Plant basal resistance - Universiteit Utrecht

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Chapter 5 occurred at time-points long preceding major tissue damage. Moreover, it was shown that apoplastic accumulation of DIMBOA signals increased callose deposition (Figure 8; Chapter 3), presumably to keep the accumulation of anti-microbial metabolites concentrated at the sites of colonization. Hence, apoplastic localisation of DIMBOA contributes to early-acting post-invasive defence, which is not only based on its biocidal activity, but also on its role as apoplastic callose-promoting signal. It would be interesting to investigate whether aphids and pathogenic fungi have co-evolved specific effectors that block apoplastic secretion of benzoxazinoid-glucosides and/or hydrolytic activity of corresponding beta-glucosidases. Further research needs to be carried out to answer unanswered questions about the cellular mechanisms regulating apoplastic accumulation of BXs. What are the mechanisms by which BX compounds are deposited into the apoplast? Are the BX compounds secreted in the glycosylated form, along with the corresponding beta-glucosidases? How is the apoplastic DIMBOA perceived by the plant cell? A better understanding of these processes may provide the means to manipulate these processes and enhance resistance against pests and diseases. Of the possibilities to be considered, the most obvious would involve a direct release of toxic BXs from the vacuole or cytoplasm to the apoplast, mediated via a plasma membrane transport system. Microscopic studies using specific antibodies against the BX-specific beta-glucosidases Glu1 and Glu2 (Cicek and Esen, 1999) could reveal whether hydrolysis of BX-glycosides takes place before or after translocation to the apoplast. Alternatively, a forward genetic analysis of PAMP-induced callose deposition could be considered. For instance, a nested association mapping (NAM) population, consisting of 25 recombinant inbred-line (RIL) populations has been developed in maize and been used to dissect resistance traits in maize (Poland et al., 2011). Using the same mapping population, similar studies can be performed to explore the molecular-genetic basis of callose-mediated defence in maize against aphids and fungi. Early acting post-invasive defence is marked by events such as reactive oxygen species accumulation and callose deposition (Luna et al., 2011). The work by Clay et al. (2009) revealed that indolic glucosinolates in Arabidopsis play a regulatory role in flagellin-induced callose deposition. This discovery pointed to the possibility that indole-derived secondary metabolites BXs in cereals could play a similar role in PAMP-induced callose deposition in maize. The results presented in Chapter 3 confirmed this hypothesis and suggest an evolutionary conserved signalling role of indolic metabolites in early post-invasive defence across the plant kingdom. SIGNALLING ROLE OF DIMBOA IN THE RHIZOSPHERE Apart from their role in aboveground plant defence, BXs also function in belowground plant defence. BXs are exuded from roots into the rhizosphere, where they act as allelochemicals, 118
119 General discussion or exert antimicrobial activities (Niemeyer, 2009). The study described in Chapter 4 describes the belowground impact of BXs in maize-rhizobacteria interactions. The plant root system not only provides support and intake of nutrient and water, but is also a source of secondary metabolites. Plant roots produce and store a blend of chemicals/metabolites, and can release them in the form of root exudates into their immediate vicinity: the rhizosphere. The presence of chemicals in root exudates has a well- documented role in manipulating and dictating the plant’s relationship with pathogenic and symbiotic microbes in the rhizosphere. A well know class of chemicals that are present in root exudates are the isoflavones. These compounds have been identified from root exudates of soybean, which attract nitrogen fixing bacteria Bradyrhizobium japonicum and a pathogen Phytopthora sojae towards roots (Bais et al., 2006). Similarly L-malic acid has been reported to be released by Arabidopsis thaliana in their root exudates, which selectively attracts the plant-beneficial rhizobacterial strain Bacillus subitilis FB17 (Rudrappa et al., 2008). In support of this, the results presented in Chapter 4 provides evidence that maize roots release DIMBOA, which stimulates chemotaxis-related gene expression and in vitro chemotaxis in P. putida KT2440 (Chapter 4; Figures 3 & 4). However, in order for rhizobacteria to establish a plant-beneficial interaction, they not only need to locate their host, but they also need to be capable of tolerating toxic chemicals that are present in the exudates. The ability to tolerate and detoxify these chemicals will provide a competitive advantage to these microbes over other strains that are incapable of tolerating these toxic chemicals in the rhizosphere. Indeed, P. putida KT2440 were tolerant to DIMBOA in comparison to other soil- borne bacteria and its exposure to DIMBOA elicited gene expression that is associated