂ delayed the accumulation of quality-related aromatic secondary metabolites including anthocyanin and eugenol.After 8 d of storage,the anthocyanin and eugenol content in elevated CO₂-treated group were only 15.75%and 39.72%of those in control group,respectively.In addition,lignin metabolism was also inhibited by elevated CO₂.Elevated CO₂ activated the glycolysis and pentose phosphate pathway by increasing the activities of PFK and G6PDH,as well as the expression of Fa PFK and Fa G6PDH,and thus accelerated the consumption of fructose,sucrose and glucose as a result.However,aromatic amino acids pathway was inhibited by elevated CO₂,which restricted carbon flow into secondary metabolism and reduced the accumulation of Phe,Tyr and Trp in strawberry fruit,leading to the inhibition of biosynthesis of aromatic secondary metabolites.4.We indientified five individual anthocyanins in strawberry fruit,and all of them were inhibited by elevated CO₂.After 8 d of storage,the contents of Cy-3-G,Pg-3-G,Pg-3-Ru,Cy-3-Ma G and Pg-3-Ma G in elevated CO₂-treated group were 36.29%,14.44%,47.45%,54.78%and 19.68%of those in control group,respectively.At later storage,elevated CO₂ delayed anthocyanin biosynthesis and decreased anthocyanin accumulation in strawberry fruit by inhibiting the activities of enzymes,as well as the expression of genes involved in phenylpropane metabolism and flavonoid biosynthesis pathway.5.Elevated CO₂ was involved in regulating the levels of ABA,SA and JA in strawberry fruit,but showed no significant effect on the content of IAA.The accumulation of ABA was significantly inhibited by elevated CO₂,which was consistent with the change of anthocyanin contents.After 8 d of storage,ABA content in elevated CO₂-treated group was 39.20%of that in control group.Elevated CO₂ decreased ABA anabolism by inhibiting NCED activity and the expression of FANCED1,FANCED2.In addition,elevated CO₂ also inhibited ABA signal transduction by down-regulating the expression of Fa Sn RK2.4 and Fa Sn RK2.6,causing the fruit to be insensitive to ABA as a result.Above all,elevated CO₂ inhibited the accumulation of anthocyanins through the dual regulation of ABA anabolism and signal transduction pathway.