Arabidopsis mlo3 mutant plants exhibit spontaneous callose deposition and signs of early leaf senescence

The family of Mildew resistance Locus O (MLO) proteins is best known for its profound effect on the outcome of powdery mildew infections: when the appropriate MLO protein is absent, the plant is fully resistant to otherwise virulent powdery mildew fungi. However, most members of the MLO protein family remain functionally unexplored. Here, we investigate Arabidopsis thaliana MLO3 , the closest relative of AtMLO2, AtMLO6 and AtMLO12 , which are the Arabidopsis MLO genes implicated in the powdery mildew interaction. The co-expression network of AtMLO3 suggests association of the gene with plant defense-related processes such as salicylic acid homeostasis. Our extensive analysis shows that mlo3 mutants are unaffected regarding their infection phenotype upon challenge with the powdery mildew fungi Golovinomyces orontii and Erysiphe pisi , the oomycete Hyaloperonospora arabidopsidis , and the bacterial pathogen Pseudomonas syringae (the latter both in terms of basal and systemic acquired resistance), indicating that the protein does not play a major role in the response to any of these pathogens. However, mlo3 genotypes display spontaneous callose deposition as well as signs of early senescence in six-or seven-week-old rosette leaves in the absence of any pathogen challenge, a phenotype that is reminiscent of mlo2 mutant plants. We hypothesize that de-regulated callose deposition in mlo3 genotypes is the result of a subtle transient aberration of salicylic acid-jasmonic acid homeostasis during development. Abbreviations : bp : base pair CFU : colony forming units dpi : days post inoculation GO : gene ontology GUS : β-glucuronidase Hpa : Hyaloperonospra arabidopsidis hpi : hours post inoculation JA : jasmonic acid JA-Ile : jasmonic acid isoleucine MAMP : microbe-associated molecular pattern MS : Murashige and Skoog Psm : Pseudomonas syringae pv. maculicola Pst : Pseudomonas syringae pv. tomato RH : relative humidity RLU : relative light units RT-PCR : reverse transcriptase polymerase chain reaction SA : salicylic acid SAG : sal-icylic acid glucoside SAR : systemic acquired resistance T-DNA : Transfer-DNA UPLC-nano ESI-MS/MS : Ultrahigh-Pressure Liquid Chromatography-Tandem Mass Spectrometry YFP : yellow fluorescent protein