Biochemical and transcriptomic changes associated with induced resistance in citrus fruits treated with sodium salts

Sodium salts are a promising alternative treatment to substitute/integrate the use of synthetic fungicides for controlling postharvest decay of citrus. In vitro studies indicated a direct activity of these salts against Penicillium attacking citrus, however, differences in their effectiveness on various citrus species were observed in in vivo trials, suggesting other modes of action. In this study, changes in enzymatic activity and gene expression level of phenylalanine ammonia lyase (PAL) in ‘Valencia late’ sweet oranges treated with Na2CO3 and NaHCO3 (3% w/v) were investigated. Both salts proved to increase PAL activity in orange tissues, upregulation being confirmed by transcriptomic analyses, particularly at 12 h after treatment. Thus, although other pathways cannot be excluded, the defense response induced by Na2CO3 and NaHCO3 in citrus fruit seems to be associated with the phenylpropanoid pathway, which has a role in the adaptation process to various stresses. This response could enhance the citrus natural reaction to wounding and pathogen attack, improving its protective effect. Thus induced resistance should be considered as one of the possible mechanisms of salts in controlling postharvest citrus decay.