College of Horticulture, Northwest A & F University, Yangling, Xianyang, 712100, Shaanxi, China.
College of Horticultural Science and Engineering, Shandong Agricultural University / State Key Laboratory of Crop Biology, Taian, 271018, Shandong, China.
BMC Plant Biol. 2021 May 8;21(1):209. doi: 10.1186/s12870-021-02974-y.
Bagging is one of the most important techniques for producting high-quality fruits. In the actual of cultivating, we found a new kind of browning in peel of apple fruit that occurs before harvest and worsen during storage period. There are many studies on metabonomic analysis of browning about storage fruits, but few studies on the mechanism of browning before harvest.
In this study, five-year-old trees of 'Rui Xue' (CNA20151469.1) were used as materials. Bagging fruits without browning (BFW) and bagging fruits with browning (BFB) were set as the experimental groups, non-bagging fruits (NBF) were set as control. After partial least squares discriminant analysis (PLS-DA), 50 kinds of metabolites were important with predictive VIP > 1 and p-value < 0.05. The most important differential metabolites include flavonoids and lipids molecules, 11 flavonoids and 6 lipids molecules were significantly decreased in the BFW compared with NBF. After browning, 11 flavonoids and 7 lipids were further decreased in BFB compared with BFW. Meanwhile, the significantly enriched metabolic pathways include galactose metabolism, ABC membrane transporter protein, flavonoid biosynthesis and linoleic acid metabolism pathways et al. Physiological indicators show that, compared with NBF, the content of malondialdehyde (MDA), hydrogen peroxide (HO) superoxide anion (O) in peel of BFW and BFB were significantly increased, and the difference of BFB was more significant. Meanwhile, the antioxidant enzyme activities of BFW and BFB were inhibited, which accelerated the destruction of cell structure. In addition, the metabolome and physiological data showed that the significantly decrease of flavonoid was positively correlated with peel browning. So, we analyzed the expression of flavonoid related genes and found that, compared with NBF, the flavonoid synthesis genes MdLAR and MdANR were significantly up-regulated in BFW and BFB, but, the downstream flavonoids-related polymeric genes MdLAC7 and MdLAC14 were also significantly expressed.
Our findings demonstrated that the microenvironment of fruit was changed by bagging, the destruction of cell structure, the decrease of flavonoids and the increase of triterpenoids were the main reasons for the browning of peel.
套袋是生产优质水果的最重要技术之一。在实际种植中，我们发现苹果果实的一种新褐变，这种褐变发生在收获前，并在贮藏期恶化。关于贮藏果实的褐变，已有许多关于代谢组学分析的研究，但对收获前褐变机制的研究较少。
本研究以 5 年生‘瑞雪’（CNA20151469.1）为试材，设置套袋无褐变果实（BFW）和套袋有褐变果实（BFB）为实验组，不套袋果实（NBF）为对照组。经偏最小二乘判别分析（PLS-DA）后，有 50 种代谢物具有重要意义，其预测 VIP 值>1，p 值<0.05。最重要的差异代谢物包括类黄酮和脂类分子，与 NBF 相比，BFW 中 11 种类黄酮和 6 种脂类分子显著减少。褐变后，BFB 中 11 种类黄酮和 7 种脂类进一步减少。同时，显著富集的代谢途径包括半乳糖代谢、ABC 膜转运蛋白、类黄酮生物合成和亚油酸代谢途径等。生理指标表明，与 NBF 相比，BFW 和 BFB 果皮丙二醛（MDA）、过氧化氢（HO）、超氧阴离子（O）含量显著增加，BFB 差异更为显著。同时，BFW 和 BFB 的抗氧化酶活性受到抑制，加速了细胞结构的破坏。此外，代谢组学和生理数据表明，类黄酮含量的显著降低与果皮褐变呈正相关。因此，我们分析了类黄酮相关基因的表达，发现与 NBF 相比，BFW 和 BFB 中类黄酮合成基因 MdLAR 和 MdANR 显著上调，但下游类黄酮相关聚合基因 MdLAC7 和 MdLAC14 也显著表达。
我们的研究结果表明，套袋改变了果实的微环境，破坏了细胞结构，降低了类黄酮含量，增加了三萜类化合物，是果皮褐变的主要原因。
