Gibberellins Interfere with Symbiosis Signaling and Gene Expression and Alter Colonization by Arbuscular Mycorrhizal Fungi in Lotus japonicus  

Abstract

Arbuscular mycorrhiza is a mutualistic plant-fungus interaction that confers great advantages for plant growth. Arbuscular mycorrhizal (AM) fungi enter the host root and form symbiotic structures that facilitate nutrient supplies between the symbionts. The gibberellins (GAs) are phytohormones known to inhibit AM fungal infection. However, our transcriptome analysis and phytohormone quantification revealed GA accumulation in the roots of Lotus japonicus infected with AM fungi, suggesting that de novo GA synthesis plays a role in arbuscular mycorrhiza development. We found pleiotropic effects of GAs on the AM fungal infection. In particular, the morphology of AM fungal colonization was drastically altered by the status of GA signaling in the host root. Exogenous GA treatment inhibited AM hyphal entry into the host root and suppressed the expression of Reduced Arbuscular Mycorrhization1 (RAM1) and RAM2 homologs that function in hyphal entry and arbuscule formation. On the other hand, inhibition of GA biosynthesis or suppression of GA signaling also affected arbuscular mycorrhiza development in the host root. Low-GA conditions suppressed arbuscular mycorrhiza-induced subtilisin-like serine protease1 (SbtM1) expression that is required for AM fungal colonization and reduced hyphal branching in the host root. The reduced hyphal branching and SbtM1 expression caused by the inhibition of GA biosynthesis were recovered by GA treatment, supporting the theory that insufficient GA signaling causes the inhibitory effects on arbuscular mycorrhiza development. Most studies have focused on the negative role of GA signaling, whereas our study demonstrates that GA signaling also positively interacts with symbiotic responses and promotes AM colonization of the host root.

Phosphate Treatment Strongly Inhibits New Arbuscule Development But Not the Maintenance of Arbuscule in Mycorrhizal Rice Roots

“…Recently, it has been shown that RAM1, a GRAS-type transcription factor, regulates the expressions of at least STR and EXO70I, which are required for the support of arbuscule branching (Park et al, 2015). RAM1 expression is also regulated by DELLA/gibberellin signaling module, and DELLA level is regulated by P status (Floss et al, 2013;Takeda et al, 2015;Park et al, 2015). We showed that the numbers of GFP-AM42-positive young arbuscules in infection fronts of P-treated roots tended to be decreased compared to those of water-treated roots, suggesting that the development of arbuscules was arrested at their young stages.…”

DELLA proteins regulate expression of a subset of AM symbiosis-induced genes inMedicago truncatula

“…2), including the AM marker genes BCP1 and SCP1, which are highly expressed in arbuscule-containing cells and the adjacent cortical cells. 7,25,26 Likewise transcripts for a Kunitz protease inhibitor, KPI106, 27 an ortholog of a Lotus japonicus AM-induced subtilase SbtM1 18 and Vapyrin [28][29][30] are all increased in roots expressing della1-D18. In contrast, in L. japonicus roots in which symbiosis signaling was constitutively activated through expression of a gain-of-function CCaMK, both SbtM1 and Vapyrin transcripts were reduced by constitutive expression of a dominant DELLA construct.…”

“…12 Genes encoding enzymes of GA biosynthesis and metabolism are differentially regulated during symbiosis 7,[13][14][15][16][17] and associated with the colonized regions of the cortex. 18 Furthermore, both bioactive and inactive GAs are present in mycorrhizal roots. All of these data suggest a role for GAs in regulating aspects of symbiotic development.…”

“…All of these data suggest a role for GAs in regulating aspects of symbiotic development. 18,19 DELLA proteins are members of a specific sub-group of the GRAS family of transcriptional regulators that possess a unique domain, the DELLA domain, located toward the N-terminal end of the protein. 20 DELLAs function as repressors of GA signaling and their mechanism of action has been elucidated through extensive studies in Arabidopsis.…”

“…All of these data suggest a role for GAs in regulating aspects of symbiotic development. 18,19 DELLA proteins are members of a specific sub-group of the GRAS family of transcriptional regulators that possess a unique domain, the DELLA domain, located toward the N-terminal end of the protein.20 DELLAs function as repressors of GA signaling and their mechanism of action has been elucidated through extensive studies in Arabidopsis. Briefly, in the presence of GA, the GA receptor (GID) interacts with the DELLAs through the DELLA domain and this subsequently results in the degradation of DELLA proteins and consequently relieves their repressive effect.…”