Functional mutants of Azospirillum brasilense elicit beneficial physiological and metabolic responses in Zea mays contributing to increased host iron assimilation

Abstract

Iron (Fe), an essential element for plant growth, is abundant in soil but with low bioavailability. Thus, plants developed specialized mechanisms to sequester the element. Beneficial microbes have recently become a favored method to promote plant growth through increased uptake of essential micronutrients, like Fe, yet little is known of their mechanisms of action. Functional mutants of the epiphytic bacterium Azospirillum brasilense, a prolific grass-root colonizer, were used to examine mechanisms for promoting iron uptake in Zea mays. Mutants included HM053, FP10, and ipdC, which have varying capacities for biological nitrogen fixation and production of the plant hormone auxin. Using radioactive iron-59 tracing and inductively coupled plasma mass spectrometry, we documented significant differences in host uptake of Fe2+/3+ correlating with mutant biological function. Radioactive carbon-11, administered to plants as 11CO2, provided insights into shifts in host usage of ‘new’ carbon resources in the presence of these beneficial microbes. Of the mutants examined, HM053 exhibited the greatest influence on host Fe uptake with increased plant allocation of 11C-resources to roots where they were transformed and exuded as 11C-acidic substrates to aid in Fe-chelation, and increased C-11 partitioning into citric acid, nicotianamine and histidine to aid in the in situ translocation of Fe once assimilated.

Plant-Growth-Promoting Bacteria Can Impact Zinc Uptake in Zea mays: An Examination of the Mechanisms of Action Using Functional Mutants of Azospirillum brasilense

“…What distinguishes ipdC from the other microbial inoculants examined in this study is its deficiency in producing auxin (indole-3-acetic acid), an important plant hormone. Our past studies showed that the HM053 mutant exhibited the highest level of auxin biosynthesis, being 2 times that of FP10 and 13 times that of ipdC [ 16 ]. We know that auxin biosynthesis in plants and Zn levels are strongly correlated [ 30 , 31 , 32 ].…”

“…Images were only used qualitatively for determining spatial patterning of 65 Zn tracer in roots and shoots; hence no attempt was made to normalize image data. Comparative whole-plant radiographic images of 59 Fe 3+ and 59 Fe 2+ were also acquired from our prior work [ 16 ], but because of the faster decay rate of this radionuclide (t ½ 44.5 day), we only needed to expose these tissues for 16 h.…”

“…The 65 Zn uptake and allocation data from the present study and 59 Fe data from our prior work [ 16 ] were analyzed by Principal Component Analysis (PCA) using XLSTAT software version 2020.3 (Addinsoft Inc., New York, NY 10001, USA).…”

“…Plant-growth-promoting bacteria (PGPB), which can help their host weather difficult conditions and potentially increase nutritional value of crops, are finding increased use in agriculture. These organisms can activate physiological and biochemical responses within their host for mutual benefit to build natural tolerances to environmental stresses and thereby reduce losses in the field [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. Several have been identified as endophytes of grass species, including Azoarcus spp.…”

“…However, its general utility in plant biology is limited by the fact that many laboratories are not equipped with the appropriate nuclear instrumentation needed to make such measurements. Here, plants were inoculated with three different functional mutant strains of Azospirillum brasilense PGPB, including HM053, a Nif + constitutively expressed mutant of the nif gene coding for nitrogen fixation enzymes that fixes excess N 2 and excretes large amounts of ammonium into the rhizosphere; ipdC , a mutant strain disrupted in the ipdC gene thus impaired in biosynthesizing the plant’s relevant hormone auxin, indole-3-acetic acid [ 16 ]; and FP10, a Nif – mutant that is deficient in fixing N 2 , and also compared plant performance for assimilating 65 Zn relative to non-inoculated controls. These studies were conducted to determine whether these microbial functions had any influence on their host’s performance.…”