Abstract
Clubroot, caused by the obligate biotrophic protist Plasmodiophora brassicae is a devastating disease of crucifers that causes substantial economic loss worldwide. The disease is characterized by the formation of galls in the root and hypocotyl of infected plants which restricts host vascular cambium development inhibiting efficient water and nutrient uptake by the plant. The pathogen-driven interference of hormonal homeostasis, particularly of cytokinin, in the root tissue is intricately linked with induction of hypertrophy and cell divisions leading to formation of galls. Levels of cytokinins and cell division generally increase at the onset of the disease which declines at the later stages of gall formation. The genes involved cytokinin biosynthesis such as cytokinin oxidase/dehydrogenases and isopentenyl transferases shows differential expressions during clubroot infection and gall expansion in root tissues. Wider understanding of the roles of cytokinins and associated genes along the development of the disease will be helpful in unravelling plants defense mechanism against clubroot disease.
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Key words: Brassica, Clubroot, Plasmodiophora brassicae, Hormone, Cytokinin
INTRODUCTION
Clubroot disease, caused by
Plasmodiophora brassicae Woronin, is a serious concern for growers of Brassicaceae crops and is one of the most common diseases of oilseed rape in North America and Europe (
Robak 1991;
Agrios 2005;
Dixon 2009;
Lüders et al. 2011). The pathogen
P. brassicae belonging to plasmodiophorids of the eukaryote supergroup Rhizaria, is an obligate biotrophic protist which is distinctively different from other plant pathogens such as fungi or oomycetes. There exists wide diversity in the population of this pathogen which enables it to overcome the resistance shown by different crop species including
Brassica oleracea or
Brassica rapa (
Jones et al. 1982;
Some et al. 1996). The pathogen has substantial variability in Korea as well (
Kim et al. 2016).
Kim et al. (2016) recently grouped 12 Korean field isolates of
P. brassicae into four pathotypes based on their reactions to the Williams’ differential set: pathotypes 1 (Gangneung 1, Gangneung 2, Goesan, Jeongseong and Hoengseong), 2 (Daejon and Keumsan), 3 (Haenam 1, Pyeongchang, Yeoncheon) and 4 (Haenam 2 and Seosan). A Japanese clubroot-resistant (CR) cultivar Akimeki exhibits resistance to all but two (Haenam 2 and Seosan) Korean field isolates. Ribosomal DNA sequencing revealed that these isolates have variable nucleotide sequences in the smaller subunit of intron 1 (
Laila et al. 2017). Due to this pathotype diversity, the disease severity of clubroot disease vary greatly among different regions. A severe outbreak of clubroot disease in Korean Chinese cabbage, the most important ingredient of the famous Korean dish, baechu kimchi, was reported in the last decade (
Cho and Kenji 2003). The outbreak of clubroot disease is predominantly being recorded to be resulted from movement of infested soil, plant materials and other farm machineries (
Strelkov and Hwang 2014).
P. brassicae is obligatory in nature and has a complex life cycle having two distinct zoosporic stages namely, formation of plasmodia and formation of resting spore inside host cells (
Fig. 1). The infection process of
P. brassicae can generally be separated into two distinct phases, primary infection and secondary infection, based on the nature of the infection and the type of root being infected. The primary infection stage is generally the first 72 hours of pathogen attack when the zoospores, released by the haploid resting spore, infect root hairs (
Fig. 1) (
Dekhuijzen 1981;
Müller and Hilgenberg 1986). Subsequently, the secondary phase of infection occurs when the zoospores invade the root cortex and steles of both the hypocotyls and roots of infected plants and develop secondary multinucleate plasmodia. Plasmodia causes rapid cell proliferation (hyperplasia) and abnormal cell enlargement (hypertrophy) which eventually results in the development of abnormal growth of root tissues, commonly known as ‘galls’. At severe cases, galls disrupt the usual water and nutrient transport in the root leading to permanent wilting of plants. The abundance and durability of spores and lack of suitable chemical control measures makes it difficult to control the disease. The extensive cell division and cell enlargement that occur in galls during secondary infection phase is due to altered hormonal balance, mainly of Auxin and cytokinin in root tissues (
Dekhuijzen and Overeem 1971;
Butcher et al. 1974). Here, we discuss the role of cytokinins and genes responsible for cytokinin biosynthesis on clubroot disease development.
