Plant basal resistance: genetics, biochemistry, and impacts on plant-biotic interactions

Abstract

Basal resistance depends largely on a diverse range of defence mechanisms that become active upon attack by pathogens or insects. These mechanisms range from rapid stomatal closure and production of reactive oxygen species, to callose deposition and defence gene induction. It is commonly assumed that the speed and intensity of these inducible defences determines the effectiveness of basal resistance. The present dissertation describes different aspects of basal resistance in Arabidopsis thaliana and Zea mays. Chapter 2 of the dissertation describes natural variation between Arabidopsis accessions in basal defence responsiveness to pathogen-associated molecular patterns (PAMPs) and the defence hormone salicylic acid (SA). Quantitative trait loci (QTL) analysis identified different loci regulating the sensitivity of PAMP-induced callose and SA-induced defence gene expression. One QTL controlling SA responsiveness was found to contribute to basal resistance against Pseudomonas syringae pv. tomato. In Chapter 3, the contribution of benzoxazinoids (BXs) in basal resistance of maize is described, using maize bx1 mutant lines that are impaired in the first dedicated step of BX biosynthesis. Compared to wild-type lines, bx1 lines displayed reduced penetration resistance against aphids and fungus. Furthermore, infestation of wild-type plants by aphids and fungi stimulated the conversion of DIMBOA-glucoside into HDMBOA-glucoside and DIMBOA, which was most pronounced in the apoplast of challenged tissues. Interestingly, these events preceded major tissue damage or symptom development, suggesting that BX-dependent basal resistance does not necessarily depend on tissue damage. Upon further investigation of wild-type and bx1 mutant lines, we observed significantly reduced callose deposition in bx1 plants after PAMP treatment. Furthermore, DIMBOA infiltration of the apoplast mimicked PAMP-induced callose in wild-type plants. Hence, DIMBOA acts as a regulatory signal in the expression of cell wall-based basal resistance of maize. BXs have also been reported to act as allelopathic signals belowground, which are further investigated in Chapter 4. Chromatographic analysis revealed that DIMBOA is the dominant BX species in root exudates of maize. To investigate the impact of BXs on root-colonizing rhizobacteria, transcriptome analysis was performed of DIMBOA-treated Pseudomonas putida KT2440. This global analysis pointed towards increased transcription of bacterial genes that are involved in break-down of aromatic metabolites and chemotaxis. The latter response was confirmed by in vitro assays, which demonstrated chemotaxis of the bacteria towards DIMBOA. Furthermore, root colonisation assays with GFP-expressing P. putida KT2440 revealed that wild-type plants allowed more bacterial colonization than BX-deficient bx1 plants, indicating that BXs can recruit rhizobacteria from the soil. Preliminary results that are presented in Chapter 5 show that root colonization by P. putida KT2440 primes aboveground basal defences against herbivores, thereby further highlighting the central and multifaceted function of DIMBOA in maize basal resistance.