Abstract
Verticillium longisporum (VL) is one of the most devastating diseases in important oil crops from the family of Brassicaceae. The fungus resides for much time of its life cycle in the extracellular fluid of the vascular system, where it cannot be controlled by conventional fungicides. To obtain insights into the biology of VL-plant interaction in the apoplast, the secretome consisting of the extracellular proteome and metabolome as well as cell wall properties were studied in the model Brassicaceae, Arabidopsis thaliana. VL infection resulted in increased production of cell wall material with an altered composition of carbohydrate polymers and increased lignification. The abundance of several hundred soluble metabolites changed in the apoplast of VL-infected plants including signalling and defence compounds such as glycosides of salicylic acid, lignans and dihydroxybenzoic acid as well as oxylipins. The extracellular proteome of healthy leaves was enriched in antifungal proteins. VL caused specific increases in six apoplast proteins (three peroxidases PRX52, PRX34, P37, serine carboxypeptidase SCPL20, α-galactosidase AGAL2 and a germin-like protein GLP3), which have functions in defence and cell wall modification. The abundance of a lectin-like, chitin-inducible protein (CILLP) was reduced. Since the transcript levels of most of the induced proteins were not elevated until late infection time points (>20 dpi), whereas those of CILLP and GLP3 were reduced at earlier time points, our results may suggest that VL enhances its virulence by rapid down-regulation and delay of induction of plant defence genes.
Highlights
Verticillium species are wide-spread soil borne fungi, which cause vascular diseases in many plant species [1]
Soluble proteins and proteins bound by ionic forces were extracted from rosette leaves yielding protein patterns partially overlapping (9 proteins) with those of previous studies applying infiltration and exudation techniques [22,29], whereas less overlap was found with proteomes from isolated Arabidopsis cell walls (2 proteins, [30]) In our study, 35 different, functionally annotated proteins with a secretory signal peptide can be classified according to Jamet et al [28] into main plant secretome categories as proteins acting on carbohydrates, proteases, oxidoreductases, proteins with interacting domains (e.g., with other proteins through leucine-rich repeat (LRR) domains, lectins interacting with sugars, or enzyme inhibitors, 21%), lipolytic enzymes (GDSLlike lipase/acylhydrolase, 7%) and unknowns (7%)
The homologs of 30% of the apoplastic washing fluids (AWFs) proteins of our study (MERI5b, b-1,3-glucanase, protease inhibitor, curculinlike, chitinase, aspartyl protease, subtilase, PRX34, PRX52, SCPL20, AGAL2) were present in Brassica oleracea xylem sap [10,33] and the homolog to GLP3 was found in leaf AWF of Brassica napus [10]
Summary
Verticillium species are wide-spread soil borne fungi, which cause vascular diseases in many plant species [1]. The economic importance and acreage of oilseed rape are increasing because of growing demand on oil crops for nutrition and bio-fuels [2,5,6]. This is accompanied by a spread of VL diseases, which may cause yield losses as high as 10 to 50% [7]. Since diseases caused by Verticillium spp. cannot be controlled by conventional fungicides, the use of resistant cultivars has been recommended [7]. Breeding of resistant cultivars requires in-depth understanding of the biology of the VL-host interaction
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