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Title: Flower morphology and floral sequence in Artemisia annua (Asteraceae)

item WETZSTEIN, HAZEL - University Of Georgia
item PORTER, JUSTIN - University Of Georgia
item JANICK, JULES - Purdue University
item Ferreira, Jorge

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2014
Publication Date: 5/8/2014
Citation: Wetzstein, H.Y., Porter, J., Janick, J., Ferreira, J.F. 2014. Flower morphology and floral sequence in Artemisia annua (Asteraceae). American Journal of Botany. 101(5):875-885.

Interpretive Summary: Malaria remains as the main killer disease in the world and has developed resistance to most quinine-based drugs. The plant known as Sweet Annie is currently the only feasible source of artemisinin, the raw material of an antimalarial drug used to treat multi-drug resistant malaria. Although the worldwide demand of artemisinin lead to an increase in the cultivated area of this pharmaceutical crop in Asia and Africa, farmers still rely on high-priced seeds (over $40.00/gram) originated from parent plants grown in regions that do not match the daylight conditions available to local farmers in tropical Africa and Asia. Seeds originated from plants selected and bred at high latitudes result in plants with precocious flowering, and insufficient leaf biomass, when grown close to the Equator (where most of the artemisinin is needed to fight malaria). Our work used light microscopy and electron scanning microscopy to record key features of flower morphology and development that will increase the understanding how these minuscule flowers are pollinated. We focused on the morphology and development of female and hermaphroditic flowers and the time difference in the maturity of male and female flower organs. This time difference favors cross-pollination, and makes self-pollination more difficult. We describe the male and female reproductive organs anthers and style, respectively, and how they evolved for wind pollination. We also report that different cultivars of the plant flower at different times, but the flowering can be synchronized by using short days. Flower synchronization using short days allows the crossing of desirable cultivar that flower at different times under natural light conditions. Farmers and breeders in tropical climates can use the information provided in this manuscript to cross plants with desirable traits, such as high leaf biomass and high artemisinin concentration, improve the efficiency of viable seed production, and select and breed plants better adapted to their climatic conditions, even if the target cultivars flower at different times.

Technical Abstract: Premise of the study: Artemisia annua produces phytochemicals possessing antimalarial, antitumor, anti-inflammatory, and anthelmintic activities. The main active ingredient, artemisinin, is extremely effective against malaria. Breeding to develop cultivars producing high levels of artemisinin can help meet worldwide demand for artemisinin and its derivatives. However, fundamental reproductive processes, such as the sequence of flowering and fertility, are not well understood and impair breeding and seed propagation programs. Methods: Capitulum structure and floral sequence were studied using light and scanning electron microscopy to describe inflorescence architecture, floret opening, and seed set. Key results: Florets are minute and born in capitula containing pistillate ray florets and hermaphroditic disk florets. Ray florets have elongated stigmatic arms that extend prior to disk floret opening. Disk florets exhibit protandry. During the staminate phase, pollen is released within a staminate tube and actively presented with projections at the tip of stigmas as the pistil elongates. During the pistillate phase, stigmatic arms bifurcate and reflex. Stigmas are of the dry type and stain positively for polysaccharides, lipids, and an intact cuticle. Floret numbers vary with genotype, and capitula are predominantly composed of disk florets. Both ray and disk florets produce filled seed. Conclusions: Gynomonoecy, early opening of ray florets, and dichogamy of disk florets promote outcrossing in A. annua. For breeding and seed development, flowering in genotypes can be synchronized under short days according to the floral developmental stages defined. Floret number and percentage seed fill vary with genotype and may be a beneficial selection criterion.