ABSTRACT Acacia mearnsii is an Australian native tree species commercially grown in Africa and South America that produces tannins and woodchips. Its seed size varies, which may impact the germination and initial growth of seedlings. In its turn, Azospirillum brasilense inoculation is known to improve the growth of several crops. However, there are few studies the effects of A. brasilense inoculation on the germination and initial growth of forestry species. The present study evaluated the influence of seed size and A. brasilense Ab-V5 and Ab-V6 inoculation on germination and initial growth of A. mearnsii. Small seeds reduced the mean germination time (MGT) by 5.2% and the time to 50% germination (T50) by 23.8% in comparison to large seeds. On the other hand, seedlings that originated from large seeds had a seedling vigor index (SVI) 18.6% higher than those that originated from small seeds. Azospirillum brasilense inoculation increased the germination percentage by 20.9%, germination speed by 91.8%, root length (RL) by 35.8%, and root fresh weight by 20.5%. Additionally, it decreased MGT by 8.2% and T50 by 37.6%. The combination of large seeds and A. brasilense inoculation increased RL by approximately 50% and SVI by 62.1%. Thus, the use of A. brasilense inoculation favors germination and seedling vigor in A. mearnsii, showing to be a promising strategy for obtaining more uniform seedlings in forest nurseries.

RESUMO Acacia mearnsii é uma espécie arbórea australiana cultivada comercialmente na África e na América do Sul para produção de taninos e madeira. A espécie apresenta tamanho variável de sementes, o que pode impactar a germinação e o crescimento inicial das mudas. Já a inoculação com Azospirillum brasilense é conhecida por melhorar o crescimento de diversas culturas. Entretanto, existem poucos estudos sobre os efeitos da inoculação de A. brasilense na germinação e no crescimento inicial de espécies florestais. O presente estudo avaliou a influência do tamanho das sementes e da inoculação de A. brasilense Ab-V5 e Ab-V6 na germinação e no crescimento inicial de A. mearnsii. Sementes pequenas reduziram o tempo médio de germinação (TMG) em 5,2% e o tempo para 50% de germinação (T50) em 23,8% em comparação às sementes grandes. Por outro lado, as mudas originadas de sementes grandes apresentaram índice de velocidade de germinação (IVG) 18,6% superior às originadas de sementes pequenas. A inoculação com Azospirillum brasilense aumentou a porcentagem de germinação em 20,9%, a velocidade de germinação em 91,8%, o comprimento da raízes (CR) em 35,8% e a massa fresca da raízes em 20,5%. Além disso, diminuiu o TMG em 8,2% e o T50 em 37,6%. A combinação de sementes grandes e de inoculação com A. brasilense aumentou o CR em cerca de 50% e o IVG em 62,1%. Assim, o uso da inoculação de A. brasilense favorece a germinação e o vigor de plântulas de A. mearnsii, mostrando-se uma estratégia promissora para obtenção de mudas mais uniformes em viveiros florestais.

Abstract The taxonomy of Burkholderia sensu lato (s.l.) has been revisited using genome-based tools, which have helped differentiate closely related species. Many species from this group are indistinguishable through phenotypic traits and 16S rRNA gene sequence analysis. Furthermore, they also exhibit whole-genome Average Nucleotide Identity (ANI) values in the twilight zone for species circumscription (95-96%), which may impair their correct classification. In this work, we provided an updated Burkholderia s.l. taxonomy focusing on closely related species and give other recommendations for those developing genome-based taxonomy studies. We showed that a combination of ANI and digital DNA-DNA hybridization (dDDH) applying the universal cutoff values of 95% and 70%, respectively, successfully discriminates Burkholderia s.l. species. Using genome metrics with this pragmatic criterion, we demonstrated that i) Paraburkholderia insulsa should be considered a later heterotypic synonym of Paraburkholderia fungorum; ii) Paraburkholderia steynii differs from P. terrae by harboring symbiotic genes; iii) some Paraburkholderia are indeed different species based on dDDH values, albeit sharing ANI values close to 95%; iv) some Burkholderia s.l. indeed represent new species from the genomic viewpoint; iv) some genome sequences should be evaluated with care due to quality concerns.

ABSTRACT The nature of the soil is shaped by the presence of roots and the soil microbiota. Bacteria, archaea and fungi profoundly alter plant growth and, in turn, depend on root exudation of carbon-rich and energy-yielding compounds. The microbial communities act as facilitators of plant processes by secreting plant growth regulators, solubilizing minerals, providing N for plant growth, altering plant immune responses and competing with plant pathogens. Characterizing and engineering the processes driven by the multiple microbial taxa that make up a “plant growth-promoting soil” represents an ecologically friendly solution that may lead to unprecedented increases in agronomic efficiency. This review outlines the characteristics of soil-plant-microbiota interactions that would lead to enhanced plant growth and the importance of characterizing the soil microbial communities with metabarcoding and shotgun metagenomics allied to machine learning analytics. Although much is still needed to be understood about soil microbial ecology, it is possible to choose the best soil management practices to take advantage of beneficial microbial activity with our current knowledge.