Plasmodiophora brassicae mediates hormone homeostasis in host plants
Plant tissue infected with
P. brassicae has been shown to modify the balance of at least four plant hormones namely, cytokinins, auxins, salicylic acid and jasmonic acid (
Schwelm et al. 2015). Phytohormone homeostasis in hypertrophied roots due to
P. brassicae invasion at the beginning of gall formation have been a focus of investigation over the past years. The pathogen-driven interference of the plant root phytohormone system, particularly the alterations in the two major hormone groups, auxins (
Butcher et al. 1974) and cytokinins (
Dekhuijzen 1981), are found to induce hypertrophy and cell divisions by reprogramming existing meristematic activity in infected host roots leading to formation of gall (
Fig. 1) (
Ludwig-Müller 1999;
Siemens 2006;
Malinowski et al. 2012;
Boivin et al. 2016).
Against clubroot, roles of key plant hormones, especially cytokinins and auxins have been extensively investigated in the past decades (reviewed in
Giron et al. 2013;
Boivin et al. 2016;
Malinowski et al. 2016). Club growth or gall formation has been speculated to be the overall result of auxin and cytokinin homeostasis (
Ludwig-Müller et al. 2009;
Schuller et al. 2014;
Jia et al. 2017;
Ludwig-Müller et al. 2017;
Ciaghi et al. 2018). Elevated levels of cytokinins and increased cell division were observed in the early stages of
P. brassicae infection in
Arabidopsis thaliana. But at the later stages of gall formation, decreased levels of cytokinins and repressed expressions of host cytokinin biosynthetic genes mainly cytokinin oxidases and dehydrogenases were observed (
Devos et al. 2006;
Siemens et al. 2006;
Malinowski et al. 2016). Cytokinin responsiveness was induced during the initial stages (3 days after inoculation, dai) of clubroot disease in the cortex and vascular tissues of the infected root and hypocotyls (visualized by ARR5:GUS expression), which was then observed in all cell layers from 5 dai onwards throughout the period of gall formation (
Devos and Prinsen 2006;
Siemens et al. 2006). Overexpression of
AtCKX harboring cytokinin-inducible promoter
ARR5 gene in
A. thaliana caused downregulation of cytokinins, whereas downregulation of
AtCKX and
AtADK were associated with higher levels of active cytokinins during
P. brassicae infection (
Siemens et al. 2006;
Ludwig-Müller et al. 2009). These further confirms the role of cytokinins in plants resistance to
P. brassicae.
In turnip (
B. rapa) as well, cytokinin levels were higher in infected roots compared to healthy roots (
Dekhuijzen 1981). Plasmodia of
P. brassicae are also believed to produce minute amounts of cytokinins, which are released into the cytoplasm of the host cells during invasion that helps in the proliferation of root tissues (
Fig. 1) (
Dekhuijzen 1981;
Müller and Hilgenberg 1986). This is further supported by the finding that isopentenyl transferase genes (
BrIPT1, 3, 5 and
7) are transiently expressed at high levels in host tissues prior to gall formation which during during gall formation were found to be downregulated, suggesting that pathogen-induced cytokinin production triggers gall formation (
Ando et al. 2005). Besides, cytokinins were also found to interfere with the production of invertase and sugar metabolism which might be crucial for the nutrition of
P. brassicae (
Siemens et al. 2011). However, laser microdissection and pressure catapulting (LMPC) analysis revealed the upregulation of genes involved in auxin and cytokinin metabolism during this process (
Schuller et al. 2014). The fluctuation in plant hormone contents does not correspond to the severity of infection in cabbage roots (
Schuller and Ludwig-Müller 2006). Transcriptome analysis during the root hair infection phase preceding infection of the cortex revealed only a small number of genes with altered expression in
A. thaliana (
Agarwal et al. 2011). By contrast, comparisons of the transcriptomes of the root cortex during two time points of the secondary infection phase revealed numerous up- and downregulated genes (
Siemens et al. 2006). Some root-specific cytokinin oxidase genes were down-regulated in root gall tissues compared with the controls (
Siemens et al. 2006), whereas some were upregulated at a later time point of infection (
Schuller et al. 2014). Understanding the detailed roles of the key cytokinin related genes mainly, cytokinin oxidase/dehydrogenases (CKXs) and isopentenyl transferases (IPTs) during clubroot infection and gall expansion in plants, especially in commercially important crops like canola and cabbage will be crucial for improving crops resistance towards the disease.
Cytokinin oxidase/dehydrogenases (CKXs) is responsible in catalyzing irreversible degradation of cytokinin (
Brownlee et al. 1975;
Armstrong 1994;
Kakimoto 2001). In this process of degradation of cytokinin and their derivatives, these enzymes remove N
6-substituted isoprene chains of isopentenyladenine and isopentenyladenine riboside (
Galuszka et al. 2000;
Motyka et al. 2003). In the catalytic process, the CKX enzymes which act as electron acceptors, display a dual catalytic mode in association with molecular oxygen and other specific substances (
Frebortova et al. 2004). These enzymes show greater thermo-stability whereas optimum pH requirement was found to be substrate dependent (
Kopecný et al. 2005). As CKXs enzyme bear both flavin adenine dinucleotide (FAD) and CK-binding domains, these enzymes might act as a substitute of flavin enzymes (
Esparza and Morris 2001). Moreover, post-translational glycosylation of CKX proteins might alter their molecular masses and localization (
Motyka et al. 2003).
