Many plants have developed adaptations to environmental extremes such as drought and high salinity. Plant resilience can enhance the vast community of associated microorganisms, collectively known as the plant microbiome. Among these beneficial bacteria, members of the Pseudomonas genus are known for their plant-growth-promoting properties.
Two strains of the Pseudomonas species, known as E102 and E141, have been the subject of a study to examine how these bacteria interact with plants to promote growth under stress by a team of researchers led by KAUST’s Ikram Blilou, Ramona Marasco, and Daniele Daffonchio (the latter two are members of Applied Microbiology International).
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The effects of these strains, which were isolated from date palm roots, have been shown to promote drought tolerance and resistance to salinity in date palms and other plants.
Root hair changes
Using Arabidopsis as a host plant, the researchers found the presence of the bacteria caused increased root hair elongation and lateral root formation. Under normal conditions, these changes did not result in an increase in root and shoot biomass. However, during mild and severe salt stress, the bacteria were able to promote and protect plant growth. Shoot and root biomass increased significantly after exposure to increasing concentrations of bacterial cells. The stress-dependent effect of these plant-growth-promoting bacteria was also confirmed in lettuce plants grown under severe salt stress.
“This effect significantly increases with the concentration of bacteria used to treat the plant,” says Marasco.
“The changes we observed suggest that the bacteria are improving plant resistance under abiotic stress by increasing the effective root surface available for water and nutrient uptake. These changes give plants an adaptive advantage under stress conditions such as salinity,” she says.
The auxin effect
Many aspects of plant growth and development, including root hair elongation and lateral root development, are triggered and regulated by plant hormones known as auxins, the best known of which is indole-3-acetic acid (IAA). IAA plays a critical role in plant growth and development, regulating key processes such as cell elongation, division and differentiation — essential for controlling and modulating root development and architecture.
Many Pseudomonas strains have an ability to produce phytohormone-like molecules, such as IAA. Although their beneficial effects have often been linked to their IAA production, it remained unclear how the bacteria influence auxin signaling and transport and how these changes further contribute to plant growth.
This study showed that the effectiveness of the bacteria in changing root architecture depends on a functional auxin signaling pathway.
Signal interference
“While Arabidopsis mutants defective in the auxin signaling pathway are unable to develop root hairs when exposed to our bacteria, in wild-type plants, the IAA produced by the bacteria can interfere with the plant’s own IAA production,” Blilou explains.
“This interference enhances the activation of the plant-auxin responsive promoter (DR5), leading to alterations in the root system architecture that we observed. These effects are further facilitated by changes in IAA transport, which promote a more rapid redistribution and circulation of the hormone,” she says.
The findings provide a model illustrating how Pseudomonas bacteria can influence root development to promote growth and enhance the adaptation of plants under salinity stress.
“The modifications depend on the plant’s auxin machinery and confer an adaptive advantage to the plant, exclusively under stress conditions, such as mild and severe salinity,” says Daffonchio.
“The results help explain the responses of plants treated with plant-growth-promoting bacteria. In future, these microorganisms could be important in mitigating the negative effects of climate change in agriculture,” he observes.
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