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).