Involvement of jasmonic acid, ethylene and salicylic acid signaling pathways behind the systemic resistance induced by Trichoderma longibrachiatum H9 in cucumber

Abstract

Background: Trichoderma spp. are effective biocontrol agents for many plant pathogens, thus the mechanism of Trichoderma-induced plant resistance is not fully understood. In this study, a novel Trichoderma strain was identified, which could promote plant growth and reduce the disease index of gray mold caused by Botrytis cinerea in cucumber. To assess the impact of Trichoderma inoculation on the plant response, a multi-omics approach was performed in the Trichoderma-inoculated cucumber plants through the analyses of the plant transcriptome, proteome, and phytohormone content.

Results: A novel Trichoderma strain was identified by morphological and molecular analysis, here named T. longibrachiatum H9. Inoculation of T. longibrachiatum H9 to cucumber roots promoted plant growth in terms of root length, plant height, and fresh weight. Root colonization of T. longibrachiatum H9 in the outer layer of epidermis significantly inhibited the foliar pathogen B. cinerea infection in cucumber. The plant transcriptome and proteome analyses indicated that a large number of differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) were identified in cucumber plants 96 h post T. longibrachiatum H9 inoculation. Up-regulated DEGs and DEPs were mainly associated with defense/stress processes, secondary metabolism, and phytohormone synthesis and signaling, including jasmonic acid (JA), ethylene (ET) and salicylic acid (SA), in the T. longibrachiatum H9-inoculated cucumber plants in comparison to untreated plants. Moreover, the JA and SA contents significantly increased in cucumber plants with T. longibrachiatum H9 inoculation.

Conclusions: Application of T. longibrachiatum H9 to the roots of cucumber plants effectively promoted plant growth and significantly reduced the disease index of gray mold caused by B. cinerea. The analyses of the plant transcriptome, proteome and phytohormone content demonstrated that T. longibrachiatum H9 mediated plant systemic resistance to B. cinerea challenge through the activation of signaling pathways associated with the phytohormones JA/ET and SA in cucumber.

Keywords: Botrytis cinerea; Cucumber; Defense response; Phytohormones; Proteome; Secondary metabolites; Transcriptome; Trichoderma longibrachiatum.

Fig. 1

Transmission electron micrographs indicated that T. longibrachiatum H9 had colonized the cucumber roots. Trichoderma hyphae developed at the root surface (a), penetrated into the root epidermis (b), and progressed towards the cortex (c). Arrows indicate Trichoderma hyphae. Bars: A, 10 μm; B, 500 μm; C, 500 μm

Fig. 2

The top 20 enriched KEGG pathways based on up-regulated DEGs in the T. longibrachiatum H9-inoculated plants in comparison to untreated plants (+T-B vs.-T-B)

Fig. 3

The top 20 enriched KEGG pathways based on up-regulated DEPs in the T. longibrachiatum H9-inoculated plants in comparison to untreated plants (+T-B vs.-T-B)

Fig. 4

The top 20 enriched KEGG pathways based on up-regulated corresponding DEGs/DEPs in the T. longibrachiatum H9-inoculated plants in comparison to untreated plants (+T-B vs.-T-B)

Fig. 5

The expression of hormone-related genes was up-regulated in cucumber plants inoculated with T. longibrachiatum H9. Among these expressed genes, LOX1, LOX2, and AOS1 were associated with JA; PAD4 was associated with SA; and ACO was associated with ET (* and ** indicate significance at P < 0.05 and 0.01)

Fig. 6

A hypothetical working process in cucumber plants with T. longibrachiatum H9 inoculation. LRRs as well as CDPKs and other elicitor-responsive proteins were induced in the Trichoderma H9-inoculated cucumber plants to deliver the signal of perception. Subsequently, MAPK cascades were activated to regulate diverse downstream signaling pathways. Furthermore, phytohormone synthesis and signaling (JA/ET and SA) were activated to translate Trichoderma-induced signaling into the activation of effective defense responses. Consequently, a whole array of defense/stress-related genes and proteins, e.g. a variety of detoxifying enzymes for ROS scavenging, were up-regulated, thus rendering the cucumber plants more resistant to subsequent B. cinerea infection. Along with other defense strategies, the production of secondary metabolites was also employed as an important plant defense strategy