Lab boost delivered by salt-tolerant PGPB strains offers hope for halophyte cultivation

Several plant growth-promoting bacterial strains are able to boost growth in halophyte plants under laboratory conditions, offering hope for developing their cultivation in marginal land.

The salt-tolerant bacteria could offer hope for scientists trying to develop halophytes that will thrive in high saline conditions and places with limited water supply.

The study by researchers at University of Aveiro in Portugal, ‘Biostimulation of Salicornia europaea L. crops with plant-growth-promoting bacteria in laboratory and field conditions: effects on growth and metabolite profile’, appears in the Journal of Applied Microbiology, an Applied Microbiology International publication.

Corresponding author Angela Cunha explained: “Halophyte plants thrive in saline environments while displaying interesting nutritional properties related to adaptations to salt and oxidative stresses.

”Their cultivation is regarded as a promising alternative to the challenges imposed by the scarcity of fresh water for irrigation, the necessity of using reclaimed or brackish water for irrigation, and the progressive salinization of soils as a consequence of sea level rise.

“Additionally, it adds value to a natural/marine resource that can be sustainably exploited as an independent activity or in association with other primary sector activities like extensive aquaculture. 

“Halophytic plants are enriched in beneficial metabolites. These compounds are of great interest for the food and pharmaceutical industries as they improve the nutritional value of these plants and display a wide range of medicinal applications.”

High salinity

However, high salinity levels can limit growth even for halophytes, Dr Cunha warned. 

“The challenge is to attain higher productivity and better nutritional properties for Salicornia crops, and in that way, increase the market value of this plant and the profit of producers. Our approach was to use salt-tolerant bacteria to stimulate plant growth and modulate the phytochemical profile.”

The lab results did meet expectations, Dr Cunha said.

“A combination of the bacteria Brevibacterium casei and Pseudomonas oryzihabitans significantly enhanced biomass production. 

“But in the field, we could not get evidence of a significant stimulation of plant growth with the same combination of bacteria. This was attributed to a low persistence of this bacteria, facing competition with the natural communities of soil.”

The team isolated bacteria with plant growth promoting activities from Salicornia europaea root. A selection of three bacteria, Brevibacterium casei EB3, Pseudomonas oryzihabitans RL18 and Bacillus aryabhattai SP20, were inoculated in S. europaea seeds, in different combinations.

Plants germinated from inoculated seeds were cultivated in the laboratory or in a saline field for six months, and fresh and dry weight were determined at the end of the experiment, for biomass quantification. 

The microbiological quality of fresh shoots for human consumption as salad greens was assessed, and the persistence of the inoculated strains in the plant rhizosphere was confirmed by next-generation sequencing (Illumina) of the 16S rDNA gene. The primary metabolite profile of biostimulated plants was characterized by GC-TOF-MS.

Laboratory conditions

In laboratory conditions, inoculation with the two strains B. casei EB3 and P. oryzihabitans RL18 caused the most significant increase in biomass production (fresh and dry weight) and caused a shift in the central metabolic pathways of inoculated plants towards amino acid biosynthesis. In the field experiment, no significant biostimulation effect was detected with any of the tested inoculants. Seed inoculation had no significant effect on the microbiological quality of the edible parts.

“Plants from the genus Salicornia hold a great potential for crop cultivation in salt-affected soils. The interest in using these plants as gourmet food and as a source for nutraceutical compounds is rising worldwide,” Dr Cunha said. 

“We found a formulation of bacteria with plant growth properties that not only highly improved plant growth, but can also be used to modulate the metabolite composition of the plant, resulting in a valorized nutritional profile.

“The effective reduction on the use of agrochemicals is imperative to achieve several of the UN sustainable goals, but sustainability relies also on the protection of the natural soil microbiome. To reduce possible negative impacts on the native soil microbial communities, the use of PGPB native to the plant microbiome is crucial. Healthy soils make healthy plants.

Engineering the rhizosphere

“Another important aspect of our work is the possibility to engineer the plant’s rhizosphere in order to accumulate specific molecules with biological activity, such as phenolic compounds, flavonoids, essential omega-3 and omega-6 fatty acids.”

The application of our biofertilizers under natural field conditions did not significantly improve plant growth, so it is still necessary to transpose this strategy from the laboratory to the field, Dr Cunha said. 

“We need to identify the major factors acting in the field, simulate them in controlled conditions, and optimize the operational parameters of biostimulation (formulation of the mixture of bacteria, mode and frequency of application) as well as the cultivation practices (seed density, irrigation mode, etc).

“The complete study of the plant metabolome is also of paramount importance to assess the full impact of inoculation on the expression of high added-value metabolites.”

The team

This research has been conducted by an interdisciplinary group of researchers from the Laboratory of Environmental Microbiology – LMICRA  (CESAM & Biology Department of the University of Aveiro, Portugal), in collaboration with experts in halophyte ecology (University of A Coruña, Spain), plant metabolomics (Agriculture University of Lisbon, Portugal) and organic chemistry (Department of Chemistry of the University of Aveiro Portugal), and also a private company producing Salicornia in tne region of Aveiro, Portugal.

The study was led by Angela Cunha. Financial support came from project Rhizomis PTDC/BIA-MIC/29736/2017 financed by Fundação para a Ciência e a Tecnologia (FCT) through the Regional Operational Program of the Center (02/SAICT/2017) with FEDER funds (European Regional Development Fund, FNR and OE) and by FCT through CESAM (UIDP/50017/2020 + UIDB/50017/2020), LAQV-REQUIMTE (UIDB/50006/2020). FCT also provided a doctoral grant to one of the co-authors (Maria João Ferreira, PD/BD/150363/2019).

The study ‘Biostimulation of Salicornia europaea L. crops with plant-growth-promoting bacteria in laboratory and field conditions: effects on growth and metabolite profile’, appears in the Journal of Applied Microbiology.