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Fruits play a crucial role in seed dispersal. They open along dehiscence zones. Fruit dehiscence zone formation has been intensively studied in Arabidopsis thaliana. However, little is known about the mechanisms and genes involved in the formation of fruit dehiscence zones in species outside the Brassicaceae. The dehiscence zone of A. thaliana contains a lignified layer, while dehiscence zone tissues of the emerging orchid model Erycina pusilla include a lipid layer. Here we present an analysis of evolution and development of fruit dehiscence zones in orchids. We performed ancestral state reconstructions across the five orchid subfamilies to study the evolution of selected fruit traits and explored dehiscence zone developmental genes using RNA-seq and qPCR. We found that erect dehiscent fruits with non-lignified dehiscence zones and a short ripening period are ancestral characters in orchids. Lignified dehiscence zones in orchid fruits evolved multiple times from non-lignified zones. Furthermore, we carried out gene expression analysis of tissues from different developmental stages of E. pusilla fruits. We found that fruit dehiscence genes from the MADS-box gene family and other important regulators in E. pusilla differed in their expression pattern from their homologs in A. thaliana. This suggests that the current A. thaliana fruit dehiscence model requires adjustment for orchids. Additionally, we discovered that homologs of A. thaliana genes involved in the development of carpel, gynoecium and ovules, and genes involved in lipid biosynthesis were expressed in the fruit valves of E. pusilla, implying that these genes may play a novel role in formation of dehiscence zone tissues in orchids. Future functional analysis of developmental regulators, lipid identification and quantification can shed more light on lipid-layer based dehiscence of orchid fruits.

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Arabidopsis seedlings developed severe hyperhydricity (HH) when 0.2% Gelrite was used to solidify the medium instead of 0.7% agar. One mM trichloroacetate (TCA, an inhibitor of wax synthesis) strongly reduced the amount of wax. TCA also strongly increased the permeability of leaves for water as shown by a decrease of the water retention capacity. One mM TCA also fully prevented HH: the visible symptoms of HH were absent and the amounts of water and air in the intercellular spaces were as in normal, nonhyperhydric seedlings. Apparently, the excess of water that caused the symptoms of HH had been removed by the increased cuticular transpiration in the presence of 1 mM TCA.

WRKY is a transcription factor family unique to plants with diverse functions in defense pathways, abiotic stress tolerance and developmental programs. Family members are characterized by the conserved WRKY domain and significant sequence variation in the remainder of the protein, which is translated into distinct functions even for closely related genes. We utilized the extensive functional characterization of the Arabidopsis thaliana WRKY family to identify tomato homologues of Arabidopsis WRKY genes that are involved in defense responses (AtWRKY 11, 29, 48, 70 and 72). In total 13 tomato WRKY homologues were identified for these genes, of which 9 were successfully over-expressed, and 12 stably silenced via RNAi in transgenic tomato lines. The transgenic lines were evaluated for their response to salt stress, powdery mildew resistance and the combination of these stresses. Lines overexpressing SlWRKY11 and SlWRKY23, and RNAi lines of SlWRKY7 and SlWRKY9 showed both increased biomass and improved salt tolerance. For SlWRKY11 and SlWRKY23 overexpression (OE) lines, this was accompanied by a moderate increase in oxidative stress tolerance. The SlWRKY6-OE line showed strongly improved salt stress tolerance, but a growth penalty under control conditions. Exceptional phenotypes were observed for the SlWRKY10-OE line (stunted growth) and the RNAi line SlWRKY23-RNAi (necrotic symptoms), but these phenotypes were partly restored to normal under salt stress. Both these lines exhibited increased resistance to powdery mildew, but this was compromised when the plants were put under salt-stress as well. Important functions for tomato WRKY genes were revealed in both the abiotic and biotic stress response and several genes should be further explored to elucidate their downstream regulatory functions that lead to increased stress tolerance.