Physical organization and phylogenetic analysis of acdR as leucine-responsive regulator of the 1-aminocyclopropane-1-carboxylate deaminase gene acdS in phytobeneficial Azospirillum lipoferum 4B and other Proteobacteria

Abstract

The phytostimulatory alphaproteobacterium Azospirillum lipoferum 4B exhibits the plant-beneficial gene acdS, which enables deamination of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Here, we show that acdS is in the vicinity of acdR, a homolog to leucine-responsive regulator lrp, in A. lipoferum 4B and most other acdS+ Proteobacteria. Unlike in Beta- and Gammaproteobacteria, acdS (and acdR) is preferentially located on symbiotic islands and plasmids in Alphaproteobacteria. In A. lipoferum 4B, acdS was mapped on a 750-kb plasmid that is lost during phenotypic variation, whereas other phytobeneficial genes such as nifH (associative nitrogen fixation) are maintained. In Proteobacteria, the phylogenies of acdR and acdS were largely but not totally congruent, despite physical proximity of the genes, regardless of whether DNA or deduced protein sequences were used. Potential Lrp, cAMP receptor protein (CRP) and fumarate-nitrate reduction regulator (FNR) binding sites were evidenced in the acdS promoter regions of strain 4B and most of 46 other acdS+ Proteobacteria. Indeed, transcriptional and enzymatic analyses done in vitro pointed to the involvement of Lrp- and FNR-like transcriptional up-regulation of ACC deaminase activity in A. lipoferum 4B. This is the first synteny, phylogenetic, and functional analysis of factors modulating acdS expression in Azospirillum plant growth-promoting rhizobacterium.

Bacterial Modulation of Plant Ethylene Levels

“…Moreover, due to intragenomic transfer events, many microorganisms may have lost acdS genes. Consistently, it has been reported that, during phenotypic variation events, A. lipoferum strain 4B readily loses the plasmid containing an acdS gene (Prigent-Combaret et al, 2008).…”

Alleviation of Abiotic and Biotic Stresses in Plants by Azospirillum

“…A model was proposed to explain how bacteria harboring the ACC deaminase activity could lower ET amount in plant cells (Glick et al 1998 ). This model is valid for certain Azospirillum strains that have ACC deaminase activity (Blaha et al 2006 ;Prigent-Combaret et al 2008 ). The ACC deaminase degrades ACC, the ET precursor.…”

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

“…4). ACC deaminase (encoded by the acdS gene) catalyses the conversion of ACC, the immediate plant precursor for ethylene, into NH 3 and α-ketobutyrate and is widely distributed in soil fungi and bacteria, especially the Proteobacteria (Blaha et al 2006;Prigent-Combaret et al 2008). Whilst the ecological significance of ACC-deaminase in soil microorganisms is largely unknown, in plant-beneficial bacteria it may serve to diminish the amount of ACC available for production of ethylene (Glick et al 2007a).…”