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Abstract
PPFMs are Pink-Pigmented Facultatively Methylotrophic bacteria of the genus, Methylobacterium. They are found ubiquitously on plant surfaces and are horizontally transmitted through seed from one generation to the next. Given their persistent association with plants, it is reasonable to ask whether their presence is of any significance to the plant. This is a question that has consumed the members of my laboratory for the past twenty five years. In this review of some of the lessons learned from these bacteria, we will present evidence that the PPFMs are a significant member of the plant microbiome, functioning as a probiotic for the plants on which they live. We also focus on ways in which this bacterium can be manipulated to alter plant performance for the benefit of agriculture. Information on the isolation and culture of the PPFM bacteria can be found in Holland et al. (2000).
PPFM bacteria are transmitted vertically through seed and seeds cured of their bacteria no longer germinate properly
Early experiments on the PPFMs demonstrated that they are seed-borne. This was something of a revelation since conventional wisdom told us that the interiors of fruits and seeds were a sterile environment A paper from Rodrigues Periera et al. (1972) convinced us not only that there were reliable mechanisms for the natural bacterization of developing seeds, but that there were some simple methods for disinfestation. The simplest of these is to heat the dry seeds in a 50°C oven for 48h. This treatment reduces the microbial population to a few percent of its natural state. Soybean, Glycine max, seeds thusly treated display lowered rates of germination and slow development. Significantly, these effects were reversed by bacterization with a culture of the PPFMs (Fig.1a). Lingering doubts about the possibility that heat treatment itself was responsible for reduced germinability were addressed when PPFMs were removed from Arabidopsis thaliana seeds by a treatment with PPFM-specific phages (previously unpublished). Phage-soaked seeds show reduced germinability and slow development; these are reversed by washing and bacterization with a fresh culture (Fig. 1b).
These experiments suggested to us that sometimes seeds may fail to germinate not because they have lost viability, but because their bacterial population has been compromised.
Experiments with seeds obtained from the National Seed...