Received: 22 February, 2007. Accepted: 22 March, 2007. Invited Review International Journal of Plant Developmental Biology ©2007 Global Science Books Developmental Studies in the Christmas Rose (Helleborus niger L.) Branka Salopek-Sondi * • Volker Magnus Rudjer Boškovi Institute, Bijenika cesta 54, P.O.Box 180, 10002 Zagreb, Croatia Corresponding author: * salopek@irb.hr ABSTRACT The Christmas rose (Helleborus niger L.) is a herbaceous, winter-green, perennial native to Southern Europe, which is also widely grown as an ornamental. In mild winters, the flowers may indeed appear at Christmas time, resembling wild roses with respect to size and color (white to pink). Reproductive development in the Christmas rose is characterized by an interesting aspect, uncommon in the world of flowering plants: after pollination and fertilization, the perianth develops a photosynthetic apparatus, and persists during fruit develop- ment. Unpollinated or depistillated flowers survive almost as long as their fruit-bearing neighbors, but do not pass through the complete greening process. Removal of the gynoecium also affects the shape of the flower and the length of the flower scape. The correlative signals which normally trigger and maintain these morphogenetic processes appear to include plant hormones synthesized in the developing fruit. Because of the size of its flowers, their long lifespan, and the changes induced by fruit development and maturation, the Christmas rose could become a useful research model for disentangling some of the complex interactions between developing seeds and the mother plant. _____________________________________________________________________________________________________________ Keywords: cytokinin, gibberellin, flower morphogenesis, perianth greening, photosynthesis, plant growth regulator, plastid metamor- phosis, source-sink relationship Abbreviations: BA, N 6 -benzyladenine; 4-Cl-IAA, 4-chloroindole-3-acetic acid; DZ, dihydrozeatin; DZ9G, dihydrozeatin 9-glucoside; DZR, dihydrozeatin riboside; DZRMP, dihydrozeatin riboside-5'-monophosphate; f. wt., fresh weight; GA x , gibberellin A x (x = 1, 3, 4 or 7); HPLC, high-performance liquid chromatography; iP, N 6 -( 2 -isopentenyl)adenine; iPR, N 6 -( 2 -isopentenyl)adenine riboside alias N 6 - ( 2 -isopentenyl)adenosine; IAA, indole-3-acetic acid; iPRMP, N 6 -( 2 -isopentenyl)adenine riboside-5'-monophosphate alias N 6 -( 2 - isopentenyl)adenosine-5'-monophosphate; Z, trans-zeatin; Z9G, trans-zeatin 9-glucoside; ZR, trans-zeatin riboside; ZRMP, trans-zeatin riboside-5'-monophosphate CONTENTS INTRODUCTION...................................................................................................................................................................................... 151 BOTANICAL BACKGROUND ................................................................................................................................................................ 152 THE LIFE CYCLE OF A CHRISTMAS ROSE FLOWER ....................................................................................................................... 153 THE STRUCTURAL AND FUNCTIONAL METAMORPHOSIS OF THE SEPALS.............................................................................. 153 IS PHOTOSYNTHETIC ACTIVITY IN CHRISTMAS ROSE SEPALS REQUIRED FOR NORMAL SEED DEVELOPMENT?........ 154 IS FLORAL PHOTOSYNTHESIS IN THE CHRISTMAS ROSE ‘SINK- REGULATED’? .................................................................... 155 CAN PLANT HORMONE TREATMENT SUBSTITUTE FOR THE ROLE OF DEVELOPING CHRISTMAS ROSE FRUIT? .......... 155 ENDOGENOUS PHYTOHORMONES .................................................................................................................................................... 156 Cytokinins ............................................................................................................................................................................................. 156 Gibberellins ........................................................................................................................................................................................... 157 THE CHRISTMAS ROSE AS A MODEL PLANT ................................................................................................................................... 157 ACKNOWLEDGEMENTS ....................................................................................................................................................................... 158 REFERENCES........................................................................................................................................................................................... 158 _____________________________________________________________________________________________________________ INTRODUCTION The Christmas rose (Helleborus niger L.) (Fig. 1) has, for centuries, played a role in folklore and herbal medicine (Damboldt and Zimmermann 1965; Mathew 1989). It has, thus, long been grown in gardens and has locally become naturalized far north of its original range. The plant is also of interest to physiologists because the flowers turn green, after fertilization, and the perianth survives until seed ri- pening. This happens in a number of other species, as well; examples studied in detail include a number of orchids (van Doorn 1997) and the dicots Chrysosplenium alternifolium L., C. oppositifolium L. (Sitte 1974), and Nuphar luteum Sibth. et Sm. (Grönegress 1974). In the araceans Spathi- phyllum wallisii Regel (Palandri 1967), Zantedeschia aethi- opica Spreng. (Pais 1972; Chaves das Neves and Pais 1980a, 1980b) and Z. elliotiana Engl. (Grönegress 1974), it is the brightly colored spathe that turns green after anthesis. With- in the above group of species, the Christmas rose with its large flowers is particularly suitable for experimental work. Also, photosynthesis in its greening perianth may substanti- ally contribute to seed filling, because leaves are frequently absent, or not fully operational, during fruit ripening. In most flowers, the perianth senesces and is subject to abscission, once it has played its part in the attraction of pollinators (Weiss 1991; O’Neill 1997; van Doorn 1997; van Doorn and Stead 1997). This highly regulated process involves complex structural and biochemical changes (Or-