Effects of silicon treatment and inoculation with Fusarium oxysporum f. sp. vasinfectum on cellular defences in root tissues of two cotton cultivars

Abstract

Background and aims: Silicon has been shown to enhance the resistance of plants to fungal and bacterial pathogens. Here, the effect of potassium silicate was assessed on two cotton (Gossypium hirsutum) cultivars subsequently inoculated with Fusarium oxysporum f. sp. vasinfectum (Fov). Sicot 189 is moderately resistant whilst Sicot F-1 is the second most resistant commercial cultivar presently available in Australia.

Methods: Transmission and light microscopy were used to compare cellular modifications in root cells after these different treatments. The accumulation of phenolic compounds and lignin was measured.

Key results: Cellular alterations including the deposition of electron-dense material, degradation of fungal hyphae and occlusion of endodermal cells were more rapidly induced and more intense in endodermal and vascular regions of Sicot F-1 plants supplied with potassium silicate followed by inoculation with Fov than in similarly treated Sicot 189 plants or in silicate-treated plants of either cultivar not inoculated with Fov. Significantly more phenolic compounds were present at 7 d post-infection (dpi) in root extracts of Sicot F-1 plants treated with potassium silicate followed by inoculation with Fov compared with plants from all other treatments. The lignin concentration at 3 dpi in root material from Sicot F-1 treated with potassium silicate and inoculated with Fov was significantly higher than that from water-treated and inoculated plants.

Conclusions: This study demonstrates that silicon treatment can affect cellular defence responses in cotton roots subsequently inoculated with Fov, particularly in Sicot F-1, a cultivar with greater inherent resistance to this pathogen. This suggests that silicon may interact with or initiate defence pathways faster in this cultivar than in the less resistant cultivar.

Keywords: Fusarium oxysporum f. sp. vasinfectum; Gossypium hirsutum; Silicon; defence; lignin; pathogen; phenolic compounds; transmission electron microscopy.

Fig. 1.

Light micrographs of Toluidine blue-stained root sections of cotton plants supplied with silicon showing defence reactions after Fusarium oxysporum f. sp. vasinfectum inoculation (silicate–Fov). (A) Infected xylem vessels (XV) in a Sicot F-1 plant showing hyphae (H) penetrating intertracheary pit membranes into adjacent xylem cells. Contact cells (CC) are occluded with material staining blue-green, indicative of polymerized phenolics (Ph). Fungal hyphae are also blue-green in colour, suggesting that they are coated with these phenolic substances (7 dpi). (B) Fungal hyphae (arrows) in a xylem vessel (XV) of a silicon-treated Sicot 189 plant. Contact cells (CC) adjacent to the xylem vessel are vacuolated and disorganized, though the infected vessel is not occluded (7 dpi).

Fig. 2.

Transmission electron micrographs of Sicot 189 cotton roots receiving no potassium silicate followed by mock inoculation (water–water). (A) Transverse section of xylem vessels (XV) with lumen, cell walls (CW) and pits (P) free of electron-dense aggregations. Contact cell (CC) adjoining xylem vessels with organelles including the nucleus (N) intact. (B) Longitudinal section showing a xylem vessel (XV) clear of inclusions, a contact cell with a large central vacuole (V) and the plasma membrane appressed to the cell wall (arrow).

Fig. 3.

Transmission electron micrographs showing defence reactions of Sicot 189 plants inoculated with Fov. (A) Transverse section of a xylem vessel (XV) showing a single hypha (H) with no evidence of plant defence reactions, in a water–Fov-treated plant (7 dpi). (B) Transverse section of a xylem vessel (XV) in a silicate–Fov-treated plant. Hyphae (H) are free of coating of amorphous material; however, xylem cell walls are covered in granular material (arrows) in infected and adjacent cells (7 dpi). (C) Transverse section of a xylem vessel (XV) in a silicate–Fov plant. Hyphae attempting to penetrate pit membranes (PM) are impeded by osmiophillic droplets (OD) (7 dpi).

Fig. 4.

Transmission electron micrographs showing defence reactions of Sicot F-1 plants inoculated with Fov. (A) Transverse section showing intact hyphae (H) approaching endodermal cells (EDC) in a plant without silicon amendment (water–Fov); minimal occlusion of cells with electron-dense material (3 dpi). (B) Transverse section from a plant treated with silicon (silicate–Fov) shows healthy (H) and degenerating (DH) hyphae surrounded by abundant osmiophillic aggregations, with plasmalemma also displaced from host cell walls (arrows). Infected cells are in close contact with densely lined and occluded endodermal cells (EDC) (3 dpi). (C) A sole hypha (H) in transverse section of a xylem vessel (XV) of cotton roots with no silicate treatment (water–Fov) adjacent to a heavily infected vessel (arrow). An adjacent xylem vessel is heavily occluded with amorphous material (OD). (D) An infected xylem vessel (XV) in a silicate–Fov-treated plant is occluded by amorphous material and the adjacent vessel possesses a thick amorphous lining (OL) directly opposing fungal structures (H) (7 dpi).

Fig. 5.

Transmission electron micrographs showing vascular defence reactions of silicon-treated plants followed by inoculation with Fov (silicate–Fov). (A) Transverse section of hyphae (H) proliferating in a xylem vessel of a Sicot F-1 plant; the vessel is completely occluded with electron-dense material and droplets (OD) (7 dpi). (B) Similar to (A) except the xylem vessel of the Sicot F-1 plant is more densely occluded. Hyphae (H) proliferate in the xylem vessel and can be seen penetrating pits to enter adjacent cells (arrows) (7 dpi). (C) Hyphae (H) in the xylem vessel (XV) of a Sicot 189 plant is relatively clear of amorphous inclusions except for thin layers coating hyphae (H), cell walls and the pit membrane (PM) (7 dpi).

Fig. 6.

Concentrations of soluble phenolic compounds in Sicot 189 and Sicot F-1 plants at (A) 3 and (B) 7 dpi. Four treatments are presented: potassium silicate followed by Fov inoculation (Silicate–Fov); no potassium silicate treatment followed by Fov inoculation (Water–Fov); potassium silicate followed by mock (water) inoculation only (Silicate–water); and no potassium silicate treatment followed by mock inoculation (Water–water). Differences between treatments were statistically assessed independently for each cultivar. Different letters above the bars indicate significant differences between treatments within each cultivar at each time point (P < 0·05, n = 3).

Fig. 7.

Lignin concentration of Sicot 189 and Sicot F-1 root tissue at (A) 3 dpi and (B) 7 dpi with Fov. Four treatments are presented: potassium silicate followed by Fov inoculation (Silicate–Fov); no potassium silicate treatment followed by Fov inoculation (Water–Fov); potassium silicate followed by mock (water) inoculation only (Silicate–water); and no potassium silicate treatment followed by mock inoculation (Water–water). Differences between treatments were statistically assessed independently for each cultivar. Different letters above the bars indicate significant differences between treatments within each cultivar at each time point (P < 0·05, n = 3).