Factors governing the regulation ofSclerotinia sclerotiorumcutinase A and polygalacturonase 1 during different stages of infection

Abstract

Sclerotinia sclerotiorum releases hydrolytic enzymes that sequentially degrade the plant cuticle, middle lamellae, and primary and secondary cell walls. The cuticle was found to be a barrier to S. sclerotiorum infection, as leaves stripped of epicuticular wax were more rapidly colonized. Consequently, the factors affecting the regulation of genes encoding polygalacturonase 1 (SsPG1) and a newly identified cutinase (SsCUTA) were examined. In vitro, SsCutA transcripts were detected within 1 h postinoculation of leaves, and expression was primarily governed by contact of mycelia with solid surfaces. Expression of SsPg1 was moderately induced by contact with solid surfaces including the leaf, and expression was restricted to the expanding margin of the lesion as the infection progressed. SsPg1 expression was induced by carbohydrate starvation but repressed by galacturonic acid. Glucose supported a basal level of SsPg1 expression but accentuated expression when provided to mycelia used to inoculate leaves. These observations were contrary to earlier reports indicating that glucose repressed SsPg1 expression while galacturonic acid induced expression. Pharmacological studies showed that disruption of calcium signalling affected SsCutA and SsPg1 expression and decreased S. sclerotiorum virulence, whereas elevated cAMP levels reduced virulence without affecting gene expression. The mechanisms involved in coordinating the expression of S. sclerotiorum hydrolytic enzymes throughout the various stages of the infection are discussed.

Changes in the Sclerotinia sclerotiorum transcriptome during infection of Brassica napus

“…Among the genes from the CE family, the gene encoding the cutinase, SsCuta (SS1G_07661), was up-regulated during the early stages of infection (from 1-24 hpi). The induction of SsCuta soon after contact with the leaf surface in the current study agrees with the previous report showing that it was induced upon contact of mycelia with hard surfaces [4] and supports the predicted role of this enzyme in degrading plant cuticle. Additionally, it is not surprising that expression of this gene declined after 24 hpi since host penetration has already been achieved by this time.…”

“…Li et al [23] reported that SsPG1 expression could be induced by contact with hard surfaces, while Bashi et al [4] reported that SsPG1 , but not SsPG2 , was moderately induced by contact with B. napus leaves and that SsPG1 expression was restricted to the expanding margin of the lesion. They suggested that since SsPG1 expression was also induced by carbon starvation and repressed by galacturonic acid that it may be involved in both early penetration events and lesion expansion.…”

“…Recent studies, however, have provided evidence for a brief biotrophic phase occurring within the apoplastic space immediately after cuticle penetration and the pathogen may, therefore, be more accurately classified as a hemi-biotroph [3]. Transition between these various developmental and pathogenic phases is governed by physical and metabolic cues including detection of contact with hard surfaces [4], glucose levels [5], cAMP levels [6], pH [7] and oxidative stress [8]. Communication between the associated signaling pathways is critical and involves numerous protein kinases [9–11] and phosphatases [12, 13].…”

“…The production of numerous types of hydrolytic and cell wall degrading enzymes (CWDE) facilitates host cuticle penetration, lesion expansion and tissue maceration [4, 21, 22]. Although pectinolytic CWDEs, such as polygalacturonases (SsPG1, SsPG3, SsPG5 and SsPG6), have captured more attention as the main group of hydrolytic enzymes involved in S. sclerotiorum virulence [23], non-pectinolytic enzymes like proteases, cellulases and glucoamylases also contribute to the infection process in this fungus [21].…”

The eukaryotic protein kinase superfamily of the necrotrophic fungal plant pathogen, Sclerotinia sclerotiorum

“…MAPKs encoded by SLT2 orthologues in Botrytis cinerea ( BMP3 ) (Rui and Hahn, ), Colletotrichum lagenarium ( MAF1 ) (Kojima et al ., ) and Magnaporthe grisea (MPS1) (Xu et al ., ) are required for germ tube elongation, leading to appressoria formation on solid surfaces and on the host plant. In S. sclerotiorum , hydrolytic enzyme synthesis, in particular the cutinases involved in the degradation of cuticular wax, is also induced by contact with solid surfaces (Bashi et al ., ), and infection ability and trichothecene production have been linked to an SLT2‐like MAPK in Fusarium species (Hou et al ., ; Zhang et al ., ). Taken together, this suggests that, in phytopathogenic fungi, a single MAPK fulfils the role of the SLT2 (cell wall integrity) and SMK1 (spore wall assembly) MAPKs found in yeast, and is involved in the detection of surface integrity, leading to infection structure (appressoria or infection cushions) formation and the deployment of virulence determinants, such as hydrolytic enzymes and toxins, that are required for infection initiation and establishment.…”