Group Sophia Sonnewald

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Molecular Physiology / Transcriptomics

The research group aims to understand sink-source regulation during plant development with special emphasis on control of potato tuber dormancy and sprout induction. In addition, sink-source regulation is studied during the interaction between plants and phytopathogenic bacteria to unravel how bacteria modify plant metabolism to their own benefit. To this end molecular, biochemical and cell-biological techniques are employed. The better understanding of underlying mechanisms will lead to new strategies to enable sprout control of potato tubers and to improve agronomic performance of crop plants.

At present the following projects are under investigation:

  • Regulation of potato tuber dormancy and sprout induction 
  • Bacteria - plant interaction to study regulation of plant primary metabolism and to elucidate mode of action of bacterial type 3 effector proteins

Regulation of potato tuber dormancy and sprouting

Potato tubers undergo a phase of dormancy after their maturation which ends with the appearance of a sprout. During the rest period tubers develop from a sink into a source organ supporting growth and development of the newly formed sprout, which gives rise to a potato plant. The sink-source transition is a complex process involving structural and metabolic changes as well as an altered gene expression. Moreover, the process is known to be under phytohormonal control.
Our main aims are to understand the molecular mechanisms controlling the length of tuber dormancy and to identify key regulatory genes as well as marker genes controlling bud breakage and sprout growth.
Breakage of dormancy coincides with the re-activation of meristematic activity in the tuber eyes. To identify genes involved in the regulation of meristematic activity and during the onset of tuber sprouting a comparative transcriptome analysis of different developmental stages during tuber life cycle and other tissues is performed. Candidate genes are being verified by qPCR, and further analysed e.g. by de-regulating their gene expression in transgenic potato plants or by analysing their promoter activity.
An in vitro sprouting system has been developed to analyse the role of phytohormones such as gibberellic acid, cytokinins or ethylene in the process of tuber sprouting. To further unravel the role of phytohormones transgenic plants with altered expression of biosynthetic or regulatory genes have been created and are analysed.
Structural changes occuring during tuber dormancy most likely include altered cell-to-cell communication which is mediated by plasmodesmata. It is assumed that these connections are blocked by callose deposition leading to a symplastic isolation of meristematic cells in a dormant state. This block has to be removed to resume cell-to-cell connections and to facilitate sprouting. Changes in the cell-to-cell communication are visualised by microscopic studies using molecular markers. In addition, the β-1,3-glucanase family which is involved in degradation of callose is under investigation.

Bacteria - plant interaction to study regulation of plant primary metabolism

The compatible interaction between Xanthomonas campestris pv. vesicatoria (Xcv) and pepper (Capsicum annuum) and tomato (Lycopersicon esculentum) plants is employed as model system to study modulation of plant primary metabolism by pathogenic bacteria. Many plant and animal pathogenic bacteria possess a type III secretion system (TTSS) mediating the translocation of effector proteins into plant host cells. These effector proteins trigger changes in plant metabolism leading either to disease in susceptible plants or to a hypersensitive response in resistant plants.
In our studies we focus on the regulation of cell wall invertase (cw-Inv) as molecular marker for changes in sink-source interaction. Induction of cw-Inv has been shown in response to different plant pathogens leading to a local accumulation of soluble sugars. These sugars most likely serve as a nutrient source for the invading pathogens. In addition, sugars are known to act as signals for up-regulation of plant defense.
Previous experiments revealed that Xcv effectors are able to suppress induction of cw-Inv and PR gene expression in a TTSS-dependent manner. The effect on gene expression was accompanied by changes in cw-Inv activity and in the content of glucose and fructose. The results suggested that Xcv encodes type III effectors which are able to suppress induction of plant defense responses and to alter plant metabolism. Biochemical, physiological and molecular studies will be performed to further analyse changes in the host metabolism by comparing responses caused by a Xcv wt strain or by a TTSS-deficient mutant which is not able to translocate effector proteins into the plant cell. In addition, transgenic tomato plants silenced in different isoforms of cw-Inv are exploited to investigate their role in more detail.
X. campestris translocates approximately 30 effector proteins into the host cell. In order to identify individual type III effectors involved in the regulation of sugar metabolism, Xcv mutants strains deficient in individual effector proteins are generated and used to infect pepper leaves and screened for their impact on cw-Inv activity. Additionally, changes in bacterial growth are monitored as well as the development of disease symptoms and the effect on protein secretion is studied. So far, six Xcv effector proteins could be identified which are involved in regulation of cw-Inv. To elucidate their in planta function different approaches are followed such as the identification interacting plant proteins (in collaboration with Group Börnke ). Furthermore, inducible expression of target genes will be used to monitor transcriptional and metabolic changes brought about by individual effector molecules.