Importance of Poly-3-Hydroxybutyrate Metabolism to the Ability of Herbaspirillum seropedicae To Promote Plant Growth

Abstract

Herbaspirillum seropedicaeis an endophytic bacterium that establishes an association with a variety of plants, such as rice, corn, and sugarcane, and can significantly increase plant growth.H. seropedicaeproduces polyhydroxybutyrate (PHB), stored in the form of insoluble granules. Little information is available on the possible role of PHB in bacterial root colonization or in plant growth promotion. To investigate whether PHB is important for the association ofH. seropedicaewith plants, we inoculated roots ofSetaria viridiswithH. seropedicaestrain SmR1 and mutants defective in PHB production (ΔphaP1, ΔphaP1ΔphaP2, ΔphaC1, and ΔphaR) or mobilization (ΔphaZ1ΔphaZ2). The strains producing large amounts of PHB colonized roots, significantly increasing root area and the number of lateral roots compared to those of PHB-negative strains.H. seropedicaegrows under microaerobic conditions, which can be found in the rhizosphere. When grown under low-oxygen conditions, only the parental strain and ΔphaP2mutant exhibited normal growth. The lack of normal growth under low oxygen correlated with the inability to stimulate plant growth, although there was no effect on the level of root colonization. The data suggest that PHB is produced in the root rhizosphere and plays a role in maintaining normal metabolism under microaerobic conditions. To confirm this, we screened for green fluorescent protein (GFP) expression under the control of theH. seropedicaepromoters of the PHA synthase and PHA depolymerase genes in the rhizosphere. PHB synthesis is active on the root surface and later PHB depolymerase expression is activated.IMPORTANCEThe application of bacteria as plant growth promoters is a sustainable alternative to mitigate the use of chemical fertilization in agriculture, reducing negative economic and environmental impacts. Several plant growth-promoting bacteria synthesize and accumulate the intracellular polymer polyhydroxybutyrate (PHB). However, the role of PHB in plant-bacterium interactions is poorly understood. In this study, applying the C4 model grassSetaria viridisand several mutants in the PHB metabolism of the endophyteHerbaspirillum seropedicaeyielded new findings on the importance of PHB for bacterial colonization ofS. viridisroots. Taken together, the results show that deletion of genes involved in the synthesis and degradation of PHB reduced the ability of the bacteria to enhance plant growth but with little effect on overall root colonization. The data suggest that PHB metabolism likely plays an important role in supporting specific metabolic routes utilized by the bacteria to stimulate plant growth.

Defining the Genetic Basis of Plant–Endophytic Bacteria Interactions

“…The importance of PHB was confirmed by field experiments with maize and wheat in which PHA-rich Azospirillum cells were more efficient in increasing crop yield [70]. The latter effect was in accordance with the observation that H. seropedicae SmR1 mutants that were not able to produce PHB were less capable of promoting plant growth, even though the long-term epiphytic and endophytic colonisation were not affected [71].…”

3-Hydroxybutyrate Is Active Compound in Flax that Upregulates Genes Involved in DNA Methylation

“…but with little effect on overall root colonization. The data suggest that PHB metabolism likely plays an important role in supporting specific metabolic routes utilized by the bacteria to stimulate plant growth [23].…”

Are we there yet? The long walk towards the development of efficient symbiotic associations between nitrogen-fixing bacteria and non-leguminous crops

“…Polyhydroxybutyrate is a carbon storage polymer that can be mobilized under stressful physiological conditions, increasing the survival of bacteria in the soil. Indeed, recently, H. seropedicae strains overproducing polyhydroxybutyrate were shown to have better colonization fitness when compared to wild-type strains [172]. This highlights the importance of studies that integrate nitrogen fixation into bacteria with better plant colonization ability.…”