The Physcomitrella patens unique alpha-dioxygenase participates in both developmental processes and defense responses

Background: Plant alpha-dioxygenases catalyze the incorporation of molecular oxygen into polyunsaturated fatty acids leading to the formation of oxylipins. In flowering plants, two main groups of alpha-DOXs have been described. While the alpha-DOX1 isoforms are mainly involved in defense responses against microbial infection and herbivores, the alpha-DOX2 isoforms are mostly related to development. To gain insight into the roles played by these enzymes during land plant evolution, we performed biochemical, genetic and molecular analyses to examine the function of the single copy moss Physcomitrella patens alpha-DOX (Pp alpha-DOX) in development and defense against pathogens. Results: Recombinant Pp alpha-DOX protein catalyzed the conversion of fatty acids into 2-hydroperoxy derivatives with a substrate preference for alpha-linolenic, linoleic and palmitic acids. Pp alpha-DOX is expressed during development in tips of young protonemal filaments with maximum expression levels in mitotically active undifferentiated apical cells. In leafy gametophores, Pp alpha-DOX is expressed in auxin producing tissues, including rhizoid and axillary hairs. Pp alpha-DOX transcript levels and Pp alpha-DOX activity increased in moss tissues infected with Botrytis cinerea or treated with Pectobacterium carotovorum elicitors. In B. cinerea infected leaves, Pp alpha-DOX-GUS proteins accumulated in cells surrounding infected cells, suggesting a protective mechanism. Targeted disruption of Pp alpha-DOX did not cause a visible developmental alteration and did not compromise the defense response. However, overexpressing Pp alpha-DOX, or incubating wild-type tissues with Pp alpha-DOX-derived oxylipins, principally the aldehyde heptadecatrienal, resulted in smaller moss colonies with less protonemal tissues, due to a reduction of caulonemal filament growth and a reduction of chloronemal cell size compared with normal tissues. In addition, Pp alpha-DOX overexpression and treatments with Pp alpha-DOX-derived oxylipins reduced cellular damage caused by elicitors of P. carotovorum. Conclusions: Our study shows that the unique alpha-DOX of the primitive land plant P. patens, although apparently not crucial, participates both in development and in the defense response against pathogens, suggesting that alpha-DOXs from flowering plants could have originated by duplication and successive functional diversification after the divergence from bryophytes.