Transcriptomic response in symptomless roots of clubroot infected kohlrabi mirrors resistant plants

Background Clubroot disease is caused by Plasmodiophora brassicae (Phytomyxea, Rhizaria) and is one of the economically most important diseases of brassica crops. The formation of the typical hypertrophied roots is accompanied by altered metabolism and hormone homeostasis of infected plants. Not all roots of an infected plant show the same phenotypic changes: while some roots remain uninfected, others develop galls of diverse sizes. Aim of this study was to analyse and compare the intra-plant heterogeneity of P. brassicae, root galls and symptomless roots of the same host plants (Brassica oleracea var. gongylodes) collected from a commercial field in Austria using transcriptome analyses. Results Symptomless roots did show transcriptomic traits that had previously described for resistant plants: Genes involved in host cell wall metabolism or salicylic acid (SA) mediated defence response were up-regulated in symptomless roots, while being down-regulated in gall tissues. Transcriptomes between symptomless roots and gall tissue were markedly different, with those differences being in accordance with visible physiological differences between the two tissues. On the pathogen side, a secreted SA methyl transferase (PbBSMT) was one of the highest expressed genes in gall tissues. Conclusions Infected and uninfected roots of the same clubroot infected plant showed transcriptomic differences which were previously only observed between clubroot resistant and susceptible hosts. We provide further evidence for the biological relevance of PbBSMT which on the one hand likely causes a decrease of SA in the galls, while the PbBSMT produced Methyl-SA potentially leads to increased pathogen tolerance in uninfected roots. The here described intra-plant heterogeneity was unexpected and highlights the need for targeted analyses of clubroot interaction using cell, tissue type, organ specific probing to identify traits that prevent the formation of clubroot disease.


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The here described intra-plant heterogeneity was unexpected and highlights the need for 35 targeted analyses of clubroot interaction using cell, tissue type, organ specific probing to 36 identify traits that prevent the formation of clubroot disease.

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Background 55 Clubroot disease is one of the most important diseases of Brassica crops worldwide 56 accounting for approximately 10% loss in Brassica vegetable, fodder, and oilseed crops 57 (Dixon, 2009). Clubroot is caused by Plasmodiophora brassicae, an obligate biotrophic 58 protist, taxonomically belonging to Phytomyxea within the eukaryotic super-group Rhizaria 59 (Bulman & Braselton, 2014, Neuhauser et al., 2014. This soil borne pathogen has a complex

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Therefore, CKs play a crucial role in disease development not only through their regulation of 108 cell division but also through their interference in the sugar metabolism and invertase 109 production, which might be crucial for the nutrition of P. brassicae (Siemens et al., 2011).

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The P. brassicae PbGH3 was also able to conjugate JA with amino acids in vitro 132 (Schwelm et al., 2015). In general, JA is associated with resistance against necrotrophic    with YG and OG (Fig. 3). Genes coding for XTHs were among the strongest down-regulated 260 DEGs in SL, with XTH24 being the strongest down-regulated transcript of all DEGs. The 261 phenylpropanoid pathway was up-regulated in SL compared with YG and OG (Fig. 3). This 262 includes the phenylalanine ammonia-lyase 1 (PAL1) homolog, a key enzyme in lignin 263 biosynthesis. Flavonoid metabolism was also induced in SL (Additional file 1: Figure S2).

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Compared with the uninfected control plant cell wall synthesis genes were up-regulated in SL 265 (Additional file 1: Figure S3) The CK and auxin metabolism was altered in YG and OG compared with SL (Fig. 4) Figure S4). However, an IAA7 gene, a repressor of auxin 285 inducible genes was down-regulated, as well as a homolog of GH3.2. Expression of PIN-286 FORMED 1 (PIN1) genes was reduced in SL (Fig. 4). Whereas SAUR (small auxin up-287 regulated RNA) and AIR12 (auxin-induced in root cultures protein 12-like) genes, were 288 found in up-and down-regulated DEGs (Fig. 4). Myrosinases and nitrilases were up-regulated 289 in SL compared with galls (Additional file 1: Figure S5). Compared with the control CK and 290 auxin metabolism were up-regulated in SL (Additional file 1: Figure S3). In old galls two 291 transcripts related to auxin synthesis and regulation were down-regulated compared with 292 young galls (Additional file 2: Table S4).

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Plant defence 313 Generally, genes for disease resistance proteins were up-regulated in SL compared with YG 314 and OG (Fig. 5).

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In SL JA related genes such as LOX2 (lipoxygenase 2), AOC (allene oxide cyclase), and 334 HPL (hyperoxide lyase) were down-regulated, while other LOX genes and the JA amido 335 synthetase genes JAR1 were up-regulated in SL compared with galls (Fig. 4). One down-336 regulated isoform of JAR1 was found between OG and YG. We found no glucosinolate 337 biosynthesis genes in the DEGs in our study.

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The    but not compared with OG (Fig. 4)  for further studies analysing intra-and inter-tissue specific pattern of clubroot infected plants.

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As genes involved in resistance responses to P. brassicae in were up-regulated in 542 symptomless roots, this might aid the identification of novel traits for resistance breeding.

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The authors declare that they have no competing interests.