Deciphering mechanisms underlying mycorrhizal induced resistance in tomato plants
- Autores: Neus Sanmartín Martínez
- Directores de la Tesis: Víctor Flors (dir. tes.), Paloma Sánchez Bel (codir. tes.)
- Lectura: En la Universitat Jaume I ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Kalliope K. Papadopoulou (presid.), Ainhoa Martinez Medina (secret.), Pierre Pétriacq (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencias por la Universidad Jaume I de Castellón
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Plants have developed a complex immune system to resist and survive under adverse environmental conditions. The association with beneficial microorganisms, such as Arbuscular Mycorrhizal Fungi (AMF), improves their tolerance to abiotic and biotic stresses. The enhancement of plant defences by AMF is known as Mycorrhiza-Induced Resistance (MIR) and the mechanisms behind are still far from being understood.
Firstly, we wanted to study how tomato mycorrhizal (AM) plants perform upon a nutritional deficiency during the early stages of the stress. AM plants become more sensitive and perceive earlier the stress and, consequently, respond more efficiently to the abiotic stress when they are subjected to incipient nutrient starvation. The high-affinity nitrate transporter NRT2.1 displays a priming profile in AM plants upon nitrogen starvation, which clearly supports a trade-off advantage to AM plants sensing and reacting more efficiently than non-mycorrhizal (NM) plants. Accordingly, AM plants show increased DELLA and repressed JAZ gene expression indicating an optimal defence status, even under detrimental growth conditions. AM plants not only respond faster to the upcoming stress but also recover faster when nitrogen is available again. Besides, there is a metabolic rearrangement upon nitrogen starvation in AM plants, and some compounds which are reduced in NM plants, such as amino acids, remained unaltered or were even increased in AM plants compared with normally fertilized plants. Note that, upon nutritional stress perception, plants regulate their growth-defence balance and prioritize the activation of resistance and attenuate their growth to promote stress tolerance. Additionally, abiotic stress tolerance seems to prevail against biotic stress defences, hence, plants dealing with nutrient starvation may become more susceptible to diseases. However, AM plants exposed to nitrogen starvation, buffer such antagonisms retaining an optimal defence status against biotic stress.
It is well known that AMF induces resistance against several pathogens. We studied the key mechanisms underlying MIR in tomato plants upon infection by the necrotrophic fungus Botrytis cinerea. We found a pronounced accumulation of callose in AM plants compared with NM plants upon infection. The inhibition of the callose synthase PMR4, the enzyme responsible for callose deposition, resulted in an impairment of MIR demonstrating the relevance of callose priming in the AMF protection against B. cinerea. Regarding carbohydrate metabolism, ADP-glucose, the main precursor of starch, presented higher levels in AM plants than NM plants, as well as the starch levels. Once the pathogen infection was present, the highest starch degradation rate seemed to feed the callose deposition priming since, despite being less infected, AM plants showed higher expression levels of sucrose synthases SUS1 and SUS3 and the invertase LIN6 than NM plants. Note that a lower B. cinerea infection means lower glucose consumption due to the pathogenic infection. Besides, the vacuolar sugar transporter SUT4 was up-regulated in AM plants upon infection suggesting that a more active sugar transport contributes to the higher sugar supply needed for callose polymerization at the pathogen penetration site. At the intracellular level, the genes codifying for proteins involved in vesicular trafficking ATL31 and SYP121 showed a priming profile in AM plants, which demonstrate the relevance of the vesicular trafficking along the sugar transport in the priming of callose. Moreover, the inhibition of vesicular trafficking, abolished callose priming and consequently MIR. Concluding, accelerated starch degradation and subsequent intracellular sugar transport contribute to callose priming and MIR.
Since AMF colonization is restricted to the roots, the enhanced resistance must involve a coordinated systemic response between roots and shoots. Our studies show that the major impact in metabolic changes at the shoot level is due to the infection with B. cinerea, whereas at the root level, the symbiosis has a major influence in the metabolic fingerprint. Several compounds were boosted in AM leaves showing a clear priming profile, such as oxylipins and lignans, which are likely involved in MIR. Following analysis of the transported metabolites from AM roots to shoots, we found the two lignans yatein and seicosolaricinresinol more accumulated in the root efflux of AM plants. Moreover, yatein was also found in higher concentrations in the leaves of mycorrhizal plants. This compound not only inhibits B. cinerea growth in vitro but also protects tomato plants after treatment, showing the implication of lignans in MIR.
Mycorrhizal colonization not only changes plant metabolome but also alters plants at proteomic level. Despite secondary metabolites involved in defence are accumulated in AM plants following infection, at proteomic level the symbiosis produces a major impact, independently of the presence of the stress. Although a general reduction of protein content is observed in AM plants, an ontology classification revealed that specific pathways, such as photosynthesis and carbohydrate metabolism, presented a higher number of proteins up-accumulated. Both pathways may participate in the enhanced resistance of AM plants since both are relevant for callose priming during MIR.
Finally, we focused on the role of volatile signals against necrotrophic pathogens during MIR. AM plants show the strongest changes in volatile emission in response to the infection by B. cinerea at 24 hpi, whereas in NM plants, these changes are observed at 48 hpi, suggesting once more that AM plants are more sensitive and therefore display faster responses to biotic challenges. Mycorrhizal symbiosis produces a general reduction in volatile organic compounds (VOCs), although some monoterpenes and green leaf volatiles present a priming profile. These volatiles can inhibit in vitro B. cinerea growth, which supports that specific VOCs may contribute to MIR.
All in all, we propose that metabolic rearrangement and enhanced callose accumulation directed by protein changes are key components of defence priming behind MIR against B. cinerea. Furthermore, the changes in specific VOCs released by AM plants likely contribute to the enhanced resistance since they show antifungal properties.
