The Role of Genetic Variation in Maize Response To Beneficial Endophytes

Growth-promoting endophytes have great potential to boost crop production and sustainability. There is, however, a lack of research on how differences in the plant host affect an endophyte’s ability to promote growth. We set out to quantify how different maize genotypes respond to specific growth-promoting endophytes. We inoculated genetically diverse maize lines with three different known beneficial endophytes: Herbaspirillum seropedicae (a gram-negative bacteria), Burkholderia WP9 (a gram-negative bacteria), and Serendipita vermifera Subsp. bescii (a Basidiomycota fungus). Maize seedlings were grown for 3 weeks under controlled conditions in the greenhouse and assessed for various growth promotion phenotypes. We found Herbaspirillum seropedicae to increase chlorophyll content, plant height, root length, and root volume significantly in different maize genotypes, while Burkholderia WP9 did not significantly promote growth in any lines under these conditions. Serendipita bescii significantly increased root and shoot mass for 4 maize genotypes, and growth promotion correlated with measured fungal abundance. Although plant genetic variation by itself had a strong effect on phenotype, its interaction with the different endophytes was weak, and the endophytes rarely produced consistent effects across different genotypes. This genome-by-genome interaction indicates that the relationship between a plant host and beneficial endophytes is complex, and it may partly explain why many microbe-based growth stimulants fail to translate from laboratory settings to the field. Detangling these interactions will provide a ripe area for future studies to understand how to best harness beneficial endophytes for agriculture.

Modulation of the Wheat Seed-Borne Bacterial Community by Herbaspirillum seropedicae RAM10 and Its Potential Effects for Tryptophan Metabolism in the Root Endosphere

Plants and their associated microbiota share ecological and evolutionary traits that are considered to be inseparably woven. Their coexistence foresees the use of similar metabolic pathways, leading to the generation of molecules that can cross-regulate each other’s metabolism and ultimately influence plant phenotype. However, the extent to which the microbiota contributes to the overall plant metabolic landscape remains largely unexplored. Due to their early presence in the seed, seed-borne endophytic bacteria can intimately colonize the plant’s endosphere while conferring a series of phytobeneficial services to their host. Understanding the dynamics of these endophytic communities is a crucial step toward the formulation of microbial inoculants that can modulate the functionality of the plant-associated microbiota for improved plant fitness. In this work, wheat (Triticum aestivum) roots non-inoculated and inoculated with the bacterium Herbaspirillum seropedicae strain RAM10 were analyzed to explore the impact of inoculant–endophyte–wheat interrelationships on the regulation of tryptophan (Trp) metabolism in the endosphere environment. Root inoculation with H. seropedicae led to phylum-specific changes in the cultivable seed-borne endophytic community. This modulation shifted the metabolic potential of the community in light of its capacity to modulate the levels of key Trp-related metabolites involved in both indole-3-acetic acid (IAA) biosynthesis and in the kynurenine pathway. Our results support a mode of action of H. seropedicae relying on a shift in both the composition and functionality of the seed-borne endophytic community, which may govern important processes such as root growth. We finally provide a conceptual framework illustrating that interactions among roots, inoculants, and seed-borne endophytes are critical to fine-tuning the levels of IAA in the endosphere. Understanding the outcomes of these interactions is a crucial step toward the formulation of microbial inoculants based on their joint action with seed-borne endophytic communities to promote crop growth and health in a sustainable manner.

Inoculação e reinoculação de Herbaspirillum seropedicae em duas variedades de arroz irrigado

O estudo consistiu em avaliar o efeito da inoculação e reinoculação da estirpe ZAE94 de Herbaspirillum seropedicae em duas variedades de arroz. A eficiência foi avaliada pela contribuição destas bactérias nos parâmetros agronômicos de produção de grãos, N%, N-total e proteína bruta dos grãos nas variedades de arroz IR42 e IAC4440. O experimento de inoculação foi conduzido em blocos ao acaso com seis repetições. Para o experimento de reinoculação, as variáveis avaliadas foram semelhantes ao experimento anterior, sendo que a parte aérea das plantas de arroz remanescentes foi retirada e o restante das plantas foi incorporado ao solo. A inoculação da estirpe ZAE 94 foi capaz de suprir até 50 kg de N ha-1 dependendo da cultivar utilizada. Não foi observado efeito da reinoculação sobre os parâmetros estudados nas condições testadas, indicando que a inoculação deve ser feita em todos os plantios. Os resultados mostraram-se promissores quanto à utilização da prática de inoculação na cultura do arroz.

