Common and divergent roles of plant hormones in nodulation and arbuscular mycorrhizal symbioses

Abstract

All of the classical plant hormones have been suggested to influence nodulation, including some that interact with the Autoregulation of Nodulation (AON) pathway. Leguminous plants strictly regulate the number of nodules formed through this AON pathway via a root-shoot-root loop that acts to suppress excessive nodulation. A related pathway, the Autoregulation of Mycorrhization (AOM) pathway controls the more ancient, arbuscular mycorrhizal (AM) symbiosis. A comparison of the published responses to the classical hormones in these 2 symbioses shows that most influence the symbioses in the same direction. This may be expected if they affect the symbioses via common components of these symbiotic regulatory pathways. However, some hormones influence these symbioses in opposite directions, suggesting a more complex relationship, and probably one that is not via the common components of these pathways. In a recent paper we showed, using a genetic approach, that strigolactones and brassinosteroids do not act downstream of the AON genes examined and argued that they probably act independently to promote nodule formation. Recently it has been shown that the control of nodulation via the AON pathway involves mobile CLE peptide signals. It is therefore suggested that a more direct avenue to determine if the classical hormones play a direct role in the autoregulatory pathways is to further examine whether CLE peptides and other components of these processes can influence, or be influenced by, the classical hormones. Such studies and other comparisons between the nodulation and mycorrhizal symbioses should allow the role of the classical hormones in these critical symbioses to be rapidly advanced.

The legume-rhizobia symbiosis is thought to have evolved ~60 mya, in part by adapting key elements from the more ancient arbuscular mycorrhizal (AM) symbiosis that is estimated to have originated over 440 mya.^1,2 Both processes play important roles in nutrient acquisition for the host, with rhizobia bacteria providing the plant with nitrogen, and mycorrhizal fungi providing nutrients including phosphorus. Common molecular elements are involved in the development and regulation of both symbioses. This includes the host Autoregulation of Nodulation (AON) and Autoregulation of Mycorrhization (AOM) pathways that control the extent of nodulation and mycorrhization, respectively.^3,4

Classical plant hormones play crucial roles in the nodulation and mycorrhization processes.^5-7 Some fundamental effects of these hormones on the gross level of colonisation are compared for the 2 symbioses in Table 1. In many cases, the hormone’s role is comparable, acting in a similar direction to positively or negatively direct the process. This is consistent with central mechanisms of the nodulation program having been derived in part from the developmental and regulatory systems of AM formation. Of particular interest are the hormones that act in a different manner. One such example is gibberellin (Table 1), which has been shown through genetic studies to have a positive role in nodulation,^13-15 but a negative role in AM formation.^6,16,17 Genes in the gibberellin biosynthetic pathway are upregulated during both the nodulation (e.g.^15,48) and AM (e.g.^49,50) processes, and this appears to occur both spatially and temporally during nodule formation.⁵¹ The regulation of these genes during AM colonisation was shown to be dependent on CCaMK,⁵⁰ an essential factor shared by both the nodulation and AM systems. These results confirm that gibberellins are actively regulated during these 2 processes even though the genetic evidence shows they affect the processes in opposite directions.

Table 1. A comparison of the influence of classical plant hormones on nodulation and arbuscular mycorrhizal development.

Hormone	Nodulation	Key references	Arbuscular mycorrhizae	Key references
Auxin	+	8 - 10	+	11 , 12
Gibberellin	+ (optimal range)	13 - 15	-	6 , 16 , 17
Ethylene	-	18 , 19	-	20 - 22
Brassinosteroid	+ (optimal range)	13 , 23	nr/+	6 , 24
Strigolactone	+	25 - 27	+	26 , 28 - 30
Cytokinin	+	31 - 34	nr	F. Frugier and P. Bonfante, pers. comm.
Abscisic acid	+/−	35 - 37	+	38 , 39
Jasmonic acid	+/−	40 , 41	+/−	42 , 43
Salicylic acid	-	44 - 46	-	47

Hormones act at various points in these developmental pathways and differences have been reported between species so the affects noted need to be qualified. Recent reviews for the role of hormones in nodulation^5,7 and mycorrhizal development⁶ should be consulted for specific affects. Hormones that positively influence colonisation are indicated by (+; those that do so only within an optimal range of hormone level are indicated), those that negatively effect colonisation are indicated by a (-), those where there is evidence in both directions (such as at different developmental stages or using different experimental systems) are indicated with (+/−), and those where there is no evidence for a role are indicated with (nr; no role).

Cytokinins may also have different roles in the 2 symbioses (Table 1). They are essential for nodule organogenesis,⁷ but studies with the cre1 cytokinin receptor mutant in Medicago truncutula suggest that they may be less critical for AM colonisation (F. Frugier and P. Bonfante, pers. comm.). This may be due in part to cytokinins having a central role in forming the nodule structure by orchestrating cell division events, which are not required for mycorrhizal colonization. Cytokinin signaling also appears to be required to trigger the production of rhizobia-induced CLE peptides^52,53 that function at the onset of the AON pathway upon initial cell divisions to control legume nodule numbers.⁵⁴

In some cases, much of what is known about the role of plant hormones in the nodulation and mycorrhization processes is based on robust application, genetic and molecular evidence from several species. However, in others evidence for a role is often indirect and is based on limited and/or fragmented findings. This includes application data alone, often involving concentrations that are not biologically relevant, and the use of different application techniques and treatment times, in addition to the use of a single species or set of growing conditions. This can be quite relevant to the interpretation of results, as different aspects can show opposite responses or maxima. For example, gibberellins appear to have an optimum level and window for nodule development⁵¹ and this may be the case for other hormones as well. Increasing the use of hormone quantification studies to help support application and gene expression findings could help to unravel a hormone’s role(s) in these symbioses. Further research is also required to better delineate hormone interactions occurring during the 2 symbioses. This includes interactions between the classical plant hormones and other key signaling molecules, such as defense related compounds and novel plant peptide signals.⁷ Indeed, interpretation issues can arise when focusing on just one signal at a time. Many findings are also limited to the laboratory and it will be interesting to determine whether they extend to field conditions.

In a recent paper,²³ we used a genetic approach to demonstrate that strigolactones and brassinosteroids do not act downstream of the AON genes examined and argued that they probably act independently to promote nodule formation. Another direct route to identify a role for hormones in AON and AOM is through monitoring their effect on factors involved in these processes, including gene expression and level of CLE peptides, as has been shown for cytokinins and the expression of AON CLE peptide encoding genes.^52,53 The reverse should also be examined, i.e., whether CLE peptides influence plant hormone synthesis or transport. Through such genetic and quantification experiments it should be feasible to more definitively show the direct role, if any, of the various plant hormones in the AON and AOM pathways (e.g.^14,23), rather than just indirect affects of the hormones on aspects of plant development.

The detailed comparison of the timing and processes affected by the classical hormones across both symbioses may provide new insights into the roles of these hormones and how the 2 symbioses have evolved. This may also throw light on the evolution of plant hormone function since, although some appear to have been present at the time of the colonisation of land by plants, and therefore when the AM symbiosis arose, other hormones appear to have evolved subsequently.⁵⁵

Foo E, Ferguson BJ, Reid JB. The potential roles of strigolactones and brassinosteroids in the autoregulation of nodulation pathway. Ann Bot. 2014;113:1037–45. doi: 10.1093/aob/mcu030.