Abstract
Main conclusion
A comprehensive analysis of the role of brassinosteroids in nodulation, including their interactions with auxin and ethylene revealed that brassinosteroids inhibit infection, promote nodule initiation but do not influence nodule organogenesis or function.
Abstract
Nodulation, the symbiosis between legumes and rhizobial bacteria, is regulated by a suite of hormones including brassinosteroids. Previous studies have found that brassinosteroids promote nodule number by inhibiting ethylene biosynthesis. In this study, we examined the influence of brassinosteroids on the various stages of infection and nodule development. We utilise pea mutants, including brassinosteroid mutants lk, lka and lkb, the ethylene insensitive ein2 mutant and the lk ein2 double mutant, along with transgenic lines expressing the DR5::GUS auxin activity marker to investigate how brassinosteroids interact with ethylene and auxin during nodulation. We show that brassinosteroids inhibit the early stages of nodulation, including auxin accumulation, root hair deformation and infection thread formation, and demonstrate that infection thread formation is regulated by brassinosteroids in an ethylene independent manner. In contrast, brassinosteroids appear to act as promoters of nodule initiation through both an ethylene dependent and independent pathway. Although brassinosteroids positively influence the ultimate number of nodules formed, we found that brassinosteroid-deficiency did not influence nodule structure including the vascular pattern of auxin activity or nitrogen-fixation capacity. These findings suggest that brassinosteroids are negative regulators of infection but positive regulators of nodule initiation. Furthermore, brassinosteroids do not appear to be essential for nodule organogenesis or function. Given the influence of brassinosteroids on discreet stages of nodulation but not nodule function, manipulation of brassinosteroids may be an interesting avenue for future research on the optimisation of nodulation.
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Acknowledgements
We sincerely thank Dr. Sonali Roy and Dr. Phillip Poole (John Innes Centre) for the kind gift of lacZ-labelled Rhizobium leguminosarum and Prof Patricia Polowick (National Research Council of Canada) for the kind gift of the DR5::GUS labelled line. We thank Karen Velandia Prieto, Michelle Lang, Tracey Winterbottom and Valerie Hecht (University of Tasmania) for technical advice and assistance. E.F. was supported in part through Australian Research Council Future Fellowship FT140100770, the work carried out in Australia was funded through Australian Research Council Discovery project DP140101709; P.N.M was supported by a Tasmania Graduate Research Scholarship.
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Communicated by Soheil S Mahmoud.
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McGuiness, P.N., Reid, J.B. & Foo, E. Brassinosteroids play multiple roles in nodulation of pea via interactions with ethylene and auxin. Planta 252, 70 (2020). https://doi.org/10.1007/s00425-020-03478-z
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DOI: https://doi.org/10.1007/s00425-020-03478-z