CKX is a small gene family that encodes CKX proteins (
Werner et al. 2006). CKX family genes and putative members were found to be functionally expressed in the members of both Brassicaceae family such as
B. rapa (
Liu et al. 2013),
A. thaliana (
Galuszka et al. 2007) and several other families. The pivotal roles of the catabolic enzyme cytokinin oxidase (CKX) in controlling the accumulation of cytokinins has been found in various tissues of many higher plants (
Mok 1994).There are seven CKX genes in
A. thaliana: AtCKX1–AtCKX7 those differ not only in their subcellular localizations and biochemical characteristics but also in their expression patterns. Experimental evidences suggest that diverse functions of CKXs might be associated with spatial and temporal expression patterns, since in transgenic Arabidopsis reduced cytokinin levels and developmental changes in roots and shoots are reported (
Werner et al. 2003;
Galuszka et al. 2007). In
B. napus, hormonal treatments altered responses of CKX genes (
Liu et al. 2018). Our unpublished data showed that five
CKX genes showed variation in expression both in roots and shoots. Moreover, expression level of genes differed greatly before and after gall development in
B. rapa indicating that
CKX genes have vital role in clubroot disease development.
Properties and functions of Isopentenyl transferases (IPTs)
In
A. thaliana, two classes of Isopentenyl transferases (IPTs) are evident based on the use of substrates:
AtIPT1,
3,
4–8 encode forms that use ADP/ATP as substrates whilst
AtIPT2 and
9 encode forms that use tRNA as substrates (
Miyawaki et al. 2006). IPTs are involved in rate limiting steps in cytokinin biosynthesis (
El-Showk et al. 2013). The IPTs synthesized from ADP/ATP are the active enzymes whilst IPTs synthesized from tRNA shows limited activity in
A. thaliana (
Gajdošová et al. 2011). Similar to CKXs, the IPTs also show spatial and temporal variation in level of expression. In
P. brassicae infected plants two genes;
IPT3 and
IPT5 exhibited abundant expression in both root and hypocotyl tissue but the expression level was greatly (8 folds) reduced in infected tissues compared to un-infected tissues (
Malinowski et al. 2016). This results speculatively indicated that the repressed expression of these two genes might be associated with transcriptional regulation rather than alterations in the proportion of specific cell types. Compared to
IPT3 and
IPT5, the expression of
IPT1 and
IPT7 was reduced and thus, it is evident that these two genes,
IPT1 and
IPT7, might have insignificant contribution in cytokinin biosynthesis. Similarly, a lower expression of
IPT2 and
IPT9 genes both in infected and un-infected tissues also indicated that those two genes also have less contribution in gall formation (
Malinowski et al. 2016).
P. brassicae has two IPT genes whose substrate specificity are yet to be characterized. However, these two genes show greater sequence homology to tRNA IPTs. Both of these two genes were expressed at both 16 and 26 days post-inoculation (dpi) suggesting that these two genes contributes in cytokinin accumulation in infected tissue (
Malinowski et al. 2016). In
ipt1;3;5;7 mutants, cytokinin biosynthesis was greatly compromised due to reduced expression of
ARR4–7 genes and as a result, secondary infection of clubroot was reduced. Microarray analysis confirmed that reduction of infection was not due to a compensatory increase in expression of other host
IPT genes, since
ipt1;3;5;7 genes showed no change in expression. Isolated plasmodia are able to take up radio labelled adenine and able to make small amounts of a compound that are designated as
tZ (
Müller and Hilgenberg 1986). However, the amount of tZ being released, stimulating type A
ARR genes expression in the
ipt1;3;5;7 mutant, is not sufficient to rescue mutant phenotype from restoring vascular cambium development. Thus, it can be postulated that the amount of cytokinin produced by
P. brassicae have minor contribution on the development of its host. In
ipt1;3;5;7 mutant disease development was restricted through restricting hyperplasia but infected cells still show hypertrophy symptom. Gibberellic acid (GA) has no effect on gall formation (
Päsold and Ludwig-Müller 2013) but brassinosteroid (BR) biosynthesis influence gall formation (
Schuller et al. 2014). Cytokinins have an important role in the development of
P. brassicae. Vascular cambial activity is required for gall to be formed and the infection by the pathogen is not dependent on this activity (
Malinowski et al. 2012).
