Alternaria alternata (Fr.) Keissler is a widespread leaf blight pathogen that disrupts many plants; including poplars. Despite its broad impact, the sex-specific responses of male and female plants to this pathogen remain poorly studied. This study investigated sex differences in the morphological; photosynthetic; and proteomic responses between male and female Populus deltoides W. Bartram ex Marshall infected with A. alternata. The results showed that the female plants had a faster onset of infection and more inhibited growth in comparison to males. In terms of photosynthetic parameters, the infected females were more severely affected, with 2 subunits in the photosynthetic electron transport chain expressed at higher levels and 12 subunits expressed at lower levels than in the infected males. Regarding the antioxidant system; the infected female plants exhibited higher reactive oxygen species (ROS) contents but lower antioxidant activities, with significantly lower expressions of 2 superoxide dismutases (SODs); 2 peroxidases (PODs); 2 ascorbate peroxidases (APXs); 2 glutathione peroxidases; and 4 glutathione S-transferases compared to the infected males. In the phenylpropanoid biosynthesis pathway, the expressions of shikimate O-hydroxycinnamoyl transferase and ferulate-5-hydroxylase were upregulated in both male and female plants after infection. However, the expression of shikimate O-hydroxycinnamoyl transferase in female plants was consistently higher, while the expression of caffeic acid 3-O-methyltransferase was lower in females compared to males. These indicate that A. alternata infection induces significant alterations in the photosynthetic capacity; antioxidant system; and phenylpropanoid biosynthetic pathway in both male and female poplars. Moreover, bimodal regulation was observed, with male poplars demonstrating greater stability in both photosynthetic and antioxidant systems.
