ABSTRACTDiverse bacterial taxa live in association with plants without causing deleterious effects. Previous analyses of phyllosphere communities revealed the predominance of few bacterial genera on healthy dicotyl plants, provoking the question of whether these commensals play a particular role in plant protection. Here, we tested two of them,MethylobacteriumandSphingomonas, with respect to their ability to diminish disease symptom formation and the proliferation of the foliar plant pathogenPseudomonas syringaepv. tomato DC3000 onArabidopsis thaliana. Plants were grown under gnotobiotic conditions in the absence or presence of the potential antagonists and then challenged with the pathogen. No effect ofMethylobacteriumstrains on disease development was observed. However, members of the genusSphingomonasshowed a striking plant-protective effect by suppressing disease symptoms and diminishing pathogen growth. A survey of differentSphingomonasstrains revealed that most plant isolates protectedA. thalianaplants from developing severe disease symptoms. This was not true forSphingomonasstrains isolated from air, dust, or water, even when they reached cell densities in the phyllosphere comparable to those of the plant isolates. This suggests that plant protection is common among plant-colonizingSphingomonasspp. but is not a general trait conserved within the genusSphingomonas. The carbon source profiling of representative isolates revealed differences between protecting and nonprotecting strains, suggesting that substrate competition plays a role in plant protection bySphingomonas. However, other mechanisms cannot be excluded at this time. In conclusion, the ability to protect plants as shown here in a model system may be an unexplored, common trait of indigenousSphingomonasspp. and may be of relevance under natural conditions.
Expression of a RING-HC protein from rice improves resistance to Pseudomonas syringae pv. tomato DC3000 in transgenic Arabidopsis thaliana