Enhancing plant’s resistance: prospects of cytokinin related transgenes
Overexpression of isopentenyl transferase genes in
PSAG12 ::IPT and PSAG13 ::IPT lines of A. thaliana showed elevated resistance to necrotrophic fungus
Botrytis cinerea, causing botrytis bunch rot or grey mould in horticultural crops (
Swartzberg et al. 2008). Overexpression of cytokinin-activated transcription factor
ARR2 have shown to enhance cytokinin levels, delay leaf senescence and increase resistance to
Pseudomonas syringae (
Choi et al. 2010). In Brassicaceae crops, transgenic studies using key cytokinin related genes remains to be done. However, transgenic
B. rapa line (Crr1a
G004 promoter::Crr1a
G004) overexpressing TIR-NB-LRR–type R genes
Crr1a induced resistance to clubroot in a susceptible cultivar (
Hatakeyama et al. 2013). The variable role of cytokinins and associated genes in different pathosystems calls for further research to identify resistant cytokinin gene mediated plant and pathogen interaction (
Hirani et al. 2015;
Nafisi et al. 2015;
Ludwig-Müller et al. 2017;
Ciaghi et al. 2018).
Cytokinin is required for P. brassicae for its development. Pathogen itself produce minute amount of cytokinin and withdraw cytokinin from its host. Vascular cambial (VC) development of the host tissue, dependent on cytokinin, is essential for secondary development and active growth of plants. However, utilization of host’s cytokinin and obstruction of VC route via forming galls by P. brassicae disrupts plants normal growth and development. Wider understanding of the mechanism of disease development and plants defense strategy require further investigation on complex homeostasis of cytokinins with other hormones such as auxins and brassinosteroids. The exact roles of the genes related with cytokinin synthesis or degradation against various pathotypes along the process of disease development remains to be determined. The genes that lie within the already identified QTLs in various crop plants especially in Brassica species may hold the key for improving resistance to clubroot disease in crop plants.
ACKNOWLEDGEMENTS
This paper was supported by Sunchon National University Research Fund in 2019.
Fig. 1The life cycle of
Plasmodiophora brassicae and the possible integration of cytokinin signaling pathways by this pathogen during clubroot formation (after
Dekhuijzen 1981;
Müller and Hilgenberg 1986). dpi = days post inoculation.
Table 1Transgenic Arabidopsis lines showing elevated resistance against Plasmodiophora brassicae and other phytopathogens upon overexpression of cytokinin related genes.
Table 1
|
Genez)
|
Line/Mutant |
Pathogen (isolate) |
Reference |
|
Cytokinin oxidase/dehydrogenases |
35S::AtCKX1
|
P. brassicae (1CK) |
, Siemens et al. 2006
|
|
|
P. brassicae (e2) |
|
|
|
P. brassicae (eH) |
|
|
|
P. brassicae (k1) |
|
|
35S::AtCKX3
|
P. brassicae (1CK) |
|
|
|
P. brassicae (e2) |
|
|
|
P. brassicae (eH) |
|
|
|
P. brassicae (k1) |
|
|
Isopentenyl transferase |
P SAG12 ::IPT
|
B. cinerea
|
, Swartzberg et al. 2008
|
|
P SAG13 ::IPT
|
B. cinerea
|
|
|
Cytokinin-activated transcription factor |
35S:ARR2
|
P. syringae
|
, Choi et al. 2010
|
References
- Agarwal A, Kaul V, Faggian R, Rookes JE, Ludwig-Müller J, Cahill DM. 2011. Analysis of global host gene expression during the primary phase of the Arabidopsis thaliana–Plasmodiophora brassicae interaction. Funct Plant Biol. 38: 462-478.
- Agrios GN. 2005. Plant Pathology. 5th edition. San Diego: Elsevier-Academic Press. pp. 922.
- Alix K, Lariagon C, Delourme R, Manzanares-Dauleux MJ. 2007. Exploiting natural genetic diversity and mutant resources of Arabidopsis thaliana to study the A. thaliana–Plasmodiophora brassicae interaction. Plant Breeding. 126: 218-221.
- Ando S, Asano T, Tsushima S, Kamachi S, Hagio T, Tabei Y. 2005. Changes in gene expression of putative isopentenyltransferase during clubroot development in Chinese cabbage (Brassica rapa L.). Physiol Mol Plant Pathol. 67: 59-67.
- Andolfo G, Iovieno P, Frusciante L, Ercolano MR. 2016. Genome-editing technologies for enhancing plant disease resistance. Front Plant Sci. 7: 1813.
