Is the carbon allocation between host and mistletoe regulated by the sink strength of the mistletoe sinker?
The relationship of mistletoes to the xylem of their hosts has long been a subject of interest to botanists. Viscum album L. was regarded as a xylem tapping, obligate hemiparasite. Recent findings however, have proven that more than 60 % of the carbon found in the mistletoes derived from the host. It is assumed that a transfer of carbon takes place preferentially in spring, during times when the host's xylem sap is rich in organic substances such as sugars. Based on anatomical investigations it is postulated that an apoplastic continuum provided by both partners facilitates selective uptake of nutrients. Generally, carbon allocation is driven by a source-sink gradient. In order to enable carbon transfer into the sinker, the mistletoe has to regulate the sink strength in its host endophytic organ. This regulation will be elucidated by partner-specific studies of the starch- and sucrose metabolism on the tissue level. This includes studies of the expression of genes coding for enzymes of the starch- and sucrose metabolism, quantification of the enzyme-specific protein and catalytic activities, as well as the quantification of regulator substances.