Gene discovery in pursuit of carbohydrate
metabolic genes in the parasitic plant
Cuscuta pentagona: Cloning of a sucrose
phosphate synthase-like gene.
Research supported by the STEM-Talent
Expansion Program, Summer 2007
Biology Department, University of Nebraska at Omaha,
Omaha, Nebraska 68182-0040, USA.
Introduction: The plant as a parasite
Just as some microbes and animals make their living parasitizing other
organisms, many species of plants make their living by parasitizing other plants.
The parasitic habit has arisen several times among flowering plant lineages. One
feature that all parasitic plants share is the use of penetrating organs, called
haustoria, to connect to the host in order to draw upon its water and nutrient
supply. Beyond the occurrence of haustoria, however, parasitic plants can vary
widely in their degree of modification, and in the degree to which they are
dependent upon their hosts. Some parasitic plants can complete their life cycles
without engaging in parasitism, while others are wholly dependent upon their
Species of Cuscuta, commonly called the dodders, lack roots, have only vestiges
of leaves, and perform very little or no photosynthesis in most tissues. As such,
they are wholly dependent upon their hosts for water and mineral nutrients, as
well as for a majority of their energy needs. In order to determine the
physiological consequences of adopting the parasitic habit, we are investigating
the core metabolic processes common to all photosynthesizing plants. These
processes include photosynthetic carbon assimilation, sugar and starch
metabolism, nitrogen assimilation, and amino acid biosynthesis. As a starting
point in these investigations, we are identifying the genes that encode the
enzymes that mediate these processes. In this study, we attempted to identify the
sucrose phosphate synthase gene, involved in carbohydrate metabolism. To
provide a point of comparison, we are also pursuing this gene in Cuscuta’s nonparasitic relative, ivy-leaf morning glory (Ipomoea hederacea).
The 2007 Biology-STEP molecular research team
Left to right: Mark Schoenbeck (instructor), James Lucas, Kimberly Meints,
Kristina Wiley, Craig Harrison, Sarah Faltin, Lacy Jacobsen, Aaron Zach,
Andria Bethelmie, Sobia Rasool, Kirk Larson, Sydney Brommer (technical
and research assistant), and Julie Kowal (lab assistant).
Figure 2. Plant cells may be either sources of, or sinks for, fixed carbon in
the form of carbohydrates. Cells may store carbohydrates in the form of starch
(left) employing enzymatic activities such as starch synthase. Alternatively, cells
may generate sucrose, a primary form of translocated carbohydrate, for
mobilization to other plant organs, through the activity of sucrose phosphate
synthase. Because dodder functions as a strong sink in order to draw
carbohydrate resources from its host, it would be of interest to understand the
functioning of starch synthase and sucrose phosphate synthase genes in the
parasite, relative to their functions in non-parasitic plants. A dodder starch
synthase gene was identified by the 2005 Biology-STEP molecular research team.
The sucrose phosphate synthase gene was chosen as an objective for this work.
Conclusions and future work
Sequence analysis of recovered clones shows a high degree of similarity between
C. pentagona sucrose phosphate synthase (SPS) and sequences reported from
Recovery and analysis of the complete SPS gene, and proximal genomic sequences
from dodder and non-parasitic relatives may reveal changes in putative promoter
Figure 1. The parasitic angiosperm Cuscuta pentagona on its host (left) and its
non-parasitic relative Ipomoea hederacea. Recent molecular investigations place
the genus Cuscuta in the family Convolvulaceae with Ipomoea. Molecular and
physiological investigations may provide clues as to the changes that
accompanied the adoption of the parasitic habit, which included the loss of
functional leaves and roots, and, likely, the modification of core metabolic
Figure 3. A flow chart describing the strategy for identifying genes involved
in core metabolic processes in dodder and ivy-leaf morning glory. The gene
discovery strategy employed a range of skills, comprising the use of bioinformatic
tools as well as standard molecular research methods.
Both the SPS and previously identified starch synthase gene fragments will be
employed to monitor levels of cognate gene transcipts, providing information on how
carbohydrate metabolic genes may be regulated in parasitic and non-parasitic plants.
Funds for this research were provided by an NSF STEP grant, NSF-033642.
Laboratory equipment was provided by NIH Grant Number P20 RR16469 from the
INBRE Program of the National Center for Research Resources.