The effect of the maize leaf cuticular landscape on bacterial colonization of the phyllosphere

Abstract

Plant cuticles serve as an interface between plant leaves and their resident bacteria, thus cuticles probably have a major influence on bacterial leaf colonization. To study this influence, we employed maize glossy mutants, that is, corn mutants altered in their cuticle. To assess specific cuticular features that might influence colonization, 11 of the mutants were characterized for leaf surface hydrophobicity, wax quantity, and the morphology of epicuticular wax crystals. We detected significant differences among all genotypes. Mutants gl1, gl3, gl4 and gl5,gl2O were selected for colonization studies with Pantoea agglomerans and Clavibacter michiganensis subsp. nebraskensis (Cmn). Although leaf surface hydrophobicity was inversely correlated to bacterial retention, the correlation was not linear, indicating that other cuticular features affect bacterial retention in addition to hydrophobicity. The percentage of the total bacteria that were tightly associated, i.e. that were not removed by sonication, was not influenced by the cuticle but was influenced by the bacterial species. The cuticle influenced population sizes in a species-dependent fashion. For example, gl1 fostered larger populations of Cmn than the wild type, but similar populations of P. agglomerans. The cuticle influenced population sizes in an environment-dependent fashion as well. Surprisingly, the mutant that had the most dramatic effect on bacterial colonization was the one that was most similar to the wild-type maize in all of the measured leaf surface properties. Mutant gl4 fostered significantly larger populations of Cmn and P. agglomerans under all conditions tested. We hypothesized that the decreased density of crystals on gl4 was responsible for the increased population sizes. In support of our hypothesis, when we reduced crystal density on the wild type by brushing with a cotton-tipped applicator stick, the altered leaves fostered larger populations than the unaltered leaves. The results of all these studies indicate that specific properties of cuticles can affect bacterial population sizes, sometimes in unpredicted ways. A better understanding of the interactions between bacteria and the cuticle will aid us in formulating disease management strategies and in predicting the effects on bacterial populations of global warming and air pollutions, which both alter plant cuticles.