Methylated Cytokinins from the Phytopathogen Rhodococcus fascians Mimic Plant Hormone Activity

Abstract

ORCID ID: 0000-0001-5449-6492 (H.S.).Cytokinins (CKs), a class of phytohormones that regulate plant growth and development, are also synthesized by some phytopathogens to disrupt the hormonal balance and to facilitate niche establishment in their hosts. Rhodococcus fascians harbors the fasciation (fas) locus, an operon encoding several genes homologous to CK biosynthesis and metabolism. This pathogen causes unique leafy gall symptoms reminiscent of CK overproduction; however, bacterial CKs have not been clearly correlated with the severe symptoms, and no virulence-associated unique CKs or analogs have been identified. Here, we report the identification of monomethylated MeCKs were recognized by a CK receptor and up-regulated type-A ARABIDOPSIS THALIANA RESPONSE REGULATOR genes. Treatment with MeCKs inhibited root growth, a hallmark of CK action, whereas the receptor mutant was insensitive. MeCKs were retained longer in planta than canonical CKs and were poor substrates for a CK oxidase/ dehydrogenase, suggesting enhanced biological stability. MeCKs were synthesized by S-adenosyl methionine-dependent methyltransferases (MT1 and MT2) that are present upstream of the fas genes. The best substrate for methylation was isopentenyl diphosphate. MT1 and MT2 catalyzed distinct methylation reactions; only the MT2 product was used by FAS4 to synthesize monomethylated N 6 -(Δ 2 -isopentenyl)adenine. The MT1 product was dimethylated by MT2 and used as a substrate by FAS4 to produce dimethylated N 6 -(Δ 2 -isopentenyl)adenine. Chemically synthesized MeCKs were comparable in activity. Our results strongly suggest that MeCKs function as CK mimics and play a role in this plant-pathogen interaction.

Infection byRhodococcus fasciansmaintains cotyledons as a sink tissue for the pathogen

“…Subsequent investigations will focus on expression of RfMT1 and RfMT2 and levels of methylated cytokinins [newly identified from Rhodococcus-infected plant tissues by Radhika et al (2015)], as well as the methylthio-and cis-forms of the cytokinins. We will then be able to compare, in pea, the impact of the methylated cytokinins (Radhika et al, 2015), with the 'trickwith-the-mix' hypothesis developed based on experiments with R. fascians and arabidopsis (Pertry et al, 2009(Pertry et al, , 2010, with the suggestion by Creason et al (2014) that only one cytokinin type (the iP-type) is necessary for disease symptoms to be manifested.…”

Phyllosphere microbiology: at the interface between microbial individuals and the plant host

“…Nonpathogenic phyllosphere‐colonising bacteria are known for their ability to modify their microenvironment. By producing plant hormone mimics, for example auxins and cytokinins, or secreting biosurfactants, microbes actively impact on the plant and gain fitness advantages, although the ramifications of these modifications for the host are often unclear (Ali et al ., ; Zhang et al ., ; Meena et al ., ; Burch et al ., ; Radhika et al ., ). First insights into the effects of nonpathogenic leaf surface colonisers on host fitness suggest that the microbiota supports the host against microbial infections.…”

Functional Genomics Insights Into the Pathogenicity, Habitat Fitness, and Mechanisms Modifying Plant Development of Rhodococcus sp. PBTS1 and PBTS2

“…The host range of these bacteria encompasses a broad spectrum of mostly herbaceous dicotyledonous plants, although some woody plants and few monocotyledons are sensitive as well (Putnam and Miller, 2007;Depuydt et al, 2008b). Key to symptom development is the bacterial production of an array of cytokinins via gene products of the fasciation (fas) operon encoded on a linear virulence plasmid in most pathogenic isolates (Crespi et al, 1992;Pertry et al, 2009Pertry et al, , 2010Francis et al, 2012;Creason et al, 2014;Radhika et al, 2015;Jameson et al, 2019) and the concomitant modification of the hormone landscape of the infected plant (Depuydt et al, 2008a(Depuydt et al, , 2009aStes et al, 2011aStes et al, , 2012Stes et al, , 2015Jameson et al, 2019). Additionally, the pathogenic model strain D188 possesses a chromosomal locus vic (virulence in chromosome) implicated in symptom persistence that encodes a malate synthase believed to be involved in the catabolism of leafy gall-specific nutrients (Vereecke et al, 2002).…”