A recent study led by Professor Kenichi Tsuda's team at Huazhong Agricultural University (HZAU) has uncovered the evolutionary origin of a major plant immune pathway, offering critical insight into how plants evolved to defend themselves.
The paper "Emergence of isochorismate-based salicylic acid biosynthesis within Brassicales" was published in the journal Proceedings of the National Academy of Sciences of the United States of America . It reveals that the isochorismate (IC)-based biosynthesis of salicylic acid (SA) – a key plant defense hormone – emerged specifically within the Brassicales order.
In Arabidopsis thaliana, a model plant in the Brassicaceae family, SA biosynthesis relies on the IC pathway involving three main proteins: ICS1, EDS5, and PBS3. Although homologs of these genes are widespread in the plant kingdom, their functional coordination and evolutionary trajectory remained unclear.
By integrating phylogenetic analysis, protein structure modeling, and functional experiments, the researchers traced the emergence of the complete IC-based pathway to between the divergence of Carica papaya and Capparis spinosa. They identified three pivotal evolutionary adaptations that enabled the functional assembly of this pathway: enhanced catalytic efficiency of ICS via reinforced salt bridges, neofunctionalization of EDS5 following gene duplication, and the appearance of IC-recognizing PBS3 proteins in specific Brassicales lineages.
This study sheds new light on the evolution of plant immune systems and offers a model for investigating the coordinated evolution of non-clustered metabolic pathways.

Evolutionary trajectory of the isochorismate (IC) pathway in Brassicales plants. [Photo/news.hzau.edu.cn]