Bioactive Cytokinins Are Selectively Secreted by Sinorhizobium meliloti Nodulating and Nonnodulating Strains

Abstract

Bacteria present in the rhizosphere of plants often synthesize phytohormones, and these signals can consequently affect root system development. In legumes, plants adapt to nitrogen starvation by forming lateral roots as well as a new organ, the root nodule, following a symbiotic interaction with bacteria collectively referred to as rhizobia. As cytokinin (CK) phytohormones were shown to be necessary and sufficient to induce root nodule organogenesis, the relevance of CK production by symbiotic rhizobia was questioned. In this study, we analyzed quantitatively, by liquid chromatography-tandem mass spectrometry, the production of 25 forms of CK in nine rhizobia strains belonging to four different species. All bacterial strains were able to synthesize a mix of CK, and bioactive forms of CK, such as iP, were notably found to be secreted in bacterial culture supernatants. Use of a mutant affected in extracellular polysaccharide (EPS) production revealed a negative correlation of EPS production with the ability to secrete CK. In addition, analysis of a nonnodulating Sinorhizobium meliloti strain revealed a similar pattern of CK production and secretion when compared with a related nodulating strain. This indicates that bacterially produced CK are not sufficient to induce symbiotic nodulation.

Euglena gracilis(Euglenophyceae) produces abscisic acid and cytokinins and responds to their exogenous application singly and in combination with other growth regulators

“…Euglena profiles of endogenous hormones suggest that methylthiol derivatives are released outside algal cells more quickly than accumulated within. This is contrary to the CK profiles observed for cultured Rhizobium bacteria, where methylthiols were the main CK type detected in the cell pellet (Kisiala et al, 2013). Glucoside derivatives were not detected in the present study or in Rhizobium.…”

“…However, we never observed bacterial contamination of the cultures, and the detected hormone profiles do not resemble those known for prokaryotes, which generally produce considerably higher levels of methylthiol-type CKs, as well as iP derivatives (Kisiala et al, 2013). For this reason, we consider all the endogenous hormone profiles to be of algal origin.…”

A Laser Dissection-RNAseq Analysis Highlights the Activation of Cytokinin Pathways by Nod Factors in the Medicago truncatula Root Epidermis

“…For CK LC-MS/MS analysis, approximately 1-cm-long root fragments were collected before or after 4 h of treatment with 10 28 M NF versus mock, after cutting off root tips (approximately 1-2 mm long) to remove a potential source of non-NF-regulated CK. Samples of 180 mg fresh weight (about 50 root systems per sample) were lyophilized and then processed as described by Kisiala et al (2013); for details see Supplemental Materials and Methods S1. In nodulation assays, composite plants expressing the pEPI:AtCKX3 or pCO: AtCKX3 construct were grown in vitro on paper-covered Fahraeus medium without nitrogen and inoculated with S. meliloti 2011 as before; infection threads and nodules were counted at 6 and 14 d post inoculation, respectively, following lacZ staining performed as described previously (Vernié et al, 2008).…”

Dynamics and origin of cytokinins involved in plant manipulation by a leaf‐mining insect

“…Production of CKs has been demonstrated in gall‐inducing bacteria (e.g., Stes et al ., , ), nodulating bacteria (e.g., Frugier et al ., ; Kisiala et al ., ) and plant‐associated fungi and viruses (e.g., Walters et al ., ; Baliji et al ., ; Morrison et al ., ). Additionally, CKs have been detected in the body, saliva or accessory glands of insects suggesting their ability to produce and deliver these effectors to the plant (Giron et al ., ; Bartlett & Connor, ; Tooker & Helms, ).…”