The Arabidopsis sepal function as defense barrier of developing flower against the biotrophic pathogen Golovinomyces cichoracearum.

Arabidopsis sepals are modified green leaf-like structures containing polyploid giant pavement cells. It has been postulated, but not demonstrated, that these cells may play a role in the defense of developing flowers against biotic and abiotic stresses. In the present work, comparative analysis of sepal, petal and rosette leaf trascriptomes showed that several stress- responsive genes, including those involved in salycilyc acid (SA) defence pathway, are differentially expressed in sepals as compared with petals and leaves. We investigated, in particular: (i) the constitutive levels of free and total SA in Arabidopsis Col-0 sepals and leaves; (ii) the level of infection in Col-0 and nahG Arabidopsis sepals and leaves inoculated with Golovinomyces cichoracearum or Botrytis cinerea. It was shown that sepals act as defense barrier for the developing reproductive organs against the biotrophic pathogen G. cichoracearum but not against the necrotrophic pathogen B. cinerea, thus emphasizing the prominent role of SA in defence pathway against biotroph.