Abstract
Environmental pollutants can generate stress in plants causing increased ethylene production that leads to the inhibition of plant growth. Ethylene production by the stressed plant may be lowered by Plant Growth-Promoting Bacteria (PGPB) that metabolizes the immediate precursor of ethylene 1-aminocyclopropane-1-carboxylate (ACC). Thus, engineering PGPB with ACC deaminase activity can be a promising alternative to mitigate the harmful effects of pollutants and thus enhance plant production. Here we show that the aromatics-degrading and metal-resistant Azoarcus sp. CIB behaves as a PGP-bacterium when colonizing rice as an endophyte, showing a 30% increment in plant weight compared to non-inoculated plants. The cloning and expression of an acdS gene led to a recombinant strain, Azoarcus sp. CIB (pSEVA237acdS), possessing significant ACC deaminase activity (6716 nmol mg−1 h−1), constituting the first PGPB of the Rhodocyclaceae family equipped with this PGP trait. The recombinant CIB strain acquired the ability to protect inoculated rice plants from the stress induced by cadmium (Cd) exposure and to increase the Cd concentration in rice seedlings. The observed decrease of the levels of reactive oxygen species levels in rice roots confirms such a protective effect. The broad-host-range pSEVA237acdS plasmid paves the way to engineer PGPB with ACC deaminase activity to improve the growth of plants that might face stress conditions.
Highlights
Ethylene is a gaseous plant hormone with essential functions in plant growth and development such as seed germination, cell expansion and elongation of stems and roots, the formation of adventitious roots and root hairs, fruit ripening, photomorphogenesis, and senescence in general [1,2,3,4,5,6]
CIB was done by biparental conjugation using the strain E. coli S17-1λpir as a donor, following a protocol previously established [44] with the following small modifications: donor cells were grown to A600 of 5, and the receptor CIB strain was grown on MC medium supplemented with pyruvate 0.2%, and concentrated to reach an A600 of 35
We investigated whether the CIB strain was able to effectively promote the growth of rice plants as endophyte
Summary
Ethylene is a gaseous plant hormone with essential functions in plant growth and development such as seed germination, cell expansion and elongation of stems and roots, the formation of adventitious roots and root hairs, fruit ripening, photomorphogenesis, and senescence in general [1,2,3,4,5,6]. A. olearius DQS-4 is closely related to strain BH72, which is capable of colonizing roots and aerial tissues of rice and green foxtail (Setaria viridis) and to promote their growth [23]. In addition to these three Azoarcus strains, we have characterized a related bacterium, Azoarcus sp. CIB is a PGP-bacterium and to increase its putative PGP abilities by the expression of an exogenous ACC deaminase, investigating the capability of the engineered CIB strain to enhance the growth of rice seedlings cultivated in Cd-polluted soils
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