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The alternative pathway activity involves in the species-specific responses to rust infection in two desert sister poplars, which may associate with AOX genes expression regulation and adaptive evolution.

Rust fungi are globally important poplar tree pathogens. However, little is known about species-specific responses to rust infection in poplars and the associated changes in alternative pathway (AP) activity. We therefore studied changes in AP activity in seedling leaves of two sister desert poplar species, Populus euphratica and Populus pruinosa, after infection by the rust fungus Melampsora pruinosae. In both species, rust infection inhibited photosynthesis while increasing oxidative stress and antioxidase activity. However, P. euphratica suffered more damage than P. pruinosa during the early phase of infection, and there were clear interspecific differences in the infection response. Notably, the AP was activated much more strongly in P. euphratica than P. pruinosa during the early phase. However, there were no significant interspecific differences in AP activity after prolonged infection. We also investigated changes in the expression of four AP-critical AOX genes in infected leaves of both species. Two genes exhibited distinctly different expression patterns between the two species in the early phase: AOX1a was expressed more strongly in P. euphratica while AOX1b was expressed more strongly in P. pruinosa. Analyses of sequence variation in the genes’ regulatory regions revealed interspecific differences in the number of likely transcription elements for these two genes that correlated with their observed expression patterns. These findings suggest that P. euphratica and P. pruinosa have developed adaptive divergences in the regulation of the genetic machinery involved in resistance to rust infections.