DynaMo Mini Symposium: Bouwmeester & Geldner

DynaMo is happy to invite to a Mini Symposium with our two distinguised guest.

Programme

10:00
Professor Dr. Harro J. Bouwmeester,Wageningen University, NL
Strigolactones: (rhizosphere) signalling molecules with surprising biological activities

11:00
Associate Professor Dr. Niko Geldner, University of Lausanne, CH
The endodermis – how plants build their inner skin

Abstracts

Harro J. Bouwmeester: Strigolactones - (rhizosphere) signalling molecules with surprising biological activities

Plants have evolved intricate signaling mechanisms to communicate with their environment, and their partners and competi-tors in which they had to evolve the right balance between giving clear enough information to their partners and keeping their enemies as much as possible uninformed. Under low phosphate conditions most land plants stimulate the formation of symbiotic associations with Arbuscular mycorrhizal (AM) fungi. This symbiosis is stimulated by signaling molecules, the strigolactones, which are exuded by the plant root. The strigolactones biosynthetically originate from carotenoid cleavage and should hence be called apocarotenoids. Strigolactone production and secretion is specifically increased in plants suffering from low phosphate availability thereby actively inviting mycorrhizal colonisation. But strigolactones are also the germination stimulants of the root parasitic Striga and Orobanche spp. Upon perception of the presence of a host root through its strigolactone secretion, seeds of these parasites will germinate and attach to the host. Nutrient shortage in agricultural fields is strongly aggravating the parasitic plant problem, and strigolactones may to a large part be responsible for this effect. Finally, it was recently discovered that the strigolactones are not only rhizosphere signaling molecules but also have an endogenous, hormonal function. The strigolactones are most likely the elusive Branch Inhibiting Signal that was postulated to exist based on genetic studies. Indeed, excessively branching mutants with mutations in two carotenoid cleavage dioxygenases, CCD7 and CCD8, in pea and rice were demonstrated not to exude strigolactones. Moreover, application of the synthetic strigolactone, GR-24, to the branching mutants could restore the wildtype phenotype. In conclusion, the apocarotenoid strigolactones are a new class of plant hormones with in- and external signaling roles. The implications for the control of Orobanche and Striga spp. will be discussed. Also the further elucidation of the biosynthetic pathway, which is still only partly resolved, and its regulation by phosphate starvation are an important challenge during the next couple of years. Because of their crucial role in rhizosphere communication and regulation of plant development, the regulation of strigolactone biosynthesis is of great interest. The genes so far identified to be involved in strigolactone biosynthesis and signaling - using highly branched/tillered mutants: Arabidopsis max (more axilary branching), rice dwarf or htd (high tillering dwarf), Petunia dad (decreased apical dominance) and pea rms (ramosus) - will be discussed. Also the further elucidation of the biosynthetic pathway, which is still only partly resolved, and its regulation by phosphate starvation are an important challenge during the next couple of years.

Niko Geldner: The endodermis – how plants build their inner skin

The Niko Geldner Lab investigates the establishment of endodermal structure and polarity. The endodermis is an invariable barrier within the root of higher vascular plants. Its barrier function is mediated by the Casparian strips, ring-like hydropho-bic cell wall thickenings that are coordinated between cells and form a supracellular network within the apoplast. Casparian strips block passage of nutrients and pathogens through the apoplast, while allowing for signal perception and nutrient up-take to take place - resembling the dual, protective/uptake function of polarised gut epithelia in animals. The molecular players and mechanisms that underlie this intricately structured cell layer have remained obscure. Recently, our group has described the developmental events leading to a differentiated endodermis and visualised a strict polarity within this cell layer (1). We found that the two polar domains in the endodermis are separated by a central membrane diffusion-barrier and we identified an unknown protein family that forms this barrier and predicts the site of Casparian Strip formation (2). Mutants in these CASP genes display disorganized deposition of Casparian Strips. In addition, we were able to demonstrate that Casparian strips are essentially a lignin-, not suberin-based structure (3). We have now obtained a number of mutants in Casparian Strip development, which display interrupted, or strongly delayed, deposition of Casparian Strips, some by interfering with CASP localization, others by executing lignin polymerisation. These mutants now provide the unprecedented opportunity to directly test the many supposed roles of the Casparian Strips and I will report on the latest results from our mutant analysis.

(1) A developmental framework for endodermal differentiation and polarity. Alassimone J, Naseer S, Geldner N. Proc Natl Acad Sci U S A. 2010 Mar 16;107(11):5214-9.
(2) A novel protein family directs Casparian Strip formation in the endodermis. Roppolo D, De Rybel B, Dénervaud Tendon V, Pfister A, Alassimone J, Vermeer JEM, Yamazaki M, Stierhof Y, Beeckman T, Geldner N.
Nature. 2011 May 19;473(7347):380-3.
(3) Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin. Naseer S, Lee Y, Lapierre C, Franke R, Nawrath C, Geldner N. Proc Natl Acad Sci U S A. 2012 Jun 19;109(25):10101-6.