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SUMMARY 12Plant growth promoting fungi include strains of Trichoderma species that are used in biocontrol, 13 and arbuscular mycorrhizal (AM) fungi, that enhance plant nutrition and stress resistance. The 14 concurrent interaction of plants with these two groups of fungi affects crop performance, but has 15 only been occasionally studied so far. Using in vivo imaging of GFP-tagged lines, we investigated 16 the cellular interactions occurring between Trichoderma atroviride PKI1, Medicago truncatula and 17 two Gigaspora species under in vitro culture conditions. T. atroviride did not activate symbiotic-18 like responses in the plant cells, such as nuclear calcium spiking or cytoplasmic aggregations at 19 hyphal contact sites. Furthermore, T. atroviride parasitized G. gigantea and G. margarita hyphae 20 through localized wall breaking and degradation -although this was not associated with significant 21 chitin lysis nor the upregulation of two major chitinase genes. T. atroviride colonized broad areas of 22 the root epidermis, in association with localized cell death. The infection of both symbionts was 23

Modern biology overlaps with chemistry in explaining the structure and function of all cellular processes at the molecular level. Plant hormone research is perfectly located at the interface between these two disciplines, taking advantage of synthetic and computational chemistry as a tool to decipher the complex biological mechanisms regulating the action of plant hormones. These small signaling molecules regulate a wide range of developmental processes, adapting plant growth to ever changing environmental conditions. The synthesis of small bioactive molecules mimicking the activity of endogenous hormones allows us to unveil many molecular features of their functioning, giving rise to a new field, plant chemical biology. In this framework, fluorescence labeling of plant hormones is emerging as a successful strategy to track the fate of these challenging molecules inside living organisms. Thanks to the increasing availability of new fluorescent probes as well as advanced and innovative imaging technologies, we are now in a position to investigate many of the dynamic mechanisms through which plant hormones exert their action. Such a deep and detailed comprehension is mandatory for the development of new green technologies for practical applications. In this review, we summarize the results obtained so far concerning the fluorescent labeling of plant hormones, highlighting the basic steps leading to the design and synthesis of these compelling molecular tools and their applications.

The biological activity of natural and novel strigolactone D-lactam analogues is assessed using a novel bioassay based on Arabidopsis transgenic lines expressing AtD14 fused to firefly luciferase.

Originally identified as allelochemicals involved in plant-parasite interactions, more recently, Strigolactones (SLs) have been shown to play multiple key roles in the rhizosphere communication between plants and mycorrhizal fungi. Even more recent is the hormonal role ascribed to SLs which broadens the biological impact of these relatively simple molecules. In spite of the crucial and multifaceted biological role of SLs, there are no data on the receptor(s) which bind(s) such active molecules, neither in the producing plants nor in parasitic weeds or AM fungi. Information about the putative receptor of SLs can be gathered by means of structural, molecular, and genetic approaches. Our contribution on this topic is the design and synthesis of fluorescent labeled SL analogs to be used as probes for the detection in vivo of the receptor(s). Knowledge of the putative receptor structure will boost the research on analogs of the natural substrates as required for agricultural applications.

As part of our ongoing work on the synthesis of a new class of plant hormones named Strigolactones (SLs) and their analogues, we became interested in tracing bioactive molecules with red emitting BODIPY fluorophores in order to unravel signaling and distribution of SLs in plants. In this paper we report on an unprecedented Heck functionalization of azadipyrromethenes (aza-DIPY) which allows for the introduction of suitable functional groups to convert aza-BODIPY in bioconjugate complexes useful for untangling biological processes.

The initiation of intracellular host cell colonization by symbiotic rhizobia in Medicago truncatula requires repolarization of root hairs, which includes the rearrangement of cytoskeletal filaments. The molecular players governing microtubule (MT) reorganization during rhizobial infections remain to be discovered. Here, we identified M. truncatula DEVELOPMENTALLY-REGULATED PLASMA MEMBRANE POLYPEPTIDE (DREPP), a member of the microtubule-binding DREPP/PCaP protein family and investigated its functions during rhizobial infections. We show that rhizobial colonization of drepp mutant roots as well as transgenic roots over-expressing DREPP is impaired. DREPP re-localizes into symbiosis-specific membrane nanodomains in a stimulus-dependent manner. This subcellular segregation coincides with DREPP-dependent MT fragmentation and a partial loss of the ability to reorganize the MT cytoskeleton in response to rhizobia, which might rely on an interaction between DREPP and the MT organizing protein SPIRAL2 (SPR2). Taken together, our results reveal that establishment of symbiotic associations in M. truncatula requires DREPP in order to regulate MT reorganization during initial root hair responses to rhizobia.

Formin-mediated bridging of cell wall, plasma membrane, and cytoskeleton in symbiotic infections of Medicago truncatula Graphical abstract Highlights d The SYMBIOTIC FORMIN 1 (SYFO1) specifically regulates symbiotic root hair curling d SYFO1 directly binds actin and polarizes in responding root hairs d SYFO1 induces membrane protrusions in cell-wall-devoid protoplasts d Cell wall association of SYFO1 is indispensable for its function in root hairs