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Interactions between cauliflower and Rhizoctonia anastomosis groups with different levels of aggressiveness.

Pannecoucque J, Höfte M - BMC Plant Biol. (2009)

Bottom Line: We demonstrated the pronounced deposition of phenolic compounds and callose against weak and non-aggressive AGs which resulted in a delay or complete block of the host colonization.Degradation of pectic compounds was observed for all pathogenic AGs, except for AG 2-2 IIIb.Ranking the AGs based on infection rate, level of induced host responses and pectin degradation revealed a strong correlation with the disease severity caused by the AGs.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Phytopathology, Faculty of Bioscience Engineering, Ghent University, Coupure Links, 653, B-9000 Gent, Belgium. joke.pannecoucque@ugent.be

ABSTRACT

Background: The soil borne fungus Rhizoctonia is one of the most important plant pathogenic fungi, with a wide host range and worldwide distribution. In cauliflower (Brassica oleracea var. botrytis), several anastomosis groups (AGs) including both multinucleate R. solani and binucleate Rhizoctonia species have been identified showing different levels of aggressiveness. The infection and colonization process of Rhizoctonia during pathogenic interactions is well described. In contrast, insights into processes during interactions with weak aggressive or non-pathogenic isolates are limited. In this study the interaction of cauliflower with seven R. solani AGs and one binucleate Rhizoctonia AG differing in aggressiveness, was compared. Using microscopic and histopathological techniques, the early steps of the infection process, the colonization process and several host responses were studied.

Results: For aggressive Rhizoctonia AGs (R. solani AG 1-1B, AG 1-1C, AG 2-1, AG 2-2 IIIb and AG 4 HGII), a higher developmental rate was detected for several steps of the infection process, including directed growth along anticlinal cell walls and formation of T-shaped branches, infection cushion formation and stomatal penetration. Weak or non-aggressive AGs (R. solani AG 5, AG 3 and binucleate Rhizoctonia AG K) required more time, notwithstanding all AGs were able to penetrate cauliflower hypocotyls. Histopathological observations indicated that Rhizoctonia AGs provoked differential host responses and pectin degradation. We demonstrated the pronounced deposition of phenolic compounds and callose against weak and non-aggressive AGs which resulted in a delay or complete block of the host colonization. Degradation of pectic compounds was observed for all pathogenic AGs, except for AG 2-2 IIIb. Ranking the AGs based on infection rate, level of induced host responses and pectin degradation revealed a strong correlation with the disease severity caused by the AGs.

Conclusion: The differences in aggressiveness towards cauliflower observed among Rhizoctonia AGs correlated with the infection rate, induction of host defence responses and pectin breakdown. All Rhizoctonia AGs studied penetrated the plant tissue, indicating all constitutive barriers of cauliflower were defeated and differences in aggressiveness were caused by inducible defence responses, including cell wall fortifications with phenolic compounds and callose.

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Ruthenium red staining of cauliflower hypocotyl cells after infection by seven different R. solani AGs and one binucleate Rhizoctonia AG. A, Cellular responses were classified into two categories (I) Representative example of pectin breakdown as indicated by faint red colour. (II) Uniform red stain of the cell walls indicating absence of pectin breakdown as observed during the interaction with R. solani AG 2-2 IIIb. Scale bars = 50 μm. B, Relative proportion of interaction sites at which pectin degradation is observed at 3, 6 and 12 dpi during the interaction with different Rhizoctonia AGs. At each time point, at least 50 interaction sites per AG were studied originating from 10 different cauliflower hypocotyls. Within one column, values followed by the same letter are not significantly different according to Kruskal-Wallis and Mann-Whitney tests (α = 0.05).
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Figure 5: Ruthenium red staining of cauliflower hypocotyl cells after infection by seven different R. solani AGs and one binucleate Rhizoctonia AG. A, Cellular responses were classified into two categories (I) Representative example of pectin breakdown as indicated by faint red colour. (II) Uniform red stain of the cell walls indicating absence of pectin breakdown as observed during the interaction with R. solani AG 2-2 IIIb. Scale bars = 50 μm. B, Relative proportion of interaction sites at which pectin degradation is observed at 3, 6 and 12 dpi during the interaction with different Rhizoctonia AGs. At each time point, at least 50 interaction sites per AG were studied originating from 10 different cauliflower hypocotyls. Within one column, values followed by the same letter are not significantly different according to Kruskal-Wallis and Mann-Whitney tests (α = 0.05).

