The application of new agricultural technologies to attain sustainable production systems is necessary. The use of plant growth-promoting bacteria to improve plant growth and health has been studied for decades. This work aimed to isolate diazotrophic endophytic bacteria associated with sweet sorghum plants and study the interaction of their inoculation in combination with chemical N-fertilization on different sorghum cultivars. A bacterial collection of 181 isolates was constructed and characterized in vitro and in vivo. From that, the strains Enterobacter sp. UYSB89 and Kosakonia sp. UYSB139 were nifH+, produce IAA, defined as true endophytes and able to promote growth of two sweet sorghum under greenhouse conditions. The evaluated cultivars responded differentially to bacterial inoculation, the nitrogen fertilization doses and their interaction. Thus, plant growth is a multifactorial consequence of the interrelation between crop practices and the plant genotypes. This knowledge is a valuable factor in terms of understanding plant-bacteria endophyte interactions to preserve environmental sustainability during the implementation of agronomic practices.
NifA is the transcriptional activator of the nif genes in
Proteobacteria. It is usually regulated by nitrogen and oxygen, allowing biological
nitrogen fixation to occur under appropriate conditions. NifA proteins have a typical
three-domain structure, including a regulatory N-terminal GAF domain, which is
involved in control by fixed nitrogen and not strictly required for activity, a
catalytic AAA+ central domain, which catalyzes open complex formation, and a
C-terminal domain involved in DNA-binding. In Herbaspirillum
seropedicae, a β-proteobacterium capable of colonizing Graminae of
agricultural importance, NifA regulation by ammonium involves its N-terminal GAF
domain and the signal transduction protein GlnK. When the GAF domain is removed, the
protein can still activate nif genes transcription; however,
ammonium regulation is lost. In this work, we generated eight constructs resulting in
point mutations in H. seropedicae NifA and analyzed their effect on
nifH transcription in Escherichia coli and
H. seropedicae. Mutations K22V, T160E, M161V, L172R, and A215D
resulted in inactive proteins. Mutations Q216I and S220I produced partially active
proteins with activity control similar to wild-type NifA. However, mutation G25E,
located in the GAF domain, resulted in an active protein that did not require GlnK
for activity and was partially sensitive to ammonium. This suggested that G25E may
affect the negative interaction between the N-terminal GAF domain and the catalytic
central domain under high ammonium concentrations, thus rendering the protein
constitutively active, or that G25E could lead to a conformational change comparable
with that when GlnK interacts with the GAF domain.
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