AcdR protein is an activator of transcription of 1-aminocyclopropane-1-carboxylate deaminase in Methylobacterium radiotolerans JCM 2831

It has been previously shown that a number of plant associated methylotrophic bacteria contain an enzyme aminocyclopropane carboxylate (ACC) deaminase (AcdS) hydrolyzing ACC, the immediate precursor of ethylene in plants. The genome of the epiphytic methylotroph Methylobacterium radiotolerans JCM2831 contains an open reading frame encoding a protein homologous to transcriptional regulatory protein AcdR of the Lrp (leucine-responsive regulatory protein) family. The acdR gene of M. radiotolerans was heterologously expressed in Escherichia coli and purified. The results of gel retardation experiments have shown that AcdR specifically binds the DNA fragment containing the promoter-operator region of the acdS gene. ACC decreased electrophoretic mobility of the AcdR-DNA complex whereas leucine had no effect on the complex mobility. The mutant strains of M. radiotolerans obtained by insertion of a tetracycline cassette in the acdS or acdR gene lost the ACC-deaminase activity but the strains with complementation of the mutation recovered this function. The acdS^- mutant but not acdR^- strain expressed the xylE reporter gene under the control of acdS promoter region thus resulting in a catechol 2,3-dioxygenase activity. This suggested that AcdR in vivo functions as activator of transcription of the acdS gene. The results obtained in this study showed that in phytosymbiotic methylotroph Methylobacterium radiotolerans AcdR mediates activation of the acdS gene transcription in the presence of an inducer ACC or 2-aminoisobutyrate and the excess of the regulatory protein assists in transcription initiation even in the absence of the inducer. The model of regulation of acdS transcription in M. radiotolerans was proposed.

Pseudomonas 1-Aminocyclopropane-1-carboxylate (ACC) Deaminase and Its Role in Beneficial Plant-Microbe Interactions

The expression of the enzyme 1-aminocylopropane-1-carboxylate (ACC) deaminase, and the consequent modulation of plant ACC and ethylene concentrations, is one of the most important features of plant-associated bacteria. By decreasing plant ACC and ethylene concentrations, ACC deaminase-producing bacteria can overcome some of the deleterious effects of inhibitory levels of ACC and ethylene in various aspects of plant-microbe interactions, as well as plant growth and development (especially under stressful conditions). As a result, the acdS gene, encoding ACC deaminase, is often prevalent and positively selected in the microbiome of plants. Several members of the genus Pseudomonas are widely prevalent in the microbiome of plants worldwide. Due to its adaptation to a plant-associated lifestyle many Pseudomonas strains are of great interest for the development of novel sustainable agricultural and biotechnological solutions, especially those presenting ACC deaminase activity. This manuscript discusses several aspects of ACC deaminase and its role in the increased plant growth promotion, plant protection against abiotic and biotic stress and promotion of the rhizobial nodulation process by Pseudomonas. Knowledge regarding the properties and actions of ACC deaminase-producing Pseudomonas is key for a better understanding of plant-microbe interactions and the selection of highly effective strains for various applications in agriculture and biotechnology.

Endophytic Microbiome of Biofuel Plant Miscanthus sinensis (Poaceae) Interacts with Environmental Gradients

Miscanthus in Taiwan occupies a cline along altitude and adapts to diverse environments, e.g., habitats of high salinity and volcanoes. Rhizospheric and endophytic bacteria may help Miscanthus acclimate to those stresses. The relative contributions of rhizosphere vs. endosphere compartments to the adaptation remain unknown. Here, we used targeted metagenomics to compare the microbial communities in the rhizosphere and endosphere among ecotypes of M. sinensis that dwell habitats under different stresses. Proteobacteria and Actinobacteria predominated in the endosphere. Diverse phyla constituted the rhizosphere microbiome, including a core microbiome found consistently across habitats. In endosphere, the predominance of the bacteria colonizing from the surrounding soil suggests that soil recruitment must have subsequently determined the endophytic microbiome in Miscanthus roots. In endosphere, the bacterial diversity decreased with the altitude, likely corresponding to rising limitation to microorganisms according to the species-energy theory. Specific endophytes were associated with different environmental stresses, e.g., Pseudomonas spp. for alpine and Agrobacterium spp. for coastal habitats. This suggests Miscanthus actively recruits an endosphere microbiome from the rhizosphere it influences.