Investigating the role of GA signaling in arbuscular mycorrhiza
Final Report Abstract
Most land plants are capable to form mutualistic interactions with arbuscular mycorrhizal (AM) fungi. The symbiosis results in the formation of tree-shaped structures, known as arbuscules, which are developed from intercellular fungal hyphae inside the inner root cortex cells. These structures are thought to be the main site of nutrient exchange between both symbiotic partners. Mechanisms regulating the development of functional arbuscules are still unknown and have to be elucidated. The present work aimed at uncovering the role of gibberellins (GA) in establishing a functional AM between Medicago truncatula and Glomus versiforme. The application of GA3 and the generation of a constitutive GA response M. truncatula mutant by RNAi of MtDELLA1 encoding a protein, which is a central player in the GA signaling pathway, result in either the complete absence or a drastic reduction of the numbers of arbuscules proposing an involvement of GA signaling via DELLA in the formation of these structures. Surprisingly, impaired arbuscule formation could not be confirmed in mtdella1 Tnt1 transposon mutants. This unexpected result indicates that functional redundancy of DELLA proteins in regulation of arbuscule formation might occur. Evidence for this hypothesis comes from the M. truncatula genome, which encodes at least 3 DELLA proteins (MtDELLA1, MtDELLA2, and MtDELLA3) and the finding that MtDELLA1 and MtDELLA2 are co-expressed. The hypothesis will be tested by analysis of the AM phenotype in mtdella1xmtdella2 mutants. Expression of a dominant MtDELLA1 version (MtΔDELLA1), which lacks the DELLA domain, led to roots that are insensitive to GA resulting in arbuscule formation. Interestingly, arbuscule formation could only be observed in GA3-treated roots expressing MtΔDELLA1 in root tissues, which include the vasculature and endodermis indicating that DELLA function in vascular tissue and/or endodermis might be sufficient for arbuscule formation. DELLA proteins are known to be localized in cell nuclei. MtDELLA1 seems to be rapidly degraded in M. truncatula roots since its accumulation as MtDELLA1-GFP could only be observed after treatment with a GA biosynthesis or proteasome inhibitor or by using MtΔDELLA1-GFP fusion constructs. A yeast two-hybrid approach shows that MtDELLA1/2 and their dominant protein versions interact with several transcription factors (TF) in Saccharomyces cerevisiae indicating that DELLA proteins might modulate the expression of genes, which are involved in arbuscule formation, by interaction with TFs. In planta experiments that strengthen this hypothesis are in progress. The results of this work are consistent with previous findings that GA/DELLA signaling is involved in arbuscule formation. Furthermore, the present study gives first insights into the mechanism how DELLA might function in regulation of arbuscule formation.