Inoculation of Herbaspirillum Seropedicae Increases Biomass in Maize Roots in the Early Stages of Plant Development

Herbaspirillum seropedicae is a plant growth-promoting bacteria isolated from diverse plant species. In this work, the main objective was to investigate the efficiency of H. seropedicae strain SmR1 in colonizing and increasing maize growth in the early stages of development under greenhouse conditions. Inoculation with H. seropedicae resulted in 10.51 and 19.43% in mean of increase of root biomass concerning non-inoculated controls, mainly in the initial stages of plant development, at 21 days after emergence (DAE). Quantification of H. seropedicae in roots and leaves was performed by quantitative PCR.. H. seropedicae was detected only in maize inoculated roots by qPCR, and a slight decrease in DNA copy number g−1 of fresh root weight was observed from 7 to 21 DAE, suggesting that there was initial effective colonization on maize plants. H. seropedicae strain SmR1 efficiently increased maize root biomass exhibiting its potencial to be used as inoculant in agricultures systems.

The Role of Genetic Variation in Maize Response to Beneficial Endophytes

Growth-promoting endophytes have great potential to boost crop production and sustainability. There is, however, a lack of research on how differences in the plant host affect an endophyte's ability to promote growth. We set out to quantify how different maize genotypes respond to specific growth-promoting endophytes. We inoculated genetically diverse maize lines with three different known beneficial endophytes: Herbaspirillum seropedicae (a gram-negative bacteria), Burkholderia WP9 (a gram-negative bacteria), and Serendipita vermifera Subsp. bescii (a Basidiomycota fungus). Maize seedlings were grown for 3 weeks under controlled conditions in the greenhouse and assessed for various growth promotion phenotypes. We found Herbaspirillum seropedicae to increase chlorophyll content, plant height, root length, and root volume significantly in different maize genotypes, while Burkholderia WP9 did not significantly promote growth in any lines under these conditions. Serendipita bescii significantly increased root and shoot mass for 4 maize genotypes, and growth promotion correlated with measured fungal abundance. Although plant genetic variation by itself had a strong effect on phenotype, its interaction with the different endophytes was weak, and the endophytes rarely produced consistent effects across different genotypes. This genome-by-genome interaction indicates that the relationship between a plant host and beneficial endophytes is complex, and it may partly explain why many microbe-based growth stimulants fail to translate from laboratory settings to the field. Detangling these interactions will provide a ripe area for future studies to understand how to best harness beneficial endophytes for agriculture.

Examining the Effects of the Nitrogen Environment on Growth and N2-Fixation of Endophytic Herbaspirillum seropedicae in Maize Seedlings by Applying 11C Radiotracing

Herbaspirillum seropedicae, as an endophyte and prolific root colonizer of numerous cereal crops, occupies an important ecological niche in agriculture because of its ability to promote plant growth and potentially improve crop yield. More importantly, there exists the untapped potential to harness its ability, as a diazotroph, to fix atmospheric N2 as an alternative nitrogen resource to synthetic fertilizers. While mechanisms for plant growth promotion remain controversial, especially in cereal crops, one irrefutable fact is these microorganisms rely heavily on plant-borne carbon as their main energy source in support of their own growth and biological functions. Biological nitrogen fixation (BNF), a microbial function that is reliant on nitrogenase enzyme activity, is extremely sensitive to the localized nitrogen environment of the microorganism. However, whether internal root colonization can serve to shield the microorganisms and de-sensitize nitrogenase activity to changes in the soil nitrogen status remains unanswered. We used RAM10, a GFP-reporting strain of H. seropedicae, and administered radioactive 11CO2 tracer to intact 3-week-old maize leaves and followed 11C-photosynthates to sites within intact roots where actively fluorescing microbial colonies assimilated the tracer. We examined the influence of administering either 1 mM or 10 mM nitrate during plant growth on microbial demands for plant-borne 11C. Nitrogenase activity was also examined under the same growth conditions using the acetylene reduction assay. We found that plant growth under low nitrate resulted in higher nitrogenase activity as well as higher microbial demands for plant-borne carbon than plant growth under high nitrate. However, carbon availability was significantly diminished under low nitrate growth due to reduced host CO2 fixation and reduced allocation of carbon resources to the roots. This response of the host caused significant inhibition of microbial growth. In summary, internal root colonization did little to shield these endophytic microorganisms from the nitrogen environment.

