Widespread production of plant growth-promoting hormones among marine bacteria and their impacts on the growth of a marine diatom

• Published in: Microbiome 2024-10, Vol.12 (1), p.205-14, Article 205
• Main Authors: Khalil, Abeeha, Bramucci, Anna R, Focardi, Amaranta, Le Reun, Nine, Willams, Nathan L R, Kuzhiumparambil, Unnikrishnan, Raina, Jean-Baptiste, Seymour, Justin R
• Format: Article
• Language: English
• Subjects: Acetic acid; Aquatic Organisms - growth & development; Aquatic Organisms - metabolism; Bacteria; Bacteria - classification; Bacteria - genetics; Bacteria - metabolism; Bacteria-phytoplankton interactions; Cytokinins; Diatoms - growth & development; Diatoms - metabolism; Gibberellins; Indoleacetic Acids - metabolism; Marine bacteria; Marine diatom; Metabolites; Organic acids; Physiological aspects; Phytoplankton - growth & development; Phytoplankton - metabolism; Plant growth promoting hormones; Plant Growth Regulators - metabolism; Seawater - microbiology; Symbiosis
• ISSN: 2049-2618; 2049-2618
• DOI: 10.1186/s40168-024-01899-6

Reciprocal exchanges of metabolites between phytoplankton and bacteria influence the fitness of these microorganisms which ultimately shapes the productivity of marine ecosystems. Recent evidence suggests that plant growth-promoting hormones may be key metabolites within mutualistic phytoplankton-bacteria partnerships, but very little is known about the diversity of plant growth-promoting hormones produced by marine bacteria and their specific effects on phytoplankton growth. Here, we aimed to investigate the capacity of marine bacteria to produce 7 plant growth-promoting hormones and the effects of these hormones on Actinocyclus sp. growth. We examined the plant growth-promoting hormone synthesis capabilities of 14 bacterial strains that enhance the growth of the common diatom Actinocyclus. Plant growth-promoting hormone biosynthesis was ubiquitous among the bacteria tested. Indeed all 14 strains displayed the genomic potential to synthesise multiple hormones, and mass-spectrometry confirmed that each strain produced at least 6 out of the 7 tested plant growth-promoting hormones. Some of the plant growth-promoting hormones identified here, such as brassinolide and trans-zeatin, have never been reported in marine microorganisms. Importantly, all strains produced the hormone indole-3 acetic acid (IAA) in high concentrations and released it into their surroundings. Furthermore, indole-3 acetic acid extracellular concentrations were positively correlated with the ability of each strain to promote Actinocyclus growth. When inoculated with axenic Actinocyclus cultures, only indole-3 acetic acid and gibberellic acid enhanced the growth of the diatom, with cultures exposed to indole-3 acetic acid exhibiting a two-fold increase in cell numbers. Our results reveal that marine bacteria produce a much broader range of plant growth-promoting hormones than previously suspected and that some of these compounds enhance the growth of a marine diatom. These findings suggest plant growth-promoting hormones play a large role in microbial communication and broaden our knowledge of their fuctions in the marine environment. Video Abstract.