Mentions: For the aggressive isolates (AG 1-1B, AG 1-1C, AG 2-1, AG 2-2 and AG 4 HGII) samples from 3 and 6 dpi were studied, while the weak and non-aggressive isolates (AG 3, AG 5 and AG K) were studied at 6 and 12 dpi. Penetration of epidermal cells by fungal hyphae occurred both by stomatal penetration and formation of infection cushions under which several penetrating hyphae were observed (Fig. 3). Hyphal penetration was found to be associated with different levels of cell wall modifications. For safranin O and aniline blue stain, cellular responses were classified into three distinct categories (Fig. 4A). In type I and type II, cell wall fortifications were detected at penetration sites of Rhizoctonia. In the case of type I, hyphae were completely surrounded by fortified cell walls, thereby restricting further colonization of the host tissue; whereas for type II cell wall depositions were detected although they could not stop the fungal growth and hyphae were observed beyond the fortified cell walls. Type III reactions, on the other hand, were characterized by the absence of cell wall depositions. Staining of the sections with ruthenium red coloured the pectic compounds red. At several interaction sites, pectic compounds were degraded as indicated by the absence of the red stain (Fig. 5A). An overview of the quantitative analysis of the host cell wall responses observed at the interaction sites of the eight Rhizoctonia AGs obtained with the three different stains is presented in Figures 4B, 4C and 5B. The majority of the type I and type II reaction sites was, besides the wall thickening, also associated with granulation of the cytoplasm in neighbouring cortical cells. These granules probably contain phenolic compounds since they stained with toluidine blue and safranin O. Eventually, these cortical cells crumpled and collapsed; all these reactions are consistent with a hypersensitive response [30].


Interactions between cauliflower and Rhizoctonia anastomosis groups with different levels of aggressiveness.

Pannecoucque J, Höfte M - BMC Plant Biol. (2009)

Ruthenium red staining of cauliflower hypocotyl cells after infection by seven different R. solani AGs and one binucleate Rhizoctonia AG. A, Cellular responses were classified into two categories (I) Representative example of pectin breakdown as indicated by faint red colour. (II) Uniform red stain of the cell walls indicating absence of pectin breakdown as observed during the interaction with R. solani AG 2-2 IIIb. Scale bars = 50 μm. B, Relative proportion of interaction sites at which pectin degradation is observed at 3, 6 and 12 dpi during the interaction with different Rhizoctonia AGs. At each time point, at least 50 interaction sites per AG were studied originating from 10 different cauliflower hypocotyls. Within one column, values followed by the same letter are not significantly different according to Kruskal-Wallis and Mann-Whitney tests (α = 0.05).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2719643&req=5

