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Abstract
Numerous Trichoderma strains have been reported to be optimal biofertilizers and biocontrol agents with low production costs and environmentally friendly properties. Trichoderma spp. promote the growth and immunity of plants by multiple means. Interfering with the hormonal homeostasis in plants is the most critical strategy. However, the mechanisms underlying plants’ responses to Trichoderma remain to be further elucidated. Auxin is the most important phytohormone that regulates almost every aspect of a plant’s life, especially the trade-off between growth and defense. The AUXIN RESPONSE FACTOR (ARF) family proteins are key players in auxin signaling. We studied the responses and functions of the PdPapARF1 gene in a hybrid poplar during its interaction with beneficial T. asperellum strains using transformed poplar plants with PdPapARF1 overexpression (on transcription level in this study). We report that PdPapARF1 is a positive regulator for promoting poplar growth and defense responses, as does T. asperellum inoculation. PdPapARF1 also turned out to be a positive stimulator of adventitious root formation. Particularly, the overexpression of PdPapARF1 induced a 32.3% increase in the height of 40-day-old poplar plants and a 258% increase in the amount of adventitious root of 3-week-old subcultured plant clones. Overexpressed PdPapARF1 exerted its beneficial functions through modulating the hormone levels of indole acetic acid (IAA), jasmonic acid (JA), and salicylic acid (SA) in plants and activating their signaling pathways, creating similar results as inoculated with T. asperellum. Particularly, in the overexpressing poplar plants, the IAA level increased by approximately twice of the wild-type plants; and the signaling pathways of IAA, JA, and SA were drastically activated than the wild-type plants under pathogen attacks. Our report presents the potential of ARFs as the crucial and positive responders in plants to Trichoderma inducing.
Highlights
With the increasing population and environmental problems, sustainable agriculture and forestry are in great demand
We examined the expression of PdPapARF1 in response to Ta536 or T. asperellum strains ACCC32492 (Ta492) or the combination of four T. asperellum strains (Ta536+Ta492+T. asperellum ACCC31650+T. asperellum T4)
Based on the phenotypes of OX1 plants, we examined the expression of poplar orthologs of genes that play important roles in adventitious root formation in Arabidopsis thaliana and poplar orthologs of auxin flux carriers including PIN1 and LAX3
Summary
With the increasing population and environmental problems, sustainable agriculture and forestry are in great demand. One important growth-promoting mechanism is modifying the levels of phytohormones, including ethylene, cytokinin, auxin, or their related compounds in plant rhizosphere and root [3,6]. More reports demonstrated that some Trichoderma species could produce or degrade in vitro indole acetic acid (IAA), namely auxin, to create optimal IAA concentrations for plant growth [9,10,11]. Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are crucial regulators of plant defense and resistance Their signaling cascades cross paths with GA and IAA through hub proteins such as DELLA and EIN3 [13,14]. The growth-promoting effects of Trichoderma spp. involving auxin signals have been attributed to the production of auxin-related compounds in vitro [9,10,11]. We studied the role of ARF1 in the interaction between a hybrid poplar Populus davidiana × P. alba var. pyramidalis (PdPap) and a beneficial T. asperellum strain
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