Chapter 22 : Thigmo Responses: The Fungal Sense of Touch

Thigmo responses play crucial roles in the fungal lifestyle. (A) Thigmonasty: On sensing exudate from nearby nematode prey, Arthobotrys oligospora hyphae produce a lateral peg and three consecutive capture cells that grow round until they contact the peg, with which they fuse to form a capture loop ( 1 , 2 ) (reprinted from taxusbaccata.hubpages.com with permission of the author). (B) Contact with a nematode entering the aperture of the loop causes immediate swelling of the capture cells to immobilize the prey (reprinted from apsnet.org with permission of the author). Invasive hyphae then germinate from the contact zone of the capture cells to penetrate and digest the nematode. (C) Thigmotropism: Hyphae of the sooty blotch fungus, C. trifolii, follow interepidermal cell depressions on the surface of the host plant, T. repens (white clover), to locate a stoma, over which a penetrative appressorium is formed (arrow) (bar, 10 μm) (reprinted from Mycological Research [ 23 ] with permission of the publisher). (D) Germ tubes of the cereal rust fungus, Puccinia hordei, grow perpendicularly across depressions in the surface of its host barley leaf to locate stomata, which are arranged in staggered arrays, and form appressoria (G, germ tube; A, appressorium; B, branch; bar, 20 μm) (reprinted from Planta [ 148 ] with permission of the publisher). (E) Thigmo differentiation: the differentiation of appressoria by U. appendiculatus is induced by topography alone when the fungus is grown on an inert polymer with topographical features that precisely mimic the surface of the host plant (bar, 11.8 μm) (reprinted from Experimental Mycology [ 4 ] with permission of the publisher).

Contact sensing stands at the threshold of a diverse range of morphological and growth transitions relevant to specific fungal lifestyles. Most transitions require a supporting chemical signal for complete induction of a normal response while, in others, this aspect has not been fully investigated.

Mechanisms involved in fungal contact resulting in recognition and/or adhesion. (A) Compatible surface chemistry between fungal and substrate surface promotes adhesion. (B) Directional forces generate cell wall stress. (C) Chemical changes sensed within the boundary layer, sometimes as a result of fungal enzyme activity against host exudate. (D) Contact-activated zone defines spatial organization of cell asymmetry. (E) Stretch-activated ion channels. (F) Plasma membrane (PM)-associated cell wall perturbation sensors coupled to intracellular signaling pathways. (F) Sensors of cell wall perturbation. (G) Cytoskeleton-coupled transmembrane proteins.

Tight contact between fungus and host during fungal mutualism. (A) Host-plant cell growth is thought to be sensed by the endophyte, Neotyphodium coenophialum, through its strong adhesion to the host intercellular matrix (H, hypha; bar, 1 μm) (reprinted from Fungal Genetics and Biology [ 28 ] with permission of the publisher). (B) The sense of stretch induces intercalary growth of the endophyte, Epichloë fesctucae, so that it elongates at the same rate as the host. Arrows indicate the lateral branches that have moved further apart as a result of intercalary growth (bar, 100 μm) (reprinted from Fungal Genetics and Biology [ 28 ] with permission of the publisher). (C, D) Arbuscular mycorrhizae form intruding hyphal arbuscules within the root cells of Medicago truncatula. Host GFP-Mtcp1 localizes to the cell plasma membrane and its derivative that surrounds the arbuscule trunk, but is not expressed in host membrane surrounding arbuscule branches (arrows), which is thought to be involved in nutrient exchange (a, arbuscules; t, arbuscule trunk; ih, intracellular hyphae; n, nucleus; bar, 20 μm) (reprinted from Plant Physiology [ 34 ] with permission of the publisher). (E) Hebeloma cylindrosporum ectomycorrhizae, labeled with wheat germ agglutininfluorescein isothiocyanate, grow as a dense outer fungal sheath or mantle surrounding the “Hartig net” (arrows) of hyphae growing between outer cortical cells of hazelnut, Tuber melanosporum (m, fungal mantle; bar, 15 μm) (reprinted from Frontiers in Plant Science [ 149 ] with permission of the publisher). (F) H. cylindrosporum forms a fungal sheath surrounding the Hartig net (arrows) of hyphae that intercalate between the cortical root cells of Pinus pinaster (CC, cortical cells; black asterisk, fungal sheath; bar, 10 μm) (reprinted from Molecular Plant-Microbe Interactions [ 35 ] with permission of the publisher).

Cell-cell contact marks the developmental tipping point between “homing” and fusion. (A to F) Healing process between the ends of a severed hypha in mycelia of the arbuscular mycorrhiza, Gigaspora margarita. (B, C) Severed hyphal compartments (open arrows) are sealed off by the formation of new septa. New hyphae emerge from behind the septa and grow chemotropically toward each other (closed arrows). (D to F) Hyphal wall contact and fusion (open arrows) permits cytoplasmic flow to be reestablished (open arrowhead). (Bars, 80 μm) (reprinted from New Phytologist [ 39 ] with permission of the publisher). (G) Mating cells of yeast S. cerevisiae, where the shmoos of two wild-type cells expressing soluble GFP have fused by degrading the cell wall and fusing the plasma membrane. A daughter cell has emerged (top). (H) In the prm1Δ null mutant, which lacks a multispanning transmembrane protein, shmoo-shmoo contact triggered cell wall degradation but not membrane fusion, stalling the mating process (reprinted from the Journal of Cell Biology [ 150 ] with permission of the publisher). (I) Conidial anastomosis tubes (CATs) emerge from conidia of N. crassa and “home” toward each other (c, conidia; gt, germ tube; bar, 6 μm). (J) CATs emerge from germ tubes prior to contact and fusion (bar, 5 μm) (reprinted from FEMS Microbiology Letters [ 44 ] with permission of the publisher).

Contact-induced hyphal tip responses in C. albicans ( 7 ). (A) Tip and Spitzenkörper asymmetry induced by contact of a hyphal tip with an obstacle in control cells expressing Mlc1-YFP (bar, 2 μm). (B, C, and D) Hyphae exhibit contour following, gap penetration, and trajectory maintenance. (B, C, bars, 2 μm; D, bar, 10 μm). (E to H) In hyphae of the rsr1Δ mutant, the Spitzenkörper tends to be centrally positioned with the apex, hyphal tips do not become asymmetrical on contact or respond normally to substrate topography by following contours or penetrating gaps (bars, 2 μm) (reprinted from Cellular Microbiology [ 7 ] with permission of the publisher).

Sensing and signaling pathways that have the potential to be involved in mechanosensing in fungi. For details and references, see the text. PM, plasma membrane.