Functional and Systems Biology
A Plant Hormone Produced by a Bacterial Species Causes Different Growth Patterns
Scientists identify superior method for assessing the ability of bacteria to cause an increase in lateral root numbers and root hairs.
Scientists identified a superior method for determining whether a bacterial species can cause an auxin root phenotype in Arabidopsis, a model land plant used in the laboratory, compared to traditional methods. Arabidopsis is inoculated with the auxin-producing microbacterium strain RU1A derived from an aquatic plant called duckweed. (Illustration by Maegan Murray | EMSL)
Science
Auxins comprise a class of plant hormones that play a crucial role in the physical development of plants. The role of auxins in plant–microbe interactions has been studied primarily using indole-3 acetic acid (IAA)-producing bacteria, which promotes plant growth. However, the bacterial biosynthetic pathway to produce IAA involves indole-related compounds (IRCs) with lesser-known functions. Through this study, scientists sought to understand changes in plant response to multiple plant-associated bacterial strains that differ in their ability to produce IRCs in a test tube. By combining analytical chemistry with bioassays to evaluate auxin performance, they identified liquid chromatography-mass spectrometry (LC-MS) as a far superior method compared to the traditional Salkowski assay for assessing the ability of bacteria to cause an increase in lateral root number and root hairs, also known as an auxin root phenotype. Additionally, they identified that when the plant gene known as AXR1 is mutated, one particular auxin-producing bacterial strain no longer colonizes within the plant tissue through the stomata.
The Impact
By determining how, when, and where bacteria produce plant hormones when interacting with plants, scientists can better understand how plants adapt to different environments. This information also helps develop methods to maximize the growth of plants to serve different purposes. Different root lengths, for example, determine the plant’s ability to absorb nutrients and other components. Altering plant morphology can help produce more biomass for its conversion to make biofuels. The team’s findings as they pertain to the AXR1 gene’s interaction with specific bacterial strains may also open many new areas of research to understand how some IAA-producing bacteria communicates with various parts of the plant, affecting the phenotype exhibited.
Summary
Through a previous project, scientists studied 47 bacterial strains isolated from several duckweed species, where they found that 79 percent produced IRCs in culture. In that study, they examined how bacteria produce phytohormones to generate different traits that are observed in the plant. They screened the collection of microbes to see if they are highly represented in producing auxin. Through this new study, they performed assays on a subset of 21 strains to evaluate the plant’s growth responses and mode of bacterial colonization. Of those strains, only four high-quantity IAA-producing bacteria strains caused an auxin root phenotype. When compared with results from the traditionally used colorimetric Salkowski assay, auxin concentration determined by LC-MS was a superior indicator of a bacteria’s ability to cause an auxin root phenotype by displaying a lower number of false positives. Use of axr1-3 mutant plants uncovered a dependence of a bacteria known as Azospirillum baldaniorum Sp245 on gene AXR1 for colonization inside the plant tissue. With wild-type plants, the pores on the leaf surface known as stomata, which are well-known to be a first line of defense when a plant detects a pathogen, are full of the Sp245 bacteria. When the gene is mutated, the bacteria is no longer able to target the stomata and instead can only colonize the leaf surface.
Contacts
Eric Lam, Rutgers University, ericl89@hotmail.com
Sarah Lebeis, University of Tennessee, lebeis.sarah@gmail.com
Sarah Gilbert, University of North Carolina at Chapel Hill, sarahg19@email.unc.edu
Funding
Duckweed research at the Lam Laboratory is provided by a Hatch project from the New Jersey Agricultural Experiment Station at Rutgers University and in part by the Department of Energy, Office of Science, Biological and Environmental Research program. A portion of this research was performed under the Facilities Integrating Collaborations for User Science (FICUS) program.
Publication
Gilbert, Sarah, Alexander Poulev, William Chrisler, Kenneth Acosta, Galya Orr, Sarah Lebeis, and Eric Lam. 2022. "Auxin-Producing Bacteria from Duckweeds Have Different Colonization Patterns and Effects on Plant Morphology." Plants 11, no. 6: 721. https://doi.org/10.3390/plants11060721