Collembola interact with mycorrhizal fungi in modifying oak morphology, C and N incorporation and transcriptomics

Abstract

Soil detritivores such as Collembola impact plant growth, tissue nutrient concentration and gene expression. Using a model system with pedunculate oak (Quercus robur) microcuttings that display a typical endogenous rhythmic growth with alternating shoot (SF) and root flushes (RF), we investigated the transcriptomic response of oak with and without mycorrhiza (Piloderma croceum) to the presence of Collembola (Protaphorura armata), and linked it to changes in resource allocation by pulse labelling the plants with ¹³C and ¹⁵N. Collembola impacted Gene Ontology (GO) terms as well as plant morphology and elemental ratios with the effects varying markedly with developmental phases. During SF Collembola increased GO terms related to primary growth and this was mirrored in increased ¹³C and ¹⁵N excess in aboveground plant compartments. During RF, Collembola increased GO terms related to plant secondary metabolism and physical fortification. Further, Collembola presence resulted in an increase in plant defence-related GO terms suggesting that Collembola in the rhizosphere prime oak shoots against the attack by fungi or herbivores. Notably, the impact of Collembola on growth, resource allocation and oak gene expression was modified by presence of P. croceum. The results indicate that oaks clearly react to the presence of Collembola in the rhizosphere and respond in a complex way by changing the expression of genes of both primary and secondary metabolism, and this resulted in concomitant changes in plant morphology and physiology.

Keywords: Collembola; Gene Ontology; Quercus robur; plant defence; plant nutrition; stable isotopes.

Figure 1.

Effects of Piloderma, Protaphorura and Stage on (a) principal root dry weight during root (RF) and shoot flush (SF) (Control n = 9, Piloderma n = 15, Protaphorura n = 6 and Protaphorura + Piloderma n = 20 in RF, and Control n = 9, Piloderma n = 7, Protaphorura n = 6 and Protaphorura + Piloderma n = 5 in SF treatments), and (b) on stem length (pooled for RF and SF) (Control n = 18, Piloderma n = 22, Protaphorura n = 12 and Protaphorura + Piloderma n = 25).

Figure 2.

Effects of (a) Protaphorura on source leaf ¹³C atom% excess during root (RF) and shoot flush (SF) (pooled for Piloderma) (RF: Control n = 6; Protaphorura n = 4; SF: Control n = 5; Protaphorura n = 6); ¹³C and ¹⁵N atom% excess in plant compartments, (b) effects of Protaphorura on stem ¹³C atom% excess during RF and SF (pooled for Piloderma) (RF: Control n = 6, Protaphorura n = 4; SF: Control n = 5, Protaphorura n = 4), (c) effects of Protaphorura and Piloderma and their combination on sink leaf ¹³C atom% excess during RF and SF (RF: Control n = 5, Piloderma n = 6, Protaphorura n = 3, Protaphorura + Piloderma n = 6; SF: Control n = 5, Piloderma n = 3, Protaphorura n = 3, Protaphorura + Piloderma n = 6), and (d) effects of Protaphorura on principal root ¹⁵N atom% excess (pooled for Piloderma) (RF: Control n = 6, Protaphorura n = 3; SF: Control n = 5, Protaphorura n = 3).

Figure 3.

Results of Gene Ontology terms over-representation analysis (FDR < 0.01). Gene Ontology (GO) terms enriched in up- and down-regulated contigs from sink leaves at shoot flush (SF) of plants treated with Piloderma croceum, Protaphorura armata, or from combined treatment are shown, with GO terms related to upregulated (red) and downregulated (blue) contigs. BP marks GO category biological process, MF molecular function and CC cellular compartment, and significance levels are marked by column lengths with maximum column length at p = 1 × 10⁻¹⁶. The absolute numbers and percentages of differentially expressed contigs under each GO term are indicated. TM, transmembrane; PK, protein kinase; SA, salicylic acid. Note that plant growth-related GO terms regulation of meristem growth and microtubule based movement are enriched in upregulated contigs at all three interaction types.