Background: Polyamines (PAs) are oxidatively deaminated at their primary or secondary amino-groups by copper-containing amine oxidases (CuAOs) or FAD-dependent amine oxidases (PAOs), respectively. Both enzymes have long been considered to be apoplastic proteins. However, three out of five PAO isoforms in Arabidopsis thaliana are localized in peroxisomes, while the other two PAOs are predicted to be cytosolic. Interestingly, most of these PAOs do not contribute to terminal PA oxidation, but instead are involved in the back-conversion pathway, producing spermidine from spermine and putrescine from spermidine, which in turn is inhibited by putrescine. This opens the question as to whether PAs are catabolized in the apoplast of Arabidopsis and if the terminal oxidation occurs in the peroxisomes. The main objective of this study was to know if these catabolic processes are mediated by CuAOs. Results: A. thaliana contains ten genes annotated as CuAOs, but only one (ATAO1) has been characterized at the protein level. Reported herein is the characterization of three genes encoding putative Arabidopsis CuAOs (AtCuAO1, AtCuAO2 and AtCuAO3). These genes encode functional CuAOs that use putrescine and spermidine as substrates. AtCuAO1, like ATAO1, is an extracellular protein, while AtCuAO2 and AtCuAO3 are localized in peroxisomes. The three genes present a different expression profile in response to exogenous treatments, such as application of abcisic acid, methyl jasmonate, salycilic acid, flagellin 22 and wounding. Conclusions: PA catabolism in the Arabidopsis apoplast is mediated predominantly by CuAOs, while in peroxisomes the co-localization of CuAO-dependent terminal catabolism with PAO-back-conversion machineries might contribute to modulating putrescine-mediated inhibition of the back-conversion, suggesting the occurrence of a tight coordination between both catabolic pathways. The expression profile of AtCuAO1-3 in response to different exogenous treatments, together with the different localization of the corresponding proteins, provides evidence for the functional diversification of Arabidopsis CuAO proteins.

The potential of plant growth-promoting rhizobacteria (PGPR) to improve plant growth and nutrient acquisition has received increased attention. This study investigated the synergistic effects of combining PGPR Paraburkholderia sp. SOS3 with the addition of inorganic N fertilizer (iN) or a combined application of inorganic N with organic manure-based fertilizer (iNoN) at low and high levels of N fertilization on Macadamia integrifolia seedling growth. We studied plant growth, mineral N in soil-leachate, and media physicochemical and microbial characteristics. Growth of seedlings fertilized with iNoN at high N level (iNoN 100) was enhanced by PGPR inoculation, with an increase of 32% in total biomass and 43% in N uptake, compared with uninoculated seedlings. No significant PGPR effect was observed on growth under low or high inorganic N treatments but PGPR significantly reduced N leaching after 3 weeks of fertilization. We found a positive relationship between media and plant δ¹⁵N and plant N uptake, and a strong increase in microbial-biomass N under the most productive fertilization treatment (iNoN 100 with PGPR), compared with the other N treatments (without or with PGPR). The results suggest that PGPR improves N acquisition by reducing mineral N loss and increasing plant N availability, but that these effects depend on the N form and N level.

Streptomyces species can form beneficial relationships with hosts as endophytes, including the phytopathogen-inhibiting strain, Streptomyces griseoaurantiacusMH191, isolated from wheat plants. Using genomic characterization and untargeted metabolomics, we explored the capacity of strain MH191 to inhibit a range of fungal phytopathogens through the production of secondary metabolites. Complete genome assembly of strain MH191 predicted 24 biosynthetic gene clusters. Secondary metabolite production was assessed following culture on six different media, with the detection of 205 putative compounds. Members of the manumycin family, undecylprodigiosin, and desferrioxamine were identified as the predominant metabolites. Antifungal activity was validated for undecylprodigiosin and manumycin. These compounds were produced from different BGCs, which showed similarity to asukamycin, undecylprodigiosin, and FW0622 gene clusters, respectively. The growth of strain MH191 on different media illustrated the metabolic regulation of these gene clusters and the strain's extended chemical potential, with the asukamycin gene cluster alone, producing a variety of antifungal metabolites. The study highlights the extended chemical capability of strain MH191, which could be exploited as a biological control agent for designing future crop protection solutions.