10. Fungiflex in the context of fungal gravitropism
The emphasis of the interpretation so far is very much on uniform chemical signalling diffusing through the whole of the Coprinopsis fruit body, from a source near the cap-stipe junction. Yet our experiments that resulted in the discovery of the Fungiflexes were initiated after our earlier studies on gravitropism which suggested a differential distribution of signalling molecules might be the cause of gravitropic curvature of Coprinopsis stipes.
Application of Fungiflex extracts does indeed mimic gravitropically-induced stipe curvature, but the distribution of the Fungiflex is entirely in the hands of the experimenter. How do our interpretations of the normal biological activity of Fungiflex fit to the biology of a disoriented stipe responding to a changed gravity vector?
We know that gravitropic bending results solely from hyphae of the lower side of the stipe preferentially extending in length significantly more than hyphae of the upper side of the disoriented stipe (Table 1; Greening & Moore, 1996; Greening, Sánchez & Moore, 1997). None of the other parameters measured differed significantly between the upper and lower flanks of the bending stipe (Table 1).
Table 1. Cell morphometrics in sections of gravitropically-responding stipes of Coprinopsis cinerea at the point of maximum curvature
|Upper flank of bend||Lower flank of bend|
Mean width of hyphae (μm)
% narrow hyphae
Cell length (μm)
Data from Greening, Sánchez & Moore, 1997.
By attaching inert markers to the stipe, we found that the outer flank of the bend initially had a faster rate of extension, although the inner flank matched this growth rate later in the response. Thus bending resulted from differential enhancement of growth rates rather than sustained differences.
In a horizontal stipe, all hyphae experience the same force of the gravitational field and so the problem of coordinating hyphal extension growth to generate a bend in the right direction becomes two-fold:
- what is the nature of the signal; and
- how is a differential growth impulse generated across the diameter of the horizontal stipe?
If we are content with the notion that the Fungiflex molecules provide the ultimate signal, we have to understand how the activity of freely-diffusing molecules can be expressed asymmetrically across the diameter of the stipe.
After a considerable amount of analysis of the gravitropic responses of both Coprinopsis cinerea and Flammulina velutipes it was concluded (Moore, Hock, Greening, Kern, Novak-Frazer & Monzer, 1996) that gravity perception in agarics depended on the nuclei acting as statoliths and exerting tension on the actin filament system that surrounds them. See the following information box for more details.
Size determinations in Coprinopsis demonstrated that cells of the stipe increase in length, not diameter, to produce the growth differential. In Flammulina a unique population of highly electron-transparent microvacuoles changes in distribution, decreasing in upper cells and increasing in the lower cells in a horizontal fruit body within a few minutes of disorientation. These are thought to contribute to vacuolar expansion which accompanies/drives cell elongation.
Application of a variety of metabolic inhibitors indicates that the secondary messenger calcium is also involved in regulating the growth differentials of gravimorphogenesis but that gravity perception is unaffected by inhibitors of calcium signalling.
In both Flammulina and Coprinopsis, gravity perception seems to be dependent on the actin cytoskeleton since cytochalasin treatment suppresses gravitropic curvature in Flammulina and, in Coprinopsis, significantly delays curvature without affecting stipe extension.
This, together with altered nuclear motility observed in living hyphae during reorientation, suggests that gravity perception involves statoliths (and the dikaryon nuclei, which are paired together in an actin cage are the best candidates for this function) acting on the actin cytoskeleton and triggering specific vesicle/microvacuole release from the endomembrane system [CLICK on the hyperlinks that follow to download into a new window a (free) PDF of the original publication (Monzer & Haindl, 1994; Monzer, Haindl, Kern & Dressel, 1994; Monzer, 1995; Novak-Frazer & Moore, 1993, 1996)].
Moore et al. (1996) suggested a plausible mechanism for gravity perception in agarics:
‘…nuclei act as statoliths, their displacement within the cytoskeleton surrounding them being communicated by some of those actin microfilaments to the endomembrane system, and maybe by similar means directly to the plasma membrane … An important point is that the initial event does not need to be a major displacement. The whole point of having a signal transduction chain is to provide for amplification of the primary input… in nature the system must be constantly monitoring orientation and correcting small disturbances to maintain vertical growth. In these circumstances statolith movement may be extremely restricted… We would envisage that statolith-activation of microfilament membrane connections could prompt export of a signalling molecule through the plasma membrane, and/or positional-dependent amplification of vesicle/microvacuole production by the endomembrane systems.’
We suggest a two-part model for fungal gravitropism that accounts for all these details and includes the concept of Fungiflex.
Fungiflex can be the uniformly diffusible proto-effector for the gravitropic response if it is assumed that the microfilament cage connections apply stress asymmetrically to the transporter molecules in the cell membrane. This asymmetry is the result of the gravity sensor (believed to be the paired nuclei within their actin cage) reacting to the direction of the gravity vector and so changing the pattern of stresses exerted by the microfilaments connected to the plasma membrane.
The important point about the Fungiflexes is that they diffuse through the extracellular matrix surrounding the hyphae, but are effective in modifying the activity of the hyphal wall construction apparatus only after translocation into the intracellular environment.
Control could be as mechanically direct as tension in the microfilaments connected to the transporter’s membrane-spanning domains. Distortions to those domains could alter the substrate affinities of the normal hexose transporter (that is, the ftr-gene-product).
Consequently, the Fungiflex molecules can be uniformly distributed, but in a disoriented stipe the activities of hexose transporter molecules are differentially regulated by the cytoskeleton across the diameter of the stipe.
- Transporters located on the ‘upper’ hyphal membrane may be activated to transport the extension-inhibiting Fungiflex 1;
- Transporters located on the ‘lower’ hyphal membrane may be activated to transport the extension-promoting Fungiflex 2 without delay.
So this is our two-part model for gravitropism in Coprinopsis, and perhaps in other agarics: (a) an asymmetry in transporter kinetics is caused by gravity-vector-induced changes in tensions in the cytoskeleton, which (b) generates an asymmetry in uptake of Fungiflex from an otherwise uniform diffusion field of that modified sugar.
What somebody needs to do now is prove it!
Copyright © David Moore & Lily Novak-Frazer 2016