- Armstrong DJ. 1994. Cytokinin oxidase and the regulation of cytokinin degradation Cytokinins: Chemistry, Activity, and Function. CRC Press. Boca Raton, FL: pp. 139-154.
- Boivin S, Fonouni-farde C, Frugier F. 2016. How Auxin and Cytokinin Phytohormones Modulate Root Microbe Interactions. 7(August): 1-12.
- Borrelli VM, Brambilla V, Rogowsky P, Marocco A, Lanubile A. 2018. The enhancement of plant disease resistance using CRISPR/Cas9 technology. Front Plant Sci. 9:
- Brownlee BG, Hall RH, Whitty CD. 1975. 3-Methyl-2-butenal: an enzymatic degradation product of the cytokinin, N-6-(delta-2 isopentenyl) adenine. Can J Biochem. 53: 37
- Butcher DN, El-Tigrani S, Ingram DS. 1974. The role of indole glucosinolates in the clubroot disesase of the Cruciferae. Physiol Plant Pathol. 4: 127-140.
- Chen Y-J, Lyngkjær MF, Collinge DB. 2012. Future prospects for genetically engineering disease-resistant plants. pp. 251-275. Sessa G, editor. Molecular plant immunity. Wiley-Blackwell. New Jersey:
- Chen YC, Kidd BN, Carvalhais LC, Schenk PM. 2014. Molecular defense responses in roots and the rhizosphere against Fusarium oxysporum. Plant Signal Behav. 9: e977710
- Cho WD, Kenji T. 2003. Occurrence of clubroot in cruciferous vegetable crops and races of the pathogen in Korea. Plant Pathol J. 19: 64-8.
- Choi J, Huh SU, Kojima M, Sakakibara H, Paek KH, Hwang I. 2010. The cytokinin-activated transcription factor ARR2 promotes plant immunity via TGA3/NPR1-dependent salicylic acid signaling in Arabidopsis. Dev Cell. 19: 284-295.
- Choi J, Daeseok C, Seungchul L, Choong-Min R, Ildoo H. 2011. “Cytokinins and plant immunity: old foes or new friends?”. Trends Plant Sci. 16: 388-394.
- Ciaghi S, Schwelm A, Neuhauser S. 2018. Transcriptomic response in symptomless roots of clubroot infected kohlrabi mirrors resistant plants. bioRxiv. http://dx.doi.org/10.1101/391516
- De Vleesschauwer D, Xu J, Höfte M. 2014. Making sense of hormone-mediated defense networking: from rice to Arabidopsis. Front Plant Sci. 5: 611
- Dekhuijzen HM, Overeem JC. 1971. The role of cytokinins in clubroot formation. Physiol Plant Pathol. 1: 151-161.
- Dekhuijzen HM. 1981. The occurrence of free and bound cytokinins in plasmodia of Plasmodiophora brassicae isolated from tissue cultures of clubroots. Physiologia Plantarum. 49: 169-176.
- Denancé N, Sánchez-Vallet A, Goffner D, Molina A. 2013. Disease resistance or growth: the role of plant hormones in balancing immune responses and fitness costs. Front Plant Sci. 4: 155
- Dervinis C, Frost CJ, Lawrence SD, Novak NG, Davis JM. 2010. Cytokinin primes plant responses to wounding and reduces insect performance. J Plant Growth Regul. 29: 289-296.
- Devos S, Laukens K, Deckers P, Van Der Straeten D, Beeckman T, Inzé D, et al. 2006. A hormone and proteome approach to picturing the initial metabolic events during Plasmodiophora brassicae infection on Arabidopsis. Mol Plant Microbe Interact. 19: 1431-1443.
- Devos S, Prinsen E. 2006. Plant hormones: a key in clubroot development. Commun Agric Appl Biol Sci. 713(Pt B): 869-872.
- Diederichsen E, Frauen M, Linders EG, Hatakeyama K, Hirai M. 2009. Status and perspectives of clubroot resistance breeding in crucifer crops. J Plant Growth Regul. 28: 265-281.
- Dixon GR. 2009. The occurrence and economic impact of Plasmodiophora brassicae and clubroot disease. J Plant Growth Regul. 28: 194-202.
- El-Showk S, Ruonala R, Helariutta Y. 2013. Crossing paths: cytokinin signalling and crosstalk. Development. 1407: 1373-1383.
- Esparza TJ, Morris RO. 2001. Molecular and biochemical characterization of a cytokinin oxidase from maize. Plant Physiol. 125: 378-386.