Efeito de bactérias promotoras de crescimento na alface crespa cultivar Veneranda

Diversos grupos de bactérias são capazes de se associar com plantas, sendo essas interações patogênicas ou prejudiciais ao tecido vegetal, ou interações benéficas que se baseiam na troca de favores entre bactéria e planta. Entre as associações benéficas destacam-se as bactérias associativas, capazes de associar às plantas de diferentes formas e com isso, estimular o desenvolvimento vegetal. As bactérias diazotróficas além de fixar nitrogênio atmosférico, produzem fitohormônios que contribuem para o crescimento e desenvolvimento das plantas. Contudo, o objetivo deste trabalho foi avaliar a resposta da alface crespa quanto a eficácia das bactérias promotoras de crescimento vegetal (BPCP). O experimento foi instalado em casa de vegetação no Polo Tecnológico Agroalimentar, município de Arapiraca/AL. A cultivar da alface crespa (Lactuca sativa L.) e as BPCP utilizadas foram Veneranda, Herbaspirillum seropedicae (BR 11175) e Glucanoacetobacter diazotrophicus (BR 11281), respectivamente. O delineamento experimental foi inteiramente casualizado (DIC) com quatro tratamentos: alface + solo (TA), alface + substrato (TS), alface + BR 11175 + solo e alface + BR 11175 + BR 11281 + solo, com 3 repetições. As variáveis analisadas no experimento foram: biomassa verde das folhas (BVF), biomassa verde do caule (BVC), biomassa verde da raiz (BVR), biomassa verde total (BVT), altura da planta (AP), comprimento do caule (CC), comprimento da raiz (CR) e número de folhas (NF). Nos resultados constatou-se que as BPCP isolada e em dupla inoculação inibiram a altura da planta, a biomassa verde do caule e a biomassa verde da raiz da alface crespa.

Transcriptional Responses of Herbaspirillum seropedicae to Environmental Phosphate Concentration

Herbaspirillum seropedicae is a nitrogen-fixing endophytic bacterium associated with important cereal crops, which promotes plant growth, increasing their productivity. The understanding of the physiological responses of this bacterium to different concentrations of prevailing nutrients as phosphate (Pi) is scarce. In some bacteria, culture media Pi concentration modulates the levels of intracellular polyphosphate (polyP), modifying their cellular fitness. Here, global changes of H. seropedicae SmR1 were evaluated in response to environmental Pi concentrations, based on differential intracellular polyP levels. Cells grown in high-Pi medium (50 mM) maintained high polyP levels in stationary phase, while those grown in sufficient Pi medium (5 mM) degraded it. Through a RNA-seq approach, comparison of transcriptional profiles of H. seropedicae cultures revealed that 670 genes were differentially expressed between both Pi growth conditions, with 57% repressed and 43% induced in the high Pi condition. Molecular and physiological analyses revealed that aspects related to Pi metabolism, biosynthesis of flagella and chemotaxis, energy production, and polyhydroxybutyrate metabolism were induced in the high-Pi condition, while those involved in adhesion and stress response were repressed. The present study demonstrated that variations in environmental Pi concentration affect H. seropedicae traits related to survival and other important physiological characteristics. Since environmental conditions can influence the effectiveness of the plant growth-promoting bacteria, enhancement of bacterial robustness to withstand different stressful situations is an interesting challenge. The obtained data could serve not only to understand the bacterial behavior in respect to changes in rhizospheric Pi gradients but also as a base to design strategies to improve different bacterial features focusing on biotechnological and/or agricultural purposes.