evolved more complex adaptations specific to the parasitic life cycle, such as complex life cycles or mech-
anisms to overcome host defenses.
Recent studies of the nuclear and chloroplast genomes of parasitic plants have proved to some extent
the main ideas in Searcy’s hypothesis.
A. State of the Nuclear and Plastid Genome of Parasitic Plants
A profound examination of the nuclear rDNA in parasitic plants shows that the ribosomal cistron is not
invariant over its entire length but is a mosaic of slowly and rapidly evolving regions [86,87]. Variable
and conserved domains exist in both 18S and 26S rDNA. High substitution rates have been detected in
nuclear 18S rDNA, mainly in the lineages that exhibit a reduction in photosynthesis and an advanced state
of nutritional dependence on the host.
The plastid genome is by far the most studied in parasitic plants: Cuscuta-Cuscutaceae [89–91],
Epifagus-Orobancheceae [15,92], Lathraea-Scrophulariaceae [14,93], Conopholis-Orobancheceae
[94,95],Orobanche-Orobancheceae [96–98].
De Pamphilis and Palmer [15] demonstrated that the root parasite Epifagus virginiana(beechdrops)
has a plastome that lacks all the genes for photosynthesis found in the chloroplast genomes of green plants
[83]. It has undergone a big reduction in size and with its 70 kb is the smallest plastid genome among
plants. The Orobancheplastid genome has also undergone an important reduction in size, resulting in a
plastid chromosome of approximately one half the length of a typical plastid genome of an autotrophic
plant, such as tobacco or Digitalis[86,87].
It has been shown that evolution of rbcL within species of Orobanchehas proceeded along divergent
pathways [97]. Intact open reading frames are present in O. corymbosaandO. fasciculata, whereas O.
cernuaandO. ramosahaverbcL pseudogenes. Rubisco function is lost, and the differences suggest that
this happened after the adaptation to heterotrophy.
B. Correlation between Nuclear, Plastid, and Mitochondrial Genomes
The genetic material of plants is compartmentalized: it is divided between the nucleus/cytosol, the plas-
tids, and the mitochondria [99]. Plants therefore not only have to control the many genes in each of these
compartments but also must coordinate the expression of genes between the three genetic compartments.
Sufficient data exist in support of the interaction between the different genetic compartments [100–106].
Nuclear genes control the expression of both plastid and mitochondrial genes, and plastome and chondri-
ome can affect nuclear gene expression [107–113].
Because holoparasitic plants are valuable model organisms that can increase our understanding of
the mode and tempo of evolutionary change at the molecular level, it may be worth checking on the state
of their mitochondrial genome too, as transfer of genes is a significant evolutionary event [114].
We examined the mitochondrial genes coxI, coxIII, atp6, atp9, atpA, 18S 5S rRNA, rrn18, as well
as the chloroplast probes pRp7-1 and pRp9-1 by Southern blot hybridization of total OrobancheDNA.
The objective was to determine whether they are present in the investigated genotypes and whether there
are differences between the genotypes and the different generations that can be used as molecular and ge-
netic markers to distinguish them unambiguously.
Figures 4, 5, and 6 represent the hybridization pattern obtained with EcoRI-digestedOrobancheDNA
probed with the coxIII mitochondrial gene, pRp7-1 chloroplast probe, and rrn18 mitochondrial gene.
C. Conservative Mitochondrial Spots—Why These Genes?
Research on the characteristics of the mitochondria of O. cernuashowed that there is no evidence to sug-
gest any basic abnormality in their biochemical properties in relation to the obligatory parasitic form of
life. The mitochondria are as functionally active as similar preparations of mitochondria from nonpara-
sitic plants insofar as the Krebs cycle, electron transport, and accessory pathways are concerned
[115,116]. However, no study of the state of different genes in the mitochondrial genome of parasitic
plants has been performed. Higher plant mitochondrial genomes have characteristics of particular inter-
est such as high frequency of recombination (by far the most unique one), large coding capacity, the en-
coding of genes present in the nuclear genome of other eukaryotes, the existence of genes that disrupt
growth and pollen development, the phenomenon of RNA editing, trans-splicing, the import of tRNAs
794 MINKOV AND LJUBENOVA