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Earth’s radiation budget and frequency and intensity of precipitation are influenced by aerosols with ice nucleation activity (INA), i.e., particles that catalyze the formation of ice. Some bacteria, fungi, and pollen are among the most efficient ice nucleators but the molecular basis of INA is poorly understood in most of them. Lysinibacillus parviboronicapiens (Lp) was previously identified as the first Gram-positive bacterium with INA. INA of Lp is associated with a secreted, nanometer-sized, non-proteinaceous macromolecule or particle. Here a combination of comparative genomics, transcriptomics, and a mutant screen showed that INA in Lp depends on a type I iterative polyketide synthase and a non-ribosomal peptide synthetase (PKS-NRPS). Differential filtration in combination with gradient ultracentrifugation revealed that the product of the PKS-NRPS is associated with secreted particles of a density typical of extracellular vesicles and electron microscopy showed that these particles consist in “pearl chain”-like structures not resembling any other known bacterial structures. These findings expand our knowledge of biological INA, may be a model for INA in other organisms for which the molecular basis of INA is unknown, and present another step towards unraveling the role of microbes in atmospheric processes.

Bacterial fruit blotch (BFB) caused byAcidovorax citrulliis one of the most important bacterial diseases of cucurbits worldwide. However, the mechanisms associated withA. citrullipathogenicity and genetics of host resistance have not been extensively investigated. We idenitfiedNicotiana benthamianaandNicotiana tabacumas surrogate hosts for studyingA. citrullipathogenicity and non‐host resistance triggered by type III secreted (T3S) effectors. TwoA. citrullistrains, M6 and AAC00‐1, that represent the two major groups amongstA. citrullipopulations, induced disease symptoms onN. benthamiana, but triggered a hypersensitive response (HR) onN. tabacumplants. Transient expression of 19 T3S effectors fromA. citrulliinN. benthamianaleaves revealed that three effectors, Aave_1548, Aave_2708, and Aave_2166, trigger water‐soaking‐like cell death inN. benthamiana.Aave_1548knockout mutants of M6 and AAC00‐1 displayed reduced virulence onN. benthamianaand melon (Cucumis meloL.). Transient expression of Aave_1548 and Aave_2166 effectors triggered a non‐host HR inN. tabacum, which was dependent on the functionality of the immune signalling component,NtSGT1. Hence, employingNicotianaspecies as surrogate hosts for studyingA. citrullipathogenicity may help characterize the function ofA. citrulliT3S effectors and facilitate the development of new strategies for BFB management.