- Frebortova J, Fraaije MP, Sebela M, Pec P, Hrbac J. 2004. Catalytic reaction of cytokinin dehydrogenase: preference for quinones as electron acceptors. Biochem J. 380: 121
- Gajdošová S, Spíchal L, Kamínek M, Hoyerová K, Novák O, Dobrev PI, et al. 2011. Distribution, biological activities, metabolism, and the conceivable function of cis-zeatin-type cytokinins in plants. J Exp Bot. 62: 2827-2840.
- Galuszka P, Frébort I, Šebela M, Peč P. 2000. Degradation of cytokinins by cytokinin oxidases in plants. Plant Growth Regul. 32: 315-327.
- Galuszka P, Popelková H, Werner T, Frébortová J, Pospíšilová H. 2007. Biochemical characterization and histochemical localization of cytokinin oxidases/dehydrogenases from Arabidopsis thaliana expressed in Nicotiana tabaccum L. J Plant Growth Regul. 26: 255-267.
- Gan SS, Amasino RM. 1995. Inhibition of leaf senescence by auto- regulated production of cytokinin. Science. 270: 1986-1988.
- Giron D, Frago E, Glevarec G, Pieterse CM, Dicke M. 2013. Cytokinins as key regulators in plant–microbe–insect interactions: connecting plant growth and defence. Funct Ecol. 27: 599-609.
- Giron D, Kaiser W, Imbault N, Casas J. 2007. Cytokinin-mediated leaf manipulation by a leaf miner caterpillar. Biol Lett. 3: 340-343.
- Grsic-Rausch S, Kobelt P, Siemens J, Bischoff M, Ludwig-Müller J. 2000. Expression and localization of nitrilase during symptom development of the clubroot disease in Arabidopsis thaliana. Plant Physiol. 122: 369-378.
- Hatakeyama K, Suwabe K, Tomita RN, Kato T, Nunome T, Fukuoka H, et al. 2013. Identification and characterization of Crr1a, a gene for resistance to clubroot disease (Plasmodiophora brassicae Woronin) in Brassica rapa L. PLoS One. 8: e54745
- Hirani AH, Li G. 2015. Understanding the genetics of clubroot resistance for effectively controlling this disease in Brassica species. Plants for the Future. IntechOpen.
- Hwang I, Sheen J, Müller B. 2012. Cytokinin signaling networks. Annu Rev Plant Biol. 63: 353-380.
- Jahn L, Mucha S, Bergmann S, Horn C, Siemens J, Staswick P, et al. 2013. The clubroot pathogen (Plasmodiophora brassicae) influences auxin signaling to regulate auxin homeostasis. Plants. 2: 726-749.
- Jameson P. 2000. Cytokinins and auxins in plant pathogen interactions-an overview. Plant Growth Regul. 32: 369-380.
- Jia H, Wei X, Yang Y, Yuan Y, Wei F, Zhao Y, et al. 2017. Root RNA-seq analysis reveals a distinct transcriptome landscape between clubroot-susceptible and clubroot-resistant Chinese cabbage lines after Plasmodiophora brassicae infection. Plant Soil. 421: 93-105.
- Jing H, Strader LC. 2019. Interplay of Auxin and Cytokinin in Lateral Root Development. Int J Mol Sci. 20: 486
- Jones DR, Ingram DS, Dixon GR. 1982. Factors affecting tests for differential pathogenicity in populations of Plasmodiophora brassicae. Plant Pathol. 31: 229-38.
- Jones JD, Witek K, Verweij W, Jupe F, Cooke D, Dorling S, et al. 2014. Elevating crop disease resistance with cloned genes. Philos Trans R Soc Lond B Biol Sci. 369: 20130087
- Kakimoto T. 2001. Identification of plant cytokinin biosynthetic enzymes as dimethylallyl diphosphate: ATP/ADP isopentenyltransferases. Plant Cell Physiol. 42: 677-685.
- Kim H, Jo EJ, Choi YH, Jang KS, Choi GJ. 2016. Pathotype Classification of Plasmodiophora brassicae Isolates Using Clubroot-Resistant Cultivars of Chinese Cabbage. Plant Pathol J. 32: 423
- Kopecný D, Pethe C, Sebela M, Houbahérin N, Madzak C, Majira A, et al. 2005. High-level expression and characterization of Zea mays cytokinin oxidase/dehydrogenase in Yarrowia lipolytica. Biochimie. 87: 1011-1022.
- Laila R, Robin AHK, Yang K, Choi GJ, Park JI, Nou IS. 2017. Detection of Ribosomal DNA sequence polymorphisms in the protist Plasmodiophora brassicae for the identification of geographical isolates. Int J Mol Sci. 18: 84
- Langner T, Kamoun S, Belhaj K. 2018. CRISPR crops: plant genome editing toward disease resistance. Annu Rev Phytopathol. 56: 479-512.