with benzoate breakdown (Chapter 4; Table I). Moreover, in vitro analysis of DIMBOA stability indicated that DIMBOA tolerance of KT2440 bacteria is based on metabolism-dependent breakdown of DIMBOA and 6-methoxy-benzoxazolin-2-one (MBOA), a product of DIMBOA degradation (Chapter 4; Figure 2). Chapter 4 finally revealed that DIMBOA-exuding roots of Bx1 igl plants allowed higher levels of P. putida KT2440 colonized than did DIMBOA-deficient bx1 igl plants. It is, therefore, likely that the increased bacterial colonization of BX-producing roots is the additive result of positive chemotaxis and tolerance to DIMBOA. Root colonization of Arabidopsis by P. putida KT2440 elicits an induced systemic resistance response against P. syringae pv. tomato DC3000 (Matilla et al., 2010). Moreover, preliminary data from our laboratory have revealed that colonization of maize roots by KT2440 bacteria primes emission of wound-inducible volatiles (Figure 3). Hence, P. putida KT2440 is a rhizobacterial strain with plant-beneficial characteristics. Together with the data presented in Chapter 4, these results suggest that BXs fulfil a signalling role in the rhizosphere to recruit and select for plant-beneficial bacteria. Interestingly, however, a recent study by Robert et al., (2012) revealed that these BX signals can also be used by the specialised root herbivore (Diabrotica virgifera) to locate their hosts. Hence, BXs are potent
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Plant basal resistance: genetics, b
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Most of the research described in t
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Promotor: Prof. dr. Ir. C.M.J. Piet
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10 CHAPTER 1 General Introduction A
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Chapter 1 (PRRs; Gomez-Gomez and Bo
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Chapter 1 by avirulent pathogens (R
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Chapter 1 Selective non-pathogenic
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Chapter 1 an endogenous plant signa
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Chapter 1 between defence signallin
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Chapter 1 rarely provides complete
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Chapter 1 genetic resources for Ara
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Chapter 1 metabolites are biosynthe
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Chapter 1 constitutively produced i
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Chapter 1 latest insights. This Cha
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ABSTRACT 33 Genetic dissection of b
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35 Genetic dissection of basal defe
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Natural variation in responsiveness
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ABSTRACT 61 Role of benzoxazinoids
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63 Role of benzoxazinoids in maize
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65 Role of benzoxazinoids in maize
Page 67 and 68: Chemical treatments and callose qua
Page 69 and 70: Aphid infestation stimulates apopla
Page 71 and 72: 71 Role of benzoxazinoids in maize
Page 81 and 82: Table S1: Primers used for RT-qPCR
Page 86 and 87: 86 CHAPTER 4 Benzoxazinoids in root
Page 88 and 89: Chapter 4 INTRODUCTION Plants have
Page 90 and 91: Chapter 4 Our study presents eviden
Page 92 and 93: Chapter 4 Maize - P. putida FBC004
Page 94 and 95: Chapter 4 P. putida is tolerant to
Page 96 and 97: Chapter 4 Impact of DIMBOA on the P
Page 98 and 99: Chapter 4 PP5286 dut deoxyuridine 5
Page 100 and 101: Chapter 4 DIMBOA-producing wild-typ
Page 102 and 103: Chapter 4 DISCUSSION The rhizospher
Page 104 and 105: Chapter 4 patterns, we considered t
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Page 108 and 109: 108 CHAPTER 5 General Discussion
Page 110 and 111: Chapter 5 QTL was caused by a polym
Page 112 and 113: Chapter 5 Figure 1: CYP81D11 regula
Page 114 and 115: Chapter 5 et al., 2009), and allele
Page 116 and 117: Chapter 5 defence against different
Page 120 and 121: Chapter 5 rhizospheric signals that
Page 122 and 123: Chapter 5 glucosinolate profiles. 1
Page 124 and 125: References References Abramovitch R
Page 126 and 127: References QTL mapping and characte
Page 128 and 129: References Gianoli E, Niemeyer HM (
Page 130 and 131: References Kliebenstein DJ, Kroyman
Page 132 and 133: References Studies in Natural Produ
Page 134 and 135: References Geniostoma antherotrichu
Page 136 and 137: References Todesco M, Balasubramani
Page 138 and 139: References Walters DR, Paterson L,
Page 140 and 141: Summary Summary Basal resistance in
Page 142 and 143: Samenvatting Samenvatting Basale re
Page 144 and 145: Samenvatting inzichten opgeleverd i
Page 146 and 147: Acknowledgments Acknowledgements In
Page 148 and 149: Curriculum vitae Shakoor was born o
Page 150: List of Publications Published arti
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