4. Fungal growth factors

The absence of Fungiflex 2 from undifferentiated primordia (in contrast to Fungiflex 1) and presence at all later stages of development suggested that its production was correlated with multihyphal differentiation. Again, there were similarities to the ‘stimulator’ described by Robert (1990) which caused stipe elongation and fruiting induction in Coprinellus congregatus. That inhibitory and stimulatory substances produced in coprinoid fungi are active at different stages of mushroom development suggests that there is also a temporal aspect to their regulation.

Unfortunately, the C. congregatus compounds were never purified or identified so their similarities, in structure and activity, with Fungiflexes 1 and 2 remain unknown.

The fruiting induction substance (FIS) extracted by Rusmin & Leonard (1978) from different developmental stages of Agaricus bisporus fruit bodies showed no temporal differences in activities; that is, all stages produced equivalent fruiting inducing activity. Technically, these two cases are not totally comparable since the first pertains to factors which enhance or inhibit stipe extension while the second deals with fruiting induction (so there is a danger we might be comparing apples with oranges).

Production of the Fungiflexes was not gravity-induced and therefore not specific to the gravitropic response since both compounds were extracted from vertical as well as horizontal stipes and their production was not meiosis-dependent, as gravicompetence seemed to be.

This does not preclude their possible roles in regulating the differential growth characteristic of gravitropic bending but suggests a general role in coordinating growth. It may be that the activity of the gravity-regulated growth factor receptors, if they exist, were only manifested after meiosis, the event which triggered gravicompetence and after which functional receptors would have become essential for establishing the vertical orientation of differentiated primordia or stipes.

The Fungiflexes were also different in terms of their activities in other mushroom species. Whereas Fungiflex 1 was non-species specific, its activity being observed in all the mushroom species examined, Fungiflex 2 activity was specific to C. cinerea, although no closely related species were tested.

This suggests that Fungiflex 1 might be a universal mushroom growth factor, similar to auxins and other growth hormones described in plants (Salisbury & Ross, 1985), although its activity in a much wider spectrum of mushroom species must be confirmed.

The universality of Fungiflex 1 activity might extend to filamentous hyphae, although this also needs to be confirmed with much purer Fungiflex 1 extracts than the ones used in the mycelial assays.

Crude stipe extracts (rich in Fungiflex 1 activity but poor in Fungiflex 2 activity) had different activities in taxonomically distinct species; specifically, extracts were inhibitory to other basidiomycete mycelia but stimulatory to ascomycete mycelia, which also exhibited a changed morphology, possibly indicative of a hormone/morphogen role for Fungiflex 1 in these fungi.

The inhibitory/stimulatory activities could not be assigned conclusively to either Fungiflex 1 or 2 and so further investigation would be needed to establish whether basidiomycete hyphae were specifically Fungiflex 1-sensitive and ascomycete hyphae Fungiflex 2-sensitive or whether the Fungiflexes had different activities in different species. These results had implications for the development of these fungal inhibitors/stimulators for products with agricultural, pharmaceutical and other applications.

Considering the fragmentary evidence for the presence of fungal growth factors (for review see Novak-Frazer, 1996) and the divergent species used to prepare extracts, it was surprising that the Fungiflexes shared similarities with the various mushroom extracts described previously, including those extracts which were analysed for their ability to elicit fruit body induction/formation.

The Fungiflexes and extracts from Agaricus bisporus, Coprinus macrorhizus, Hypholoma fasciculare, Armillaria matsutake (Hagimoto & Konishi, 1960; Konishi, 1967; Urayama, 1969), Lentinula edodes, Flammulina velutipes, and Pleurotus ostreatus (Urayama, 1969; Gruen, 1982) were all found to be less than 12,000 MW, heat stable, acid/base stable and mostly soluble in polar solvents including water.

These substances also all enhanced stipe elongation with the notable exceptions of Fungiflex 1 and the substances extracted from Coprinus congregatus primordia (Robert & Bret, 1987; Robert, 1990), which inhibited stipe elongation. Although these growth controlling substances are probably not identical, they possess remarkably similar characteristics and may be similar types of compounds.

They might comprise a family of hormones/growth factors with slightly different chemical properties in each species, but with enough similarities to be cross-reactive, as Fungiflex 1 activity seems to suggest and as in the case of the Agaricus bisporus extracts, which induced fruiting in Schizophyllum commune (Rusmin & Leonard, 1978). This is not unlike the situation in plants where 'auxin' is actually a family of related compounds based on indole-3-acetic acid (Salisbury & Ross, 1985) and is active on a very wide variety of plants.

The similarity of the Fungiflexes to other fungal growth factors may again suggest a general role in controlling extension of the fruit body stipe. During rapid elongation, which occurs soon after meiosis in Coprinopsis cinerea (Hammad et al., 1993a), the growth of the expanding cap and elongating stipe must be coordinated. The expansion of the whole fruit body must also proceed accurately in a vertical direction so that the gill surface is properly oriented when sporulation occurs a few hours later. The Fungiflex factors may be responsible for continually correcting the direction of growth of the mushroom as well as coordinating the expansion of the cap and elongation of the stipe during the latter stages of fruit body development.

Gravitropism as a case study for putative Fungiflex activity

Although the involvement of Fungiflexes in bringing about the differential growth characteristic of gravitropic bending has not been proved, the manner in which these putative growth factors affected differential extension in vertical stipes paralleled events which normally occurred during the gravitropism of horizontal stipes.

Gravitropism consists of the upward bending of a horizontal stipe until the stipe apex reaches a minimum angle of about 35° (to the horizontal), which acts as a trigger for the activation of the compensation mechanism (Kher et al., 1992; Moore et al., 1994a; Moore et al., 1996), initiated to dampen bending so that the stipes return to the vertical without overshooting.

The initial upward bending response in a horizontal stipe, is due to the selective inhibition of extension of hyphae in the upper flank of the bending stipe (Greening, 1995; Greening & Moore, 1996), and may be regulated by the activity of Fungiflex 1, which caused ‘bending by contraction’ in vertical stipes. Meanwhile, the bend compensation mechanism, as a result of resumption of growth on the upper side, may be regulated by the activity of Fungiflex 2, which caused enhanced hyphal extension in the bioassays with vertical stipes.

Whether the Fungiflexes actually functioned in this manner to bring about gravitropic bending requires further experimentation with purified compounds. Clearly, the fact that different regions of the same gravitropically responding stipe extend differently implies not only that the relative position of hyphae in the stipe is recognised but that there must be a mechanism by which differential growth is coordinated. Models incorporating the information from studies of gravitropism and the observed activities of the Fungiflexes may explain how these growth control substances might function. But before we speculate about this mechanism we should define what we know about the chemistry of the Fungiflexes.

Copyright © David Moore & Lily Novak-Frazer 2016