- Liu P, Zhang C, Ma J-Q, Zhang L-Y, Yang B, Tang X-Y, et al. 2018. Genome-Wide Identification and Expression Profiling of Cytokinin Oxidase/Dehydrogenase (CKX) Genes Reveal Likely Roles in Pod Development and Stress Responses in Oilseed Rape (Brassica napus L.). Genes. 9: 168
- Liu Z, Lv Y, Zhang M, Liu Y, Kong L, Zou , et al. 2013. Identification expression and comparative genomic analysis of the ipt and ckx gene families in Chinese cabbage (Brassica rapa ssp. Pekinensis). BMC Genomics. 14: 594
- Lüders W, Abel S, Friedt W, Kopahnke D, Ordon F. 2011. Phenotypic and molecular characterisation of new clubroot resistance genes derived from the primary gene pool of Brassica napus. Nachwuchswissenschaftlerforum. 2011.
- Ludwig-Müller J, Rolfe SA, Scholes JD. 2009. Metabolism and plant hormone action during clubroot disease metabolism and plant hormone action during clubroot disease. J Plant Growth Regul. 28: 229
- Ludwig-Müller J. 1999. Plasmodiophora brassicae, the causal agent of clubroot disease: a review on molecular and biochemical events in pathogenesis. J Plant Dis Protect. 106: 109-127.
- Ludwig-Müller J, Auer S, Jülke S, Marschollek S. 2017. Manipulation of auxin and cytokinin balance during the Plasmodiophora brassicae–Arabidopsis thaliana interaction. Auxins and Cytokinins in Plant Biology. Humana Press. New York, NY: pp. 41-60.
- Malinowski R, Novák O, Borhan MH, Spíchal L, Strnad M, Rolfe SA. 2016. The role of cytokinins in clubroot disease. Eur J Plant Pathol. 145: 543-557.
- Malinowski R, Smith JA, Fleming AJ, Scholes JD, Rolfe SA. 2012. Gall formation in clubroot-infected Arabidopsis results from an increase in existing meristematic activities of the host but is not essential for the completion of the pathogen life cycle. Plant J. 71: 226-238.
- Mapes CC, Davies PJ. 2001. Cytokinins in the ball gall of Solidago altissima and in the gall forming larvae of Eurosta solidaginis. New Phytol. 151: 203-212.
- Miyawaki K, Tarkowski P, Matsumoto-Kitano M, Kato T, Sato S, Tarkowska D, et al. 2006. Roles of Arabidopsis ATP/ADP isopentenyltransferases and tRNA isopentenyltransferases in cytokinin biosynthesis. Proc Natl Acad Sci U S A. 103: 16598-16603.
- Mok DWS, Mok MC. 2001. Cytokinin metabolism and action. Annu Rev Plant Physiol Plant Mol Biol. 52: 89-118.
- Mok MC. 1994. Cytokinins and plant development Cytokinins: chemistry, activity, and function. CRC Press. Boca Raton: pp. 155-166.
- Motyka V, Vaňková R, Čapková V, Petrášek J, Kamínek M, Schmülling T. 2003. Cytokinin-induced upregulation of cytokinin oxidase activity in tobacco includes changes in enzyme glycosylation and secretion. Physiol Plant. 117: 11-21.
- Müller P, Hilgenberg W. 1986. Isomers of zeatin and zeatin riboside in clubroot tissue: evidence for trans-zeatin biosynthesis by Plasmodiophora brassicae. Physiol Plant. 66: 245-250.
- Nafisi M, Fimognari L, Sakuragi Y. 2015. Interplays between the cell wall and phytohormones in interaction between plants and necrotrophic pathogens. Phytochemistry. 112: 63-71.
- Naseem M, Dandekar T. 2012. The role of auxin-cytokinin antagonism in plant-pathogen interactions. PLoS Pathog. 8: e1003026
- Naseem M, Wölfling M, Dandekar T. 2014. Cytokinins for immunity beyond growth, galls and green islands. Trends Plant Sci. 19: 481-484.
- Päsold S, Siegel I, Seidel C, Ludwig-Müller J. 2010. Flavonoid accumulation in Arabidopsis thaliana root galls caused by the obligate biotrophic pathogen Plasmodiophora brassicae. Mol Plant Pathol. 11: 545-562.
- Päsold S, Ludwig-Müller J. 2013. Reduction of clubroot Plasmodiophora brassicae formation in Arabidopsis thaliana after treatment with prohexadione-calcium, an inhibitor of oxoglutaric acid-dependent dioxygenases. Plant Pathol. 62: 1357-1365.
