有益根棲細菌Bacillus cereus C1L之揮發性化合物改變阿拉伯芥根分泌物組成進而促進根部纏聚

Abstract

植物根部會分泌碳源至根圈，從而吸引特定的微生物纏聚。根棲細菌的纏聚可促使其發揮對植物的有益影響，如促進植物生長和引起植物系統抗性。而有益根棲細菌釋放的揮發性化合物(volatile compounds, VCs)可作為跨界交流的訊號分子來調控植物的生理反應。目前已有研究發現，根棲細菌纏聚可改變植物根分泌物組成，並進一步招募有益根棲細菌，因此可推測有益根棲細菌產生的揮發性化合物可能作為訊號以調控植物根分泌物組成，從而影響根棲細菌的行為和纏聚能力。本研究利用定菌系統研究有益根棲細菌Bacillus cereus C1L釋放的揮發性化合物對阿拉伯芥根分泌物的影響。研究發現B. cereus C1L揮發性化合物處理的阿拉伯芥根分泌物增加了C1L在阿拉伯芥根部的纏聚。透過揮發性化合物成分減少的C1L突變株處理，以及回補或單獨處理揮發性化合物成分證明C1L揮發性化合物中的成分acetoin,2,3-butanediol和dimethyl disulfide可以促進C1L在阿拉伯芥根部的纏聚。在植物方面，阿拉伯芥根部二次代謝物香豆素(coumarins)的生合成和分泌會被C1L揮發性化合物誘導。利用香豆素生合成缺失阿拉伯芥突變株進行纏聚試驗並回補香豆素，證明了C1L揮發性化合物增強C1L在阿拉伯芥根部纏聚的現象需要香豆素產生。體外試驗證明香豆素不影響C1L生長但明顯增進其群游移動能力。總而言之，本研究發現了有益根棲細菌透過揮發性化合物影響植物生理而增進其根部纏聚能力的機制，為植物宿主-微生物的互動提供了更多分子層面的瞭解，並促進微生物在農業上的應用。

Plant roots exude carbon sources into rhizosphere, which attract colonization of specific microbes. Colonization of rhizobacteria is critical for their beneficial effects on plants such as plant growth promotion and induced systemic resistance. Volatile compounds (VCs) emitted from beneficial rhizobacteria function as inter-kingdom signal molecules mediating physiological responses of plants. As known, rhizobacterial colonization changes root exudate composition and further recruits beneficial rhizobacteria. Presumably, VCs from beneficial rhizobacteria could act as signals to alter root exudates that affect rhizobacterial behaviors and colonization ability. In this study, a gnotobiotic system was used to assay the effect of the VCs emitted from beneficial rhizobacterium Bacillus cereus C1L on Arabidopsis root exudates. B. cereus C1L VCs-treated Arabidopsis root exudates increased C1L colonization on Arabidopsis roots. Through treating with a C1L mutant strain that reduced the production of volatile constituents as well as supplementation assay and individual treatment of the volatile chemicals, the acetoin, 2,3-butanediol and dimethyl disulfide in C1L VCs were demonstrated to promote C1L colonization on roots. In the aspects of plants, biosynthesis and export of the secondary metabolite coumarins could be induced by C1L VCs in Arabidopsis roots. Colonization assays using coumarin defected mutants of Arabidopsis and coumarin supplementation demonstrated that C1L VCs-enhanced C1L colonization on Arabidopsis roots was partly dependent on coumarin production. Coumarins did not affect C1L growth in vitro but significantly increase its swarming motility. In conclusion, this research discovered the mechanism of beneficial rhizobacteria to enhance their root colonization through affecting plants physiology using VCs, which increases our knowledge of molecular interactions between plant hosts and microbes, and facilitates the microbial application in agriculture.