flesh softening
The
enzymes involved in cell wall reconstruction are the key factors for
keeping cell shape and maintaining mechanical strength of peach fruits
(Hayama et al., 2006). We found that 5
genes had higher FPKM values in CT fruits, including beta-glucosidase
(BGlu), polygalacturonase (PG, Prupe.4G261900 and Prupe.4G262200),
pectate lyase (PLY) and pectin methylesterase (PME)
(Supplementary Figure S5 ). The mean FPKM value of PG
(Prupe.4G262200) was 6,359.39, significantly higher than other genes
(P -value < 0.01). Notably, PG (Prupe.4G261900), PLY
(Prupe.1G239900) and PME (Prupe.7G192800) displayed a markedly distinct
expression pattern that HT treated fruits had extremely low expression
levels at all time points. Expansin protein (EXP) is connected with cell
enlargement and cell wall change induced by plant hormones such as
gibberellin, cytokinin, ethylene and brassinosteroids
(Sampedro and Cosgrove, 2005). We found
that EXP2 (Prupe.1G276700) had nearly constant expression levels at all
time points in HT condition, and EXP1 (Prupe.2G263600) had opposite
responses to CT and HT in transcriptional level during storage. Overall,
these genes cooperate to prevent flesh softening in HT.
Expression of genes related
toenzymatic
browning
Enzymatic browning impairs the sensory properties of fruits such as
color and flavor, as a result of oxidative polymerization of phenolic
compounds catalyzed by polyphenol oxidase (PPO) and peroxidase (POD)
(Nicolas et al., 1994). Besides, the
relationships between fruit browning and antioxidant enzymes such as
lipoxygenase (LOX, EC 1.13.11.12), superoxide dismutase (SOD, EC
1.15.1.1) and catalase (CAT, EC 1.11.1.6), have been reported previously
(Inzé and Montagu, 1995). There is no
obvious trend in mRNA levels of POD genes during shelf life, while two
genes (Prupe.1G29900, Prupe.3G278900) were up-regulated in CT condition,
contrary to Prupe.4G042100 (PPO), Prupe.4G021100 (POD) and
Prupe.2G005300 (LOX) (Supplementary Figure S6 ). Particularly,
the expression levels of SOD1 and SOD2 were increased at the middle
stage in CT condition and mostly higher than HT condition. Enzymatic
browning is a strong and complex response to HT according to our
research.