Phytochrome mediated responses in Agrobacterium fabrum: growth, swimming, plant infection and interbacterial competition

Abstract

The soil bacterium Agrobacterium fabrum C58 infects plants by a unique 15 DNA transfer mechanism. A. fabrum has two phytochrome photoreceptors, Agp1 and Agp2. We 16 found that DNA transfer into plants by A. fabrum is down regulated by light and that 17 phytochrome knockout mutants have diminished DNA transfer rates. The regulation pattern 18 matches with that of bacterial conjugation reported earlier. Growth, swimming and interbacterial 19 competition were also affected in phytochrome knockout mutants, although these effects were 20 often not affected by light. We can thus distinguish between light-regulated and light-independent 21 phytochrome responses. In microarray studies, transcription of only 4 genes was affected by 22 light, indicating that most light responses are regulated post-transcriptionally. In a mass 23 spectrometery-based proteomic study, 24 proteins were different between light and dark grown 24 bacteria, whereas 382 proteins differed between wild type and phytochrome knockout mutants, 25 pointing again to light-dependent and light-independent roles of Agp1 and Agp2. 26 27 stranded DNA is covalently attached to the protein that makes the nick, which is VirD2 (21) in A. 59 fabrum plant infection and TraA (22) in conjugation. During conjugation, the entire plasmid is 60 transferred. The complex of protein and single-strand DNA is transferred through pili formed by 61 the type IV secretion system into the target cells. 62We also tested the impact of phytochromes in A. fabrum on growth, motility, and 63 interbacterial competition and also performed microarray and proteome studies. These data show 64 that protein levels are affected for few gene products and suggests regulatory mechanisms 65 independent on transcription or translation. On the organismal level and on the level of single 66 proteins, we found phytochrome effects that are light dependent and such that are light 67 independent. 68

Phytochromes in Agrobacterium fabrum

“…The effects on A. fabrum motility were difficult to analyze because broad pH variations of the medium and sophisticated data analyses were required to see effects. There was no evidence for a red light effect on motility in these experiments, but in certain pH conditions, motility was different between mutants and wild type ( Xue et al, 2020 ). In case of cell growth, a comparable effect was found.…”

“…Conjugation has many features in common with gene transfer into plants: in both cases, (i) single stranded DNA is transferred, (ii) there is a covalent link between a transfer protein and the adduct between DNA and the transfer protein is transmitted, and (iii) a type IV secretion system is involved in the transfer. We, therefore, investigated the gene transfer into plants again ( Xue et al, 2020 ). In those experiments, tumor induction in Arabidopsis roots, tumor induction in Nicotiana stems, and DNA transfer into Nicotiana leaves were light controlled and dependent on phytochromes.…”

“…Stems with or without tumors were photographed after 6 weeks. (B) A. fabrum with pBIN-GUS plasmid was used for infection of N. bentamiana leaves, same strains as in (A,C) as in (B) , quantification by fluorescence based on MUG conversion ( Xue et al, 2020 ).…”

A light life together: photosensing in the plant microbiota

“…3.5 fold) in the WT strain. More important with respect to plants are the results of plant root infection: formation of root tumours by A. fabrum is strongly down-regulated by RL, with respect to dark conditions, and almost totally repressed in the agp1 − /agp2 − mutant (both in dark and under RL) [79]. This latter result indicates that plant phyproteins, possibly present in the roots, do not have significant effects on light-regulation of tumour formation.…”

“…Absence of Agp1 and/or Agp2 also affect swimming, inter-bacterial competition, and the expression of diverse proteins, but in many cases without light regulation [79].…”