Accumulation of Abnormal Amyloplasts in Pulp Cells Induces Bitter Pit in Malus domestica

Abstract

Apple bitter pit primarily occurs during fruit ripening and storage; however, its formation mechanism remains unclear. Although it is considered that Ca2+ deficiency causes metabolic disorders in apples, there have been few studies on the mechanism of the bitter pit from the perspective of cell structure. At the fruit ripening stage, the fruit with a bitter pit on the tree was taken as the research material. In this study, the microscopic observation revealed numerous amyloplasts in the pulp cells of apples affected with bitter pit, but not in the healthy pulp. Furthermore, the results of fluorescence staining and transmission electron microscopy (TEM) revealed that the bitter pit pulp cells undergo programmed cell death (PCD), their nuclear chromosomes condense, and amyloplast forms autophagy. The cytoplasmic Ca2+ concentration in the healthy fruits was lowest near the peduncle, followed by that in the calyx, whereas it was highest at the equator. In contrast, the cytoplasmic Ca2+ concentration in apple fruits showing bitter pit disorder was lowest near the peduncle and highest in the calyx. Moreover, the cytosolic Ca2+ concentration in the flesh cells of apples with the bitter pit was much lower than that in the healthy apple flesh cells; however, the concentration of Ca2+ in the vacuoles of fruits with the bitter pit was higher than that in the vacuoles of healthy fruits. In summary, bitter pit pulp cells contain a large number of amyloplasts, which disrupts the distribution of Ca2+ in the pulp cells and causes PCD. These two processes lead to an imbalance in cell metabolism and induce the formation of a bitter pit.

Transcriptomics Reveals the ERF2-bHLH2-CML5 Module Responses to H2S and ROS in Postharvest Calcium Deficiency Apples

“…Transcriptome data of calcium sufficient and calcium deficient apple peels were published in the NCBI database ( , accessed on 15 September 2021) by Qiu et al [ 56 ]. The three sequenced conditions in this transcriptome were T01, diseased peel of calcium-deficient apples; T02, healthy peel of calcium-deficient apples; and T03, peel of calcium-sufficient apples, and each condition had two replicates.…”

Plumbagin resurrect colistin susceptible against colistin-resistant Pseudomonas aeruginosa in vitro and in vivo

“…The cell membrane permeability test was used to investigate the synergistic antibacterial mechanism of plumbagin and colistin ( Qiu et al, 2021 ). TL2314 was selected as the experimental strain.…”

Comparative Physiological and Transcriptomic Analysis Reveal MdWRKY75 Associated With Sucrose Accumulation in Postharvest Apples With Bitter Pits

“…According to transcriptome data of calcium-de cient "Fuji" apple fruit (T01), calcium-de cient apple healthy esh (T02) and healthy apple fruit (T03) at fruit ripening period [5], eight expression patterns of all differentially expressed genes (DEGs) were obtained. We selected a total of 42 DEGs from pro les 0, 1, 6 and 7 (Fig.…”

“…Normal cell metabolism requires the cell membrane to directly contact free calcium to maintain proper cell membrane stability and function [3]. Metabolic changes in Ca 2+ content in these compartments might be related to Ca 2+ depletion in apoplastic solution, which can weaken plasma membrane structures, leading to cell death and bitter pit symptoms [4][5].…”