Waldemar Röhrig


B4 Re-programming of plant metabolism by type III effectors from Xanthomonas campestris

Principal investigator
Sophia Sonnewald

Andreas Burkovski

Communication between plant cells and their interaction with plant pathogens: Analysis of sugars and major metabolites as signaling molecules for growth regulation and defense response.

In plants, sugars act both as a metabolic resource as well as a signal molecule, in which in particular glucose regulates many aspects of organismal growth and functions through metabolic effects and gene regulatory mechanisms. As the predominant transport form of carbohydrates, sucrose forms the interface between photosynthetically active source tissues and heterotrophic sink tissues. Partitioning between source and the different competing sink tissues is a tightly controlled process, and is amongst others regulated by the activity of the cell wall bound invertase (cw-Inv). Cw-Inv is an extracellular enzyme catalyzing the cleavage of sucrose into glucose and fructose, which are taken up by the cells to fuel the energy metabolism. In addition hexoses also act as signaling molecules to integrate environmental and endogenous changes.
According to previous works, there is increasing evidence that plant pathogens interfere with the sink-source balance of leaves by induction of cw-Inv activity and hence transforming an infected leaf from a source to a sink tissue. In order to study the role of cw-Inv and the resulting effects of altered hexose concentrations, our group uses the pathogenic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) as a tool to modulate cw-Inv activity. In previous work of our group, effector proteins which are delivered by the bacterial type III secretion system (TTSS) into the cell could be identified as suppressors of cw-Inv, which suggests that the activity of cw-Inv is also used for defensive purposes by the plant. Transgenic tomato plants, which were silenced for the major leaf cw-Inv isoforms, showed an increased sucrose to hexose ratio in the apoplast. This change resulted in delayed symptom development, inhibition of photosynthesis and senescence and only a weak induction of PR gene expression. The different response compared to wild type is thought to be most likely caused by the absence of hexose signals.
In this project, we will elucidate how changes in apoplastic sucrose to hexose ratios are sensed and perceived. Furthermore, we will investigate the effects on cellular metabolism and gene expression. In addition to the utilization of Xcv bacteria to change cw-Inv activity, we aim to use Xcv knockout strains deficient of defined sugar transporters to modulate apoplastic sugar levels and to identify putative signaling components by comparative transcript profiling. A promising candidate is the recently proposed Arabidopsis hexokinase1 gene (HXK1), which is thought to act as an important glucose sensor or transducer and we aim to unravel its signaling or sensing role in plant-pathogen interaction. For this purpose we will generate transgenic HXK-deficient tomato plants and investigate their responses to Xcv infection or exogenously applied sugars. Besides the transgenic HXK plants we also will use the already existing cw-Inv silenced plants for our transcript profiling experiments.
In a second more diverse approach, we will investigate the nutritional strategy of Xcv in planta which might provide clues on the changes during the transition of source to sink tissues. By understanding how Xcv adapts and reacts to the changing conditions in the plant apoplast and how these changes are sensed and utilized by the bacterium, we also might be able to identify regulatory mechanisms for survival and the beneficial reprogramming of the plant metabolism.

Figure:Model of sugar-sensing mechanisms in plants. (a) The HXK1 glucose sensor is mainly associated with mitochondria, possibly as part of a glycolytic metabolon. In addition, HXK1 is found in high-molecular-weight complexes in the nucleus where it controls transcription and proteasome-mediated degradation of the EIN3 TF. Other HXK and HKL proteins are also associated with the outer membrane of plastids, including chloroplasts, or cytosol. HXK can also be found in the chloroplast stroma. (b) Sucrose (and other disaccharides) appears to be sensed at the plasma membrane, possibly by transporter homologs. (c) G-protein coupled receptor signaling by RGS1 and GPA1 is involved in glucose control of seed germination and seedling development, possibly in a hexokinase-independent way. (d) SnRK1 proteins play an important role in plant sugar and starvation signaling, although the significance of the regulation of these proteins by sucrose (Suc) and G6P is still unclear. (e) Important regulatory effects are reported for trehalose (Tre) and T6P, apparently downstream of SnRK1. In the nucleus, several types of transcription factors are involved in sugar-regulated transcription. (Figure taken from Rolland, Baena-Gonzales and Sheen 2006).



Kirsch, F., Horst, K., Röhrig, W., Rychlik, M. and Buettner, A. (2012). Tracing metabolite profiles in human milk: studies on the odorant 1,8-cineole transferred into breast milk after oral intake. Metabolomics DOI 10.1007/s11306-012-0466-9.

Pfister, A.S., Hadjihannas, M. V., Röhrig, W., Schambony, A. and Behrens J. (2012). Amer2 interacts with EB1 and APC and controls microtubule stability and cell migration. J Biol Chem 287, 35333-35340.



July 2012 4th Annual Retreat, Erlangen School of Molecular Communication, Kloster Banz, Bad Staffelstein, Germany
Hide and Seek: The role of apoplasmic metabolites during Plant-Pathogen-Interaction
October 2011 First International SFB 796 Conference: Mechanisms of viral host cell manipulations: from plants to humans, Bamberg, Germany
Sugar signaling and perception in plant development and plant pathogen interaction