Plant basal resistance - Universiteit Utrecht

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Chapter 4 DISCUSSION The rhizosphere is an energy-rich niche that is characterised by a rapid turnover of chemical compounds from plant root exudates (Jones et al., 2009). Before rhizobacteria can exploit these compounds in the rhizosphere, they must first locate their host and tolerate potentially toxic allelochemicals in root exudates. In this study, we provide evidence that rhizosphere-colonising P. putida cells are tolerant of the N-heteroaromatic allelochemical DIMBOA (Figure 2), which is exuded in relatively high quantities from roots of young maize seedlings (Figure 1). Since BXs are nitrogen-containing metabolites, it might be expected that constitutive DIMBOA exudation by seedlings provides significant ecological benefits, outweighing the metabolic cost. Apart from allelopathatic activity by DIMBOA (Niemeyer, 1988, 2009), our study revealed that DIMBOA can also acts as a below-ground semiochemical for recruitment of plant-beneficial rhizobacteria from a competitive soil environment (Figure 5B). Interestingly, mycorrhization of maize was recently reported to boost DIMBOA production (Song et al., 2011). Since mycorrhization is known to cause major qualitative changes in rhizobacterial communities (Linderman, 1988), it is tempting to speculate that increased DIMBOA exudation from mycorrhizal roots contributes to this so-called mycorrhizosphere effect. The BX content of maize roots has been studied extensively, because of their demonstrated roles as allelochemicals (Niemeyer and Perez, 1994; Niemeyer, 2009). Recent studies have identified the glucosides HDMBOA-glc and DIMBOA-glc as the principal BXs in roots and root exudates of maize (Glauser et al., 2011; Robert et al., 2012). DIMBOA was identified in both studies as only a minor component of the total root BX content. A possible explanation for this discrepancy lies in the different method of BXs extraction. In our study, entire root systems were incubated in water for 7 hours, whereas Robert et al. (2012) used a direct sampling method with a 50% (v/v) methanol extraction buffer from the root surface. Hence, the latter method analyzed root-exuded BXs directly, while our method assessed root-exuded BXs after prolonged incubation of the root system in water. Since BX glucosides are readily hydrolysed in water and DIMBOA is considerably more stable than HDMBOA (Maresh et al., 2006), it may not be surprising that our study identified the aglycone DIMBOA as the dominant BX from root exudates. Considering that soils constitute a mostly watery environment, we propose that the more breakdown-resilient DIMBOA compound functions as the long-distance BX signal that recruits beneficial rhizobacteria. The P. putida strain used in our studies was originally isolated from horticultural soil (Nakazawa, 2002), and is a competitive coloniser of rhizospheres of economically important crops (Molina et al., 2000). Using in vivo expression techniques (IVET), Ramos- González et al. (2005) identified 29 genes that are induced following 14 days of growth in the maize rhizosphere, including some with annotated functions in chemotaxis and 102
103 Recruitment of beneficial bacteria by maize roots detoxification. However, despite the similarities in general cellular functions, there were no overlapping genes between this IVET study and our transcriptome analysis. A more recent transcriptome study of P. putida KT2440 identified gene induction as the dominant response after 6 days of colonisation in the maize rhizosphere (Matilla et al., 2007), which is in agreement with our finding that DIMBOA enhances P. putida gene expression. In total, Matilla et al. (2007) revealed enhanced expression of 93 genes in the maize rhizosphere, including genes with predicted functions in general metabolism, transcriptional regulation, transport, chemotaxis and DNA metabolism. With the exception of the ISPpu14 transposase Orf1 (PP5398), there is again no overlap between this study and our transcriptome analysis. This is not surprising, since our analysis was specifically focussed on the bacterial response to DIMBOA, and not to the multitude of responses that are required for rhizopshere competence, such as attachment to the maize root surface and metabolism of the wide range of compounds besides DIMBOA in root exudates. Furthermore, the transcriptional response reported in our study was expressed within 1 hour of exposure to DIMBOA. It is, therefore, likely that these gene expression patterns are specific to the initial stages of the interaction: the bacterial response to chemical cues from the host plant in the soil before they attach and establish themselves in the rhizosphere. Since our ultimate objective was to study the maize-bacterium interaction, rather than quantitative gene expression in P. putida KT2440 per se, we made no further attempts to confirm our in vitro transcription profiling with a complementary technique. It remains therefore difficult to establish unequivocally that specific genes identified as DIMBOA-inducible in vitro are in fact responsible for the biological interactions described in this study. Nevertheless, it is still instructive to consider these genes in the light of what is already known about environmentally responsive P. putida genes. Moreover, the DIMBOA-inducible gene expression patterns associated with tolerance to N-heteroaromatic compounds and bacterial motility led us to conduct follow- up experiments, which revealed a novel signalling mechanism during the initial phases of the maize - P. putida interaction. Motility is an essential trait for rhizosphere competence (Lugtenberg and Dekkers, 1999). Our transcriptome analysis identified two DIMBOA-inducible genes that can be associated with bacterial chemotaxis, and a third gene with a putative function in DIMBOA transport (Table I). The DIMBOA-responsive gene cheY (PP4340) is a chemotactic response regulator in bacteria (Wolanin et al., 2003). Furthermore, benzoate chemotaxis in P. putida PRS2000 depends on a methyl-accepting chemotaxis transducer (M-ACT) and the aromatic acid:H + symporter (AAHS) PcaK (Harwood et al., 1994). Interestingly, our transcriptome analysis included the M-ACT homologue PP4888, and two genes, PP2241 and PP2604, belonging to the Major Facilitator Superfamily (MFS) of AAHS transporters (Pao et al., 1998). Of these two genes, PP2604 shares common features with pcaK (The STRING v.9 database; Szklarczyk et al., 2011). On the basis of these motality-related transcriptional
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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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Page 51 and 52: 51 Genetic dissection of basal defe
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Page 61 and 62: ABSTRACT 61 Role of benzoxazinoids
Page 63 and 64: 63 Role of benzoxazinoids in maize
Page 67 and 68: Chemical treatments and callose qua
Page 69 and 70: Aphid infestation stimulates apopla
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
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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 118 and 119: Chapter 5 occurred at time-points l
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
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