Plant growth promotion and biocontrol traits of Vitis vinifera "Glera" bacterial endophytes in the sustainable management of viticulture
Year: 2018
Type: article
Abstract: This thesis describes the results obtained during the three years PhD course which was focused on plant-bacteria interactions, in particular Vitis vinifera cultivar Glera and endophytes, previously isolated from the same plant species. The project aimed to investigate the possible beneficial effects that four selected endophyte strains, belonging to Pantoea and Bacillus genera, can give to Glera rooting cuttings and their potential utility as environmental-friendly alternative approach in plant growth promotion, stress alleviation, and biocontrol. The project was prompted to reduce the extensive use of agro-chemicals in agriculture and to promote a sustainable management of viticulture. As a first step, P. agglomerans (GL83) and B. licheniformis (GL174) selected for their valuable plant growth promoting traits were GFP-tagged and used to inoculate in vitro Glera apical cuttings in order to confirm their endophytic nature. Exploiting laser scanning confocal microscopy and a cultivation dependent method the colonization of stem endosphere of Glera apical cuttings after 20 and 30 days post inoculation (dpi) was demonstrated. The analysis was then extended at 45, 52 and 60 dpi in Glera cuttings which were in the meantime transferred in hydroponic non-sterile medium. Both strains were mostly visualized inside the xylem vessels of the stem until 60 dpi confirming the strains endophytic attitude and persistence. Then, the four strains were directly applied in the field by inoculating rootstocks (Kober 5BB) and Glera buds with each bacteria suspension (GL83, GL174, GL189, GL452 and the consortium of these four) both during the hydration step of rooting cuttings production line and by administering the bacterial suspensions at the root level, after plants have developed roots. Then, both the morphological and physiological plant parameters were recorded sampling plants at different time-point of their development. GL83 and GL174 were the most promising grapevine bio-fertilizer strains, especially when rootstocks and buds were inoculated during the hydration step of rooting cuttings production line. Inoculated plants presented improved leaf physiological parameters such as photosynthesis rate and transpiration flux linked to an increase of root and shoot fresh biomasses compared to the control plants. Moreover, an experiment aimed to simulate a fertilization procedure using 1-year-old Glera grapevine plants, which were bio-fertilized once a week for seven consecutive weeks with GL83 and GL174 confirmed these strains as promising bio-fertilizer candidates. Furthermore, GL83 was able to relieve the effects of drought stress in inoculated Glera rooting cuttings. In fact, stressed-inoculated plants presented enhanced physiological parameters compared to stressed-control plants and, in addition, GL83 plant growth promoting traits seemed to be water stress dependent. Another issue investigated during these three PhD years was the role of Bacillus cyclic lipopeptides (LPs) families as elicitors of plant defence responses, through induced systemic resistance (ISR) pathway. LPs are amphiphilic compounds produced by some beneficial bacterial strains freely living in the soil or undergoing mutualistic interactions with plants. The natural functions of LPs suggest their role in antagonism toward other microorganisms, like plant pathogens, and their involvement in motility and attachment to surfaces encouraging bacteria biofilm formation and development. So far, the mechanisms underlying the induced physiological responses in host plants, upon the exposure to these compounds remain unclear. Challenging Arabidopsis cell suspension cultures with the commercial Bacillus LPs families (surfactin, fengycin and iturin) both the early cell response and the later events like defence-gene expression were evaluated. Through Evans blue test no cell death increase was detected challenging Arabidopsis cell culture with three different concentrations of the LPs families. The administration of the highest concentration of surfactin (50 μg/ml) to A. thaliana cell cultures expressing the Ca2+-sensitive photoprotein aequorin in the cytosol evoked a cytosolic Ca2+ transient, suggesting that the perception of surfactin could be mediated by Ca2+, which is one of the main important intracellular messenger that generate a wide range of different spatial and temporal signals depending on the stimulus perceived. Then, to evaluate the possible long-term effects generated upon the perception of LPs, a semi-quantitative PCR defence-gene expression analysis was performed at 2 and 6 hours after LPs cell culture-exposure. Lipoxygenase 1 (LOX1), Pathogen-related protein 1 (PR1), Phenylalanine ammonia lyase (PAL1), Non-expressor of Pathogenesis-Related genes 1 (NPR1) and Mitogen-Activated Protein Kinase 3 (MAPK3) were the main genes investigated which are considered reliable markers of ISR and systemic acquired resistance (SAR). The results showed that ~3-fold increase of LOX1 and PAL1 expression was recorded in treated samples respect to controls. A ~2-fold increase in the NPR1 gene expression was observed comparing samples at 2 h with control. No differences in PR1 and MAPK3 gene expression were detected between samples and controls. The up-regulation of the LOX1 and NPR1 could suggest that surfactin elicits the activation of jasmonate-mediated pathway that could culminate in ISR. Instead, up-regulation of PAL1 without PR1 expression may indicate an initial SAR triggering, afterwards inhibited by some steps of ISR pathway. A UPLC-MS analysis performed at the Gembloux Agro-Bio Tech (University of Liège, Belgium) revealed that some selected Glera Bacillus endophytes produce surfactin and fengycin. Moreover, a quite uncommon type of surfactin called pumilacidin was detected and semi-purified. An in vitro antagonism assays against some plant pathogen bacteria showed that pumilacidin had a quite strong antibacterial effect. This is a preliminary result and other tests against, for example, infectious fungi such as B. cinerea need to be performed. Nevertheless, this is an important starting point for developing new sustainable practices with potential application in disease control. In conclusion, among grapevine cultivable endophytic strains, GL83 and GL174 showed to be promising candidates as bio-fertilizer enhancing both the grapevine growth and health: GL83 was able to promote growth and to confer an increased tolerance to drought stress whereas GL174 seemed to have a protecting action against some fungal grapevine pathogens. The possibility of endophyte exploitation to help plants to cope with abiotic as well as biotic environmental stresses, interfering with plant morphology and physiology or priming their systemic responses against pathogens, opens new scenarios for an environmentally friendly shift in agricultural practices with much lower impact of synthetic agro-chemicals. Results obtained from this work are aligned with the huge amount of studies, grown exponentially in the last years, in which the endophytes abilities and properties are often assayed in a single plant species or within groups of closely related plant genotypes. In addition, investigations are usually performed using microbial species that are relatively easy to cultivate leaving out that plant biome is always characterized by wide and complex interactions. The plant phenotype is determined not only by the plant responses to the environment but it is also orchestrated by the associated microbiota, the responses of the microbiota to the environment, and the complex interactions between individuals in the endosphere microbial plant community. Future exciting challenges, based on classical approaches and new valuable technologies (i.e. next-generation sequencing) applied to greenhouse and field conditions would allow to explore and characterize the contributions of genetic and metabolic elements involved in the interactions between host plants and endophytes, providing new ecological and evolutionary insights and a better knowledge of the plant-microbiome-environment relationships.
Author: Filippo Zanella
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W2903552828