Abstract
Most terrestrial plants are able to form a root symbiosis with arbuscular mycorrhizal (AM) fungi for enhancing the assimilation of mineral nutrients. AM fungi are obligate symbionts that depend on host plants as their sole carbon source. Development of an AM association requires a continuous signal exchange between the two symbionts, which triggers coordinated differentiation of both partners, to enable their interaction within the root cells. The control of the AM symbiosis involves a finely-tuned process, and an increasing number of studies have pointed to a pivotal role of several phytohormones, such as strigolactones (SLs), gibberellic acids (GAs), and auxin, in the modulation of AM symbiosis, through the early recognition of events up to the final arbuscular formation. SLs are involved in the presymbiotic growth of the fungus, while auxin is required for both the early steps of fungal growth and the differentiation of arbuscules. GAs modulate arbuscule formation in a dose-dependent manner, via DELLA proteins, a group of GRAS transcription factors that negatively control the GA signaling. Here, we summarize the recent findings on the roles of these plant hormones in AM symbiosis, and also explore the current understanding of how the DELLA proteins act as central regulators to coordinate plant hormone signaling, to regulate the AM symbiosis.
Highlights
In natural ecosystems, many plants are able to grow in nutrient-poor soils by living together with microbes, for mutual benefit, an instance which is referred to as ‘symbiosis’
Consistent with this, the overexpression of the unique rice DELLA SLR1, resulted in a substantially increased arbuscular mycorrhizal (AM) colonization, as compared to the wild-type rice plants [41], and the expression of a dominant DELLA protein, non-degradable by gibberellic acids (GAs) (D18DELLA1) in the M. truncatula, promoted arbuscule formation and counteracted the negative effects of the GA application [40]. These results provide direct evidence that GAs modulate AM colonization via the DELLA proteins, which could, in turn, promote arbuscule formation, through the suppression of GA-signaling. These findings provide the evidence to support that there exists a precise mechanism in plants to finely tune the GA-signaling and the protein amount of the DELLAs, during the establishment of the AM symbiosis
DELLA proteins can interact with IPD3/CYCLOPS, a component of the common symbiosis signaling pathway (CSSP), to activate the expression of the REDUCED ARBUSCULAR MYCORRHIZA1 (RAM1), a GRAS-domain transcription factor that is required for the arbuscule branching, and could fine-tune the plant biosynthesis and transfer of lipids to the fungal arbuscules [45,47] (Figure 2)
Summary
Many plants are able to grow in nutrient-poor soils by living together with microbes, for mutual benefit, an instance which is referred to as ‘symbiosis’. AM fungi colonize the root interior, which involves morphological changes of both plant cells and fungal hyphae. The formation of intracellular fungal structures and the level of fungal proliferation within the roots are dynamically tuned by the plant, which may prevent excessive colonization and carbon loss, maintaining the symbiotic benefits, at mutualistic levels. To achieve this regulation, continual signaling-exchange between the symbionts and an extensive transcriptional reprogramming and cellular remodeling in the plant, are required [10]. We provide an overview of the recent progress about the roles of these phytohormones and their cross-talks involving the DELLA regulators, in controlling AM development
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