Successive decomposition of pinecones by fungi ofAuriscalpium and Strobilurus
The successive decomposition of substrates by microbial communities is a
common phenomenon (Johnston, Boddy, & Weightman, 2016; Niemela et al.,
1995). Often, the microbial community structure, including the relative
abundances of saprophytic fungi, changes significantly during the
successive decomposition process (Fukasawa & Matuoka, 2015). Though
occupying the same cological niche, species in these communities may
develop unique but complementary strategies to partition the resources
in the substrates, leading to temporal niche differentiation and
divergence (Friedemann et al., 2016). Here, part of the resource
partition is temporal changes of fungi with different fungi use
different sets of nutrients within the pinecone. Similar phenomena have
been found in other substrates such as plant litters and deadwoods
(Baldrian et al., 2016; Edman & Eriksson, 2016; Herzog, Hartmann, Frey,
Stierli, & Brunner, 2019). In the processe of biodegradation, microbial
coordinations with different ecological strategies and certain orders
are evident (Herzog et al., 2019; Holmer, Renvall, & Stenlid, 1997).
For
example, successions of fungi in temperate forests were considered to be
reflected in sugar utilizing fungi, followed by wood structural decaying
fungi, and finally residual decaying fungi in some cases (Stokland,
Siitonen, & Jonsson, 2012). This change may be explained in part by
nutrients released by the primary decomposers that enabled the
colonization of secondary decomposers (Boddy, 2000). However, the
successive decomposition of substrates such as deadwood and plant litter
requires the action and interaction of many fungi with their fungal
community showing a high degree of complexity (Baldrian et al., 2016;
Voříšková & Baldrian, 2013) For example, Zhang and Wei (2016) had
carried out relevant research on fungi in the same forest, in which
different fungi will appear on rotten wood in the same state, or even on
the same rotten wood. At the same time,
a
kind of fungus can also exist in different periods of rotten wood (Zhang
& Wei, 2016; Figure S1). Some fungi can only appear in one period, but
most fungi can produce fruitbodies at several stages of rotten wood
(Zhang & Wei, 2016; Figure S1). Similarily, Niemela et al., (1995)
reported the succession of more than one hundred species of lignicolous
Basidiomycetes on fallen trunks in Picea obovata and P .sylvestris . Our study revealed that fungi in Auriscalpiumand Strobilurus possess clear differences in the type and number
of CAZymes and lignocellulolytic genes
(Figures
2c,3a,4g–h and S3b). Our results indicate that even though they
colonize the same pinecones, there are significant divergence and niche
differentiation in the utilization of substrates in pinecones between
the fungi of the two genera, which leads to the dynamic changes of their
emergences on the pinecones.
During the initial decomposition of pinecones, Auriscalpium fungi
are the primary colonizers, likely related to their ability to break
down resin and their
strong
capacity to decompose lignin and hemicellulose (Figure 4a–c; Table 2).
Such abilities are common among WR fungi (Floudas, Bentzer, Ahrén,
Johansson, & Tunlid, 2020). For example, the fungi ofCeriporiopsis subvermispora , Phellinus pini ,Ganoderma australe , and Phlebia tremellosa specifically
degrade lignin and hemicellulose among WR fungi (Weng, Peng, & Han,
2021). The most compelling evidence supporting the early colonizing
ability of Auriscalpium fungi is that their number of peptidases
S8 and S53 is far greater than that in Strobilurus fungi.
Peptidases S8 and S53 are among the top 10 most up-regulated enzymes inR . microporus in the presence of latex (Oghenekaro et al.,
2020) (Table 2). The genomic evidence is consistent withAuriscalpium fungi capable of colonizing newly fallen cones and
decomposing proteins in resin rapidly. Polo, Pereira, Mazzafera,
Flores-Borges, & Meneau, (2020) showed that lignin and hemicellulose
are in the outermost layer of plant cell wall which prevents the
cellulolytic enzymes reaching the cellulose and protect plants from
microbes. Our analyses demonstrated that the number of genes coding for
lignin oxidases (AA2) and hemicellulase (GH3) in Auriscalpium are
significantly higher than those in Strobilurus (Figure 4g–h;
Table S10), and these genes may be related to lignin and hemicellulase
decompositions in the outermost layer of pinecones. Once the outer layer
is breached, the condition is now more favorable for the subsequent
invasion of Strobilurus fungi. With increasing decay, the
nutritional composition, physical structure, chemical composition and
other aspects of the pinecones have changed, which result in the
succession changes of fungal community.
After decomposition by Auriscalpium fungi, the proportions of
lignin and hemicellulose in the pinecone would decrease and those of
cellulose and pectin proportionally would increase (Figure 4a–c). The
subsequent colonization by Strobilurus fungi relies on the
residual components of the cones suitable for their growth and replacing
the corresponding fungi of Auriscalpium (Figure 5). Similarly,
comparing Auriscalpium and Strobilurus grown on the same
pinecone, fungi of Strobilurus show decreasing trends of in the
number of genes coding for ligninases and hemicellulases, but with
higher number of genes coding for cellulase and pectinase, which is
broadly consistent with the changes of cone components (Figure 4a–f).Strobilurus pachcystidiatus and A . microsporum also
show the same pattern, but the differences are not particularly evident,
which may relate to the fact that both could grow on newly fallen cones.
However, S . orientalis grew on the highly rotten cones
after decomposition by A . microsporum or S .pachcystidiatus and it showed a more obvious decrease in the
number of ligninase-encoding genes and an increase in the number of
cellulase-encoding genes than S . pachcystidiatus (Figure
4f).
In addition, in the field, we observed that the fungi ofAuriscalpium can decompose cones independently, especially in
tropical areas, but the successive decomposition of the two genera is
more common. However, we did not observe the decomposition of cones by
fungi of Strobilurus independently. In each distribution areas of
fungi in Strobilurus , fungi in Auriscalpium could be
collected in different periods, and the fungi in Strobiluruscollected all grow on the cones with high degree of decay. For
successive decomposition of P . armandii’ s cones, in
addition to the most common combination of A .microsporum -S . pachcystidiatus -S .orientalis , we also observed the combinations of A .microsporum -S . pachcystidiatus and A .microsporum -S . orientalis . Therefore,
various situations may occur in
the field (Figure 5). Although Strobilurus fungi always appear on
cones with a high degree of decomposition, however, the results of our
field observation show that compared with Auriscalpium fungi, theStrobilurus fungi can occupy the cone for a long time and fully
decompose the cone. There have been reported on the positive correlation
between the large amount of CAZymes in the genome and the degradation of
plant biomass (Adams et al., 2011), so we speculate that fungi inStrobilurus are the main decomposer with the type and number of
CAZymes
in Strobilurus being richer than those of Auriscalpium(Figures 2b–c and 3a). In the CAZymes comparisons between the two
genera, only seven CAZyme gene families have significantly more genes in
the Auriscalpium fungi than in Strobilurus , while the
other 36 gene families have more genes in Strobilurus fungi than
in Auriscalpium fungi (Table S3), which broadly supports that the
subsequent decomposers are main components of substrate decompositions
(Song et al., 2012).