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Article|01 Mar 2020|OPEN
Photooxidative stress activates a complex multigenic response integrating the phenylpropanoid pathway and ethylene, leading to lignin accumulation in apple (Malus domestica Borkh.) fruit
Carolina A. Torres1,2, Constanza Azocar3,4, Patricio Ramos4,5, Ricardo Pérez-Díaz1,4, Gloria Sepulveda1,2 & María A. Moya-León4
1Facultad de Ciencias Agrarias, Universidad de Talca, Talca, Chile
2Department of Horticulture, Tree Fruit Research & Extension Center, Washington State University, Wenatchee, WA, USA
3Universidad Andres Bello, Facultad Ciencias Biologicas, Santiago, Chile
4Instituto de Ciencias Bioló gicas, Universidad de Talca, Talca, Chile
5Núcleo Cientifíco Multidisciplinario-DI, Universidad de Talca, Talca, Chile

Horticulture Research 7,
Article number: 20022 (2020)
doi: 10.1038/hortres.2020.22
Views: 491

Received: 14 Jun 2019
Revised: 13 Dec 2019
Accepted: 07 Jan 2020
Published online: 01 Mar 2020

Abstract

Photooxidative stress, when combined with elevated temperatures, triggers various defense mechanisms leading to physiological, biochemical, and morphological changes in fruit tissue. Furthermore, during sun damage, apple fruit undergo textural changes characterized by high flesh firmness compared to unexposed fruit. Fuji and Royal Gala apples were suddenly exposed to sunlight on the tree and then sampled for up to 29 days. Cell wall components and lignin biosynthetic pathway analyses were carried out on the fruit tissue. At harvest, Fuji apples with different sun exposure levels, such as exposed to direct sunlight (Exp), shaded (Non-Exp), and with severe sun damage (Sev), were also characterized. In fruit suddenly exposed to sunlight, the expression levels of phenylpropanoid-related genes, phenylalanine ammonia lyase (MdPAL), chalcone synthase (MdCHS), and flavanone-3-hydroxylase (MdF3H), were upregulated in the skin and flesh of Exp and Sev. Exposure had little effect on the lignin-related genes caffeic acid O-methyltransferase 1 (MdCOMT1) and cinnamyl alcohol dehydrogenase (MdCAD) in the skin; however, the expression of these genes was highly induced in the flesh of Exp and Sev in both cultivars. Lignin deposition increased significantly in skin with sun injury (Sev); in flesh, this increase occurred late during the stress treatment. Additionally, the ethylene biosynthesis genes 1-aminocyclopropane-1-carboxylate synthase (MdACS) and 1-aminocyclopropane-1-carboxylate oxidase (MdACO) were highly expressed in the skin and flesh tissues but were more upregulated in Sev than in Exp during the time-course experiment, which paralleled the induction of the phenylpropanoid pathway and lignin accumulation. At harvest, flesh from Sev fruit exhibited higher firmness than that from Non-Exp and Exp fruit, although no differences were observed in the alcohol-insoluble residues (AIR) among groups. The fractionation of cell wall polymers revealed an increase in the uronic acid contents of the water-soluble pectin fraction (WSF) in Exp and Sev tissues compared to Non-Exp tissues, while the other pectin-rich fractions, that is, CDTA-soluble (CSF) and Na2CO3-soluble (NSF), were increased only in Sev. The amount of hemicellulose and cellulose did not differ among fruit conditions. These findings suggest that increases in the flesh firmness of apples can be promoted by photooxidative stress, which is associated with the induction of lignin accumulation in the skin and flesh of stressed fruit, with the involvement of stress phytohormones such as ethylene.