Abstract
Plant hormones play key roles in nodulation and arbuscular mycorrhizal (AM) associations. These two agriculturally and ecologically important symbioses enable plants to gain access to nutrients, in particular, nitrogen in the case of nodulation and phosphorous in the case of AM. Work over the past few decades has revealed how symbioses with nitrogen-fixing rhizobia, restricted almost exclusively to legumes, evolved in part from ancient AM symbioses formed by more than 80% of land plants. Although overlapping, these symbiotic programs also have important differences, including the de novo development of a new organ found only in nodulation. One emerging area of research is the role of two plant hormone groups, the gibberellins (GAs) and brassinosteroids (BRs), in the development and maintenance of these symbioses. In this review, we compare and contrast the roles of these hormones in the two symbioses, including potential interactions with other hormones. This not only focuses on legumes, most of which can host both symbionts, but also examines the role of these in AM development in non-legumes. GA acts by suppressing DELLA, and this regulatory module acts to negatively influence both rhizobial and mycorrhizal infection but appears to promote nodule organogenesis. While an overall positive role for BRs in nodulation and AM has been suggested by studies using mutants disrupted in BR biosynthesis or response, application studies indicate that BR may play a more complex role in nodulation. Given the nature of these symbioses, with events regulated both spatially and temporally, future studies should examine in more detail how GAs and BRs may influence precise events during these symbioses, including interactions with other hormone groups.
Highlights
While the application of fertilizer to crop plants may compensate for poor nutrient availability in the soil, there may be negative impacts such as the use of non-renewable resources, cost, enhancing the growth of weeds, or runoff into waterways which may cause eutrophication
Arbuscular mycorrhizal symbioses can increase plant growth and yield in soils poor in essential nutrients, phosphorus (Baum et al, 2015; Yang et al, 2015). Another important plant microbe symbiosis almost completely restricted to legumes is nodulation (Mus et al, 2016), where nitrogenfixing rhizobial bacteria are housed in nodules in the root system
This review focuses on how two families of plant growth hormones, the gibberellins (GAs) and brassinosteroids (BR), influence the development of rhizobial and arbuscular mycorrhizal (AM) symbioses
Summary
Work over the past few decades has revealed how symbioses with nitrogen-fixing rhizobia, restricted almost exclusively to legumes, evolved in part from ancient AM symbioses formed by more than 80% of land plants. Overlapping, these symbiotic programs have important differences, including the de novo development of a new organ found only in nodulation. We compare and contrast the roles of these hormones in the two symbioses, including potential interactions with other hormones This focuses on legumes, most of which can host both symbionts, and examines the role of these in AM development in non-legumes.
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