- Pieterse CMJ, Van der Does D, Zamioudis C, Leon-Reyes A, Van Wees SCM. 2012. Hormonal modulation of plant immunity. Annu Rev Cell Dev Biol. 28: 489-521.
- Robak J. 1991. Variability of the pathotypes of Plasmodiophora brassicae Wor, Present in Poland and Their Pathogenicity to the Cultivars and Breeding Lines of Brassica oleracea [in Polish]. Habilitation monograph. Skierniewice: InstytutWarzywnictwa. pp. 59.
- Robert-Seilaniantz A, Grant M, Jones JDG. 2011. Hormone crosstalk in plant disease and defense: more than just jasmonate-salicylate antagonism. Annu Rev Phytopathol. 49: 317-343.
- Saharan V, Jain D, Pareek S, Pal A, Kumaraswamy RV, Jakhar SK, et al. 2016. Viral, fungal and bacterial disease resistance in transgenic plants. Advances in Plant Breeding Strategies: Agronomic, Abiotic and Biotic Stress Traits. Springer. Cham: pp. 627-656.
- Sakakibara H. 2006. Cytokinins: activity, biosynthesis, and translocation. Annu Rev Plant Biol. 57: 431-449.
- Sasaki T, Suzaki T, Soyano T, Kojima M, Sakakibara H, Kawaguchi M. 2014. Shoot-derived cytokinins systemically regulate root nodulation. Nat Commun. 5: 4983
- Schuller A, Ludwig-Müller J. 2006. A family of auxin conjugate hydrolases from Brassica rapa: characterization and expression during clubroot disease. New Phytol. 171: 145-158.
- Schuller A, Kehr J, Ludwig-Müller J. 2014. Laser microdissection coupled to transcriptional profiling of Arabidopsis roots inoculated by Plasmodiophora brassicae indicates a role for brassinosteroids in clubroot formation. Plant Cell Physiol. 55: 392-411.
- Schwachtje J, Baldwin IT. 2008. Why does herbivore attack reconfigure primary metabolism? Plant Physiol. 146: 845-851.
- Schwelm A, Fogelqvist J, Knaust A, Jülke S, Lilja T, Bonilla-Rosso , et al. 2015. The Plasmodiophora brassicae genome reveals insights in its life cycle and ancestry of chitin synthases. Sci Rep. 5: 11153
- Siemens J, González MC, Wolf S, Hofmann C, Greiner S, Du Y, et al. 2011. Extracellular invertase is involved in the regulation of clubroot disease in Arabidopsis thaliana. Mol Plant pathol. 12: 247-262.
- Siemens J, KellerISarx J, Kunz S, Schuller A, Nagel W, et al. 2006. Transcriptome analysis of Arabidopsis clubroots indicate a key role for cytokinins in disease development. Mol Plant Microbe Interact. 19: 480-494.
- Some A, Manzanares MJ, Laurens F, Baron F, Thomas G, Rouxel F. 1996. Variation for virulence on Brassica napus L. amongst Plasmodiophora brassicae collections from France and derived single-spore isolates. Plant Pathol. 45: 432-439.
- Stone GN, Schönrogge K. 2003. The adaptive significance of insect gall morphology. Trends Ecol Evol. 18: 512-522.
- Strelkov SE, Hwang SF. 2014. Clubroot in the Canadian canola crop: 10 years into the outbreak. Canadian Journal of Plant Pathology. 36(sup1): 27-36.
- Swartzberg D, Kirshner B, Rav-David D, Elad Y, Granot D. 2008. Botrytis cinerea induces senescence and is inhibited by autoregulated expression of the IPT gene. Eur J Plant Pathol. 120: 289-297.
- Tytgat TO, Verhoeven KJ, Jansen JJ, Raaijmakers CE, Bakx-Schotman T, McIntyre , et al. 2013. Plants know where it hurts: root and shoot jasmonic acid induction elicit differential responses in Brassica oleracea. PLoS One. 8: e65502
- Vysotskaya LB, Korobova AV, Kudoyarova GR. 2008. Abscisic acid accumulation in the roots of nutrient-limited plants: its impact on the differential growth of roots and shoots. J Plant Physiol. 165: 1274-1279.
- Werner T, Köllmer I, Bartrina I, Holst K, Schmülling T. 2006. New insights into the biology of cytokinin degradation. Plant Biol. 8: 371
- Werner T, Motyka V, Laucou V, Smets R, Onckelen HV, Schmülling T. 2003. Cytokinin-deficient transgenic Arabidopsis plants show multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristem activity. Plant Cell. 15: 2532-2550.
- Yin K, Qiu JL. 2019. Genome editing for plant disease resistance: applications and perspectives. Philos Trans R Soc Lond B Biol Sci. 374: 20180322
