MORPHOGENESIS IN THE PARASITIC PLANT VISCUM MINIMUM (VISCACEAE) IS HIGHLY ALTERED, HAVING APICAL MERISTEMS BUT LACKING ROOTS, STEMS, AND LEAVES

摘要

Premise of research. The parasitic plant Viscum minimum allows its embryo to die after it infects its host. The plant then exists completely inside its host, Euphorbia polygona (Euphorbiaceae), which has a soft succulent cactus-like body. Other parasitic plants that grow entirely within their hosts have extremely modified, very simple bodies, and we suspected that V. minimum does as well. Our objective was to examine the body of V. minimum inside its host and determine the ways its morphogenesis differs from that of ordinary plants such as Arabidopsis. Methodology. Plants of E. polygona infected with V. minimum were dissected, fixed, embedded in wax, and prepared for LM. Because the host is so soft and easy to microtome, numerous samples were examined from many sites in the infected plants. Pivotal results. Morphogenesis of V. minimum is highly altered and differs greatly from that of ordinary nonparasitic plants. Viscum minimum has no leaves, roots or stems; no epidermis or endodermis. Its vessel elements and sieve tube members form just small patches rather than continuous vessels or sieve tubes. Viscum minimum grows by multicellular apical meristems varying from 2 to 9 cells wide that produce axes of pure parenchyma, lacking the patterns of tissues typical of stems or roots. Conclusions. Parasitic plants evolved from photosynthetic ancestors, so their morphogenic genes must be orthologous to those of model plants such as Arabidopsis, Zea, and others. But morphogenic programs in V. minimum must be highly modified, so molecular genetic studies of morphogenesis in V. minimum would greatly expand our understanding of morphogenesis in all plants.