Abstract
Calcium deficiency usually causes accelerated quality deterioration in postharvest fruit, whereas the underlining mechanism is still unclear. Here, we report that calcium deficiency induced the development of bitter pit on the surface of apple peels compared with the healthy appearance in control apples during postharvest storage. Physiological analysis indicates that calcium-deficient peels contained higher levels of superoxide anion (O2•−), malondialdehyde (MDA), total phenol, flavonoid contents and polyphenol oxidase (PPO) activity, and reduced calcium, H2S production, anthocyanin, soluble protein content, and peroxidase (POD) activity compared with those in calcium-sufficient peels. The principal component analysis (PCA) results show that calcium content, ROS, and H2S production were the main factors between calcium-deficient and calcium-sufficient apple peels. Transcriptome data indicated that four calmodulin-like proteins (CMLs), seven AP2/ERFs, and three bHLHs transcripts were significantly differentially expressed in calcium-deficient apple peels. RT-qPCR and correlation analyses further revealed that CML5 expression was significantly positively correlated with the expression of ERF2/17, bHLH2, and H2S production related genes. In addition, transcriptional co-activation of CML5 by ERF2 and bHLH2 was demonstrated by apple transient expression assays and dual-luciferase reporter system experiments. Therefore, these findings provide a basis for studying the molecular mechanism of postharvest quality decline in calcium-deficient apples and the potential interaction between Ca2+ and endogenous H2S.
Highlights
Calcium (Ca) is one of the essential and abundant elements for the growth of plants
Plant hormone signal transduction was the most abundant differentiallyHe2Oxp2 creosnsteedntgenes (DEGs) found in metabolism category followed by plant–pathogen interaction, biosynthesis of amino acids, and phenylpropanoid biosynthesis. These results suggested that the DEGs in the T03/T01 comparison were mainly enriched in metabolic processes related to plant signal transduction and plant pathogen infestation
It can be concluded that the production of reactive oxygen species (ROS), total phenols and flavonoids was increased in calcium-deficient peels compared to calcium-sufficient peels, but calcium content, H2S production, anthocyanin content, and POD activity were reduced
Summary
Calcium (Ca) is one of the essential and abundant elements for the growth of plants. It determines the yield and quality of agricultural crops, and plays a pivotal role in maintaining plant cell structure, resistance to adverse stress and signal transduction [1,2,3,4]. Calcium deficiency results in ROS accumulation and causes damage to membranes, reduces the cell wall, weakens tissue stiffness, and increases water loss, which in turn leads to leaf wilting and shortens the shelf life of harvested fruit [8,9,10]. It causes various physiological disorders, such as reduction in fruit size, low firmness and thin peel, tip burn, and oxidative stress [5,10,11,12]. Bitter pit is one of the major post-harvest disorders associated with apple production and can cause up to a 50%
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