Autoactivation of mycorrhizal symbiosis signaling through gibberellin deactivation in orchid seed germination.

Orchids parasitically depend on external nutrients from mycorrhizal fungi for seed germination. Previous findings suggest that orchids utilize a genetic system of mutualistic arbuscular mycorrhizal (AM) symbiosis, in which the plant hormone gibberellin (GA) negatively affects fungal colonization and development, to establish parasitic symbiosis. Although GA generally promotes seed germination in photosynthetic plants, previous studies have reported low sensitivity of GA in seed germination of mycoheterotrophic orchids where mycorrhizal symbiosis occurs concurrently. To elucidate the connecting mechanisms of orchid seed germination and mycorrhizal symbiosis at the molecular level, we investigated the effect of GA on a hyacinth orchid (Bletilla striata) seed germination and mycorrhizal symbiosis using asymbiotic and symbiotic germination methods. Additionally, we compared the transcriptome profiles between asymbiotically and symbiotically germinated seeds. Exogenous GA negatively affected seed germination and fungal colonization, and endogenous bioactive GA was actively converted to the inactive form during seed germination. Transcriptome analysis showed that B. striata shared many of the induced genes between asymbiotically and symbiotically germinated seeds, including GA metabolism- and signaling-related genes and AM-specific marker homologs. Our study suggests that orchids have evolved in a manner that they do not use bioactive GA as a positive regulator of seed germination and instead autoactivate the mycorrhizal symbiosis pathway through GA inactivation to accept the fungal partner immediately during seed germination.

Diversity and composition of flower-visiting insects and related factors in three fruit tree species.

Animal-mediated pollination is an essential ecosystem service for the production of many fruit trees. To reveal the community composition of flower-visiting wild insects which potentially contribute to fruit production and to examine the effects of geographic location, local meteorological conditions and locally introduced domesticated pollinators on them, we investigated the community composition of insects visiting the flowers (hereafter, "visitors") of apple, Japanese pear and Oriental persimmon for 1‒3 years at 20 sites around Japan. While most of the variation (82%) of the community composition was explained by tree species with a slight contribution by geographic distance (2%), maximum temperature and tree species contributed 62% and 41% of the variation in total abundance of the visitors, respectively. Though the dominant families of the visitors varied spatiotemporally, the community composition of the visitors of apple and Japanese pear clearly differed from that of Oriental persimmon. While Andrenidae and Syrphidae together accounted for 46%‒64% of the visitors of apple and Japanese pear, Apidae represented 57% of the visitors of Oriental persimmon. The taxonomic richness, diversity and evenness of the visitors were best predicted by locally introduced domesticated pollinators and local meteorological conditions of wind speed and maximum temperature. Amongst these selected factors, locally introduced domesticated pollinators could have the largest impact. It seemed to be strongly related to the reduction of taxonomic richness, diversity and evenness of the visitors, accounting for 41‒89% of the variation. Results suggested that the community composition and total abundance of potential pollinators were predominantly determined by tree species and temperature, but locally introduced domesticated pollinators could have a determinantal pressure on the taxonomic diversity of the community.