The role of cytokinins in clubroot disease

Abstract

Clubroot (Plasmodiophora brassicae) is a pathogen of Brassicaceae that causes significant reductions in yield as a consequence of gall formation in the root and hypocotyl of infected plants. The pathogen hijacks host vascular cambium development, and cytokinins are implicated in this process. This paper uses transcriptomics and metabolomics to investigate changes in cytokinin metabolism during gall formation of clubrootinfected Arabidopsis thaliana. RNASeq analysis of infected tissue showed that host cytokinin metabolism was strongly down-regulated both at the onset and late stages of gall formation. Expression of host genes associated with cytokinin biosynthesis, signalling, degradation and conjugation was strongly repressed. Analysis of cytokinin precursors, active components and conjugates by microanalytical techniques was consistent with these transcriptional responses. Two isopentenyltransferase genes associated with cytokinin biosynthesis are present in the P. brassicae genome and are expressed throughout gall formation. The impact of pathogen-derived cytokinins on the total cytokinin content of infected tissue and host gene expression was minimal in wild type plants. However, infection of ipt1;3;5;7 mutants that are severely restricted in their ability to synthesise active cytokinins led to an increase in expression of host cytokininresponsive genes. We interpret these results as indicating that P. brassicae can synthesise small amounts of cytokinin, but this has little impact on the host plant as the ipt1;3;5;7 phenotype is not rescued. Intriguingly, plasmodial development was slowed and spore viability reduced in these mutants indicating a potential role for cytokinins in plasmodial development.

Transcriptomic response in symptomless roots of clubroot infected kohlrabi (Brassica oleracea var. gongylodes) mirrors resistant plants

“…A. thalinana plants 10 dpi (Siemens et al, 2006), in a different study expression of 603 AHK and AHP genes did not differ between infected and uninfected Arabidopsis at 604 later time points (Malinowski et al, 2016). Similar to these findings, in our much longer 605 infected kohlrabi plants AHK and AHP genes were not differential expressed between 606 SL and root galls.…”

Hypoxia response in Arabidopsis roots infected by Plasmodiophora brassicae supports the development of clubroot

“…6). We then followed standard procedures (details in Additional files 1, 2, 3 and 4) to process and normalize the publically available RNAseq data from Malinowski et al [19] (Arrayexpress: E-MTAB-4176), for which Arabidopsis responses to clubroot infection were evaluated in roots and hypocotyls at 16 and 26 dpi (3 biological replicates). A focused analysis (Fig.…”

“…6Transcriptional regulation of a set of 23 hypoxia and N-end rule-regulated genes, in Arabidopsis plants infected with gall-inducing pathogens. Data are from available transcriptome datasets from the literature [4, 10, 19–22] and are expressed here as log2 ratios between inoculated vs non-inoculated conditions for disease responses [4, 19–22], or as indicated for hypoxia and N-end rule driven responses [10]. Stars indicate datasets where only statistically significant regulations are given …”

Not in your usual Top 10: protists that infect plants and algae

“…When galls are formed, the expression of host cytokinin biosynthetic genes is repressed, as is the expression of host cytokinin oxidases and dehydrogenases (Devos et al ., ; Siemens et al ., ). Plasmodiophora brassicae ‐produced cytokinins probably play a minor role in cytokinin homeostasis in infected tissues (Malinowski et al ., ). Arabidopsis mutants of auxin conjugate synthesis, as well as auxin receptors, were more susceptible to the pathogen (Jahn et al ., ), whereas nitrilase mutants were more tolerant (Grsic‐Rausch et al ., ).…”