Most land plant species live in symbiosis with arbuscular mycorrhizal fungi. These fungi differentiate essential functional structures called arbuscules in root cortical cells from which mineral nutrients are released to the plant. We investigated the role of microRNA393 (miR393), an miRNA that targets several auxin receptors, in arbuscular mycorrhizal root colonization. Expression of the precursors of the miR393 was down-regulated during mycorrhization in three different plant species: Solanum lycopersicum, Medicago truncatula, and Oryza sativa. Treatment of S. lycopersicum, M. truncatula, and O. sativa roots with concentrations of synthetic auxin analogs that did not affect root development stimulated mycorrhization, particularly arbuscule formation. DR5-GUS, a reporter for auxin response, was preferentially expressed in root cells containing arbuscules. Finally, overexpression of miR393 in root tissues resulted in down-regulation of auxin receptor genes (transport inhibitor response1 and auxin-related F box) and underdeveloped arbuscules in all three plant species. These results support the conclusion that miR393 is a negative regulator of arbuscule formation by hampering auxin perception in arbuscule-containing cells.

Background Biological control is an environmentally sound and effective means of reducing pathogeninduced damage to agriculture using natural antagonists. Paenibacillus is a cosmopolitan and ubiquitously occurring bacterial genus with antagonistic activity against phytopathogens. Many species and strains with promising potential for plant growth promotion and biocontrol of pathogens have been identified since Paenibacillus was first described 20 years ago. Nevertheless, important questions regarding the colonization of plants, and the mode of action of Paenibacillus remain unanswered. Scope This review focuses on the occurrence of Paenibacillus in microbial metagenomes, the endophytic lifestyle of Paenibacillus, and the function of Paenibacillus-derived volatile organic compounds (VOCs) combining actual literature with our own results.Conclusions This review provides new insights into the endophytic lifestyle of Paenibacillus and discusses strain-specific and system-dependent growth promotion effects on plants. VOCs, in particular pyrazine derivatives emitted by Paenibacillus, showed high activity against other organisms. This suggests that VOCs play an important role in communication and interaction. Overall, Paenibacillus strains demonstrate promising potential not only for sustainable agriculture and biological control, but also as a source for novel bioactive volatiles.