Figure 5: Ruthenium red staining of cauliflower hypocotyl cells after infection by seven different R. solani AGs and one binucleate Rhizoctonia AG. A, Cellular responses were classified into two categories (I) Representative example of pectin breakdown as indicated by faint red colour. (II) Uniform red stain of the cell walls indicating absence of pectin breakdown as observed during the interaction with R. solani AG 2-2 IIIb. Scale bars = 50 μm. B, Relative proportion of interaction sites at which pectin degradation is observed at 3, 6 and 12 dpi during the interaction with different Rhizoctonia AGs. At each time point, at least 50 interaction sites per AG were studied originating from 10 different cauliflower hypocotyls. Within one column, values followed by the same letter are not significantly different according to Kruskal-Wallis and Mann-Whitney tests (α = 0.05).
Mentions: For the aggressive isolates (AG 1-1B, AG 1-1C, AG 2-1, AG 2-2 and AG 4 HGII) samples from 3 and 6 dpi were studied, while the weak and non-aggressive isolates (AG 3, AG 5 and AG K) were studied at 6 and 12 dpi. Penetration of epidermal cells by fungal hyphae occurred both by stomatal penetration and formation of infection cushions under which several penetrating hyphae were observed (Fig. 3). Hyphal penetration was found to be associated with different levels of cell wall modifications. For safranin O and aniline blue stain, cellular responses were classified into three distinct categories (Fig. 4A). In type I and type II, cell wall fortifications were detected at penetration sites of Rhizoctonia. In the case of type I, hyphae were completely surrounded by fortified cell walls, thereby restricting further colonization of the host tissue; whereas for type II cell wall depositions were detected although they could not stop the fungal growth and hyphae were observed beyond the fortified cell walls. Type III reactions, on the other hand, were characterized by the absence of cell wall depositions. Staining of the sections with ruthenium red coloured the pectic compounds red. At several interaction sites, pectic compounds were degraded as indicated by the absence of the red stain (Fig. 5A). An overview of the quantitative analysis of the host cell wall responses observed at the interaction sites of the eight Rhizoctonia AGs obtained with the three different stains is presented in Figures 4B, 4C and 5B. The majority of the type I and type II reaction sites was, besides the wall thickening, also associated with granulation of the cytoplasm in neighbouring cortical cells. These granules probably contain phenolic compounds since they stained with toluidine blue and safranin O. Eventually, these cortical cells crumpled and collapsed; all these reactions are consistent with a hypersensitive response [30].

Bottom Line: We demonstrated the pronounced deposition of phenolic compounds and callose against weak and non-aggressive AGs which resulted in a delay or complete block of the host colonization.Degradation of pectic compounds was observed for all pathogenic AGs, except for AG 2-2 IIIb.Ranking the AGs based on infection rate, level of induced host responses and pectin degradation revealed a strong correlation with the disease severity caused by the AGs.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Phytopathology, Faculty of Bioscience Engineering, Ghent University, Coupure Links, 653, B-9000 Gent, Belgium. joke.pannecoucque@ugent.be

ABSTRACT

Background: The soil borne fungus Rhizoctonia is one of the most important plant pathogenic fungi, with a wide host range and worldwide distribution. In cauliflower (Brassica oleracea var. botrytis), several anastomosis groups (AGs) including both multinucleate R. solani and binucleate Rhizoctonia species have been identified showing different levels of aggressiveness. The infection and colonization process of Rhizoctonia during pathogenic interactions is well described. In contrast, insights into processes during interactions with weak aggressive or non-pathogenic isolates are limited. In this study the interaction of cauliflower with seven R. solani AGs and one binucleate Rhizoctonia AG differing in aggressiveness, was compared. Using microscopic and histopathological techniques, the early steps of the infection process, the colonization process and several host responses were studied.

Results: For aggressive Rhizoctonia AGs (R. solani AG 1-1B, AG 1-1C, AG 2-1, AG 2-2 IIIb and AG 4 HGII), a higher developmental rate was detected for several steps of the infection process, including directed growth along anticlinal cell walls and formation of T-shaped branches, infection cushion formation and stomatal penetration. Weak or non-aggressive AGs (R. solani AG 5, AG 3 and binucleate Rhizoctonia AG K) required more time, notwithstanding all AGs were able to penetrate cauliflower hypocotyls. Histopathological observations indicated that Rhizoctonia AGs provoked differential host responses and pectin degradation. We demonstrated the pronounced deposition of phenolic compounds and callose against weak and non-aggressive AGs which resulted in a delay or complete block of the host colonization. Degradation of pectic compounds was observed for all pathogenic AGs, except for AG 2-2 IIIb. Ranking the AGs based on infection rate, level of induced host responses and pectin degradation revealed a strong correlation with the disease severity caused by the AGs.

Conclusion: The differences in aggressiveness towards cauliflower observed among Rhizoctonia AGs correlated with the infection rate, induction of host defence responses and pectin breakdown. All Rhizoctonia AGs studied penetrated the plant tissue, indicating all constitutive barriers of cauliflower were defeated and differences in aggressiveness were caused by inducible defence responses, including cell wall fortifications with phenolic compounds and callose.

Show MeSH
Related in: MedlinePlus