Laboratory of Ann Hirsch


Facts About Sweetclover


Sweetclover (Melilotus Adans.) has been known as a important  crop since the times of the ancient Greeks, who used this legume for  medicinal purposes as well as for flavoring food. Besides being used as a  source of honey (Harman, 1992), the major use of sweetclover today is as a forage and  soil improving crop. 

Most sweetclovers used for agricultural purposes are biennial. The  biggest use of sweetclover for pasturing is in the northern United States  and southern Canada. Details on sweetclover production and management can be obtained from a number of sources, including the North Dakota State University Extension Service and Ohio State University. Sweetclovers grow on soils  where other legumes fail because the plants tolerate wet or dry conditions  as well as soils with low nutrient availability. Sweetclover species are  also remarkably salt tolerant, and some are even adapted to grow in subartic  climates. However, sweetclover requires a soil pH of 6.5 or more for good  root nodulation and growth. 

Several seed companies sell large quantities of either white (M.  alba) or yellow (M. officinalis) seeds (see Seed Sources). If the seed sources  are prefaced by the word "common", this means that they consist  of mixed varieties. An annual variety of M. alba is "Hubam";  more recently developed varieties are "Israel" and "Floranna".  The low coumarin varieties are biennials ("Polara" for M.  alba and "Norgold" for M. officinalis). 

Recently, a group of scientists have been involved with utilizing  sweetclover, specifically the annual white sweetclover (Melilotus alba annua Desr.) as a genetic system to study the interaction of this legume  with its nitrogen-fixing symbiont Rhizobium meliloti. The  importance of model organisms such as yeast, the fruit fly Drosophilamelanogaster or the flowering plant Arabidopsis  thaliana has become clear in recent years as more and more  information is gleaned from the study of these "simple"  organisms. Melilotus alba has the potential to be a model for  studying the genes involved in plant-microbe interactions because it has a  number of distinct advantages over many other small-seeded, diploid  legumes. 

  1. Like Medicago and Trigonella species, Melilotus is nodulated by Rhizobium meliloti, one of the most well studied Rhizobium species from the perspective of molecular biology and  genetics. To learn more about nitrogen  fixation, connect to this website. Also, unlike other legumes, Melilotus is very small making it very useful for screening for mutants. 

  2. A long-standing investigation of the genetics of Melilotus species is in existence due to the pioneering efforts of H.J. Gorz and  F.A. Haskins (now Professors Emeriti of the University of Nebraska). A  1965 paper by Smith and Gorz summarized the genes that were known up to  then (see the Sweetclover Mutant List)

    Many of the original lines and mutants from Gorz and Haskins as well as  all the species in the genus and older and newer cultivars are stored at  the North Central Region Plant Introduction Center at Iowa State  University in Ames, Iowa 50011. David Brenner is  the curator. The Plant Introduction Center has a 900 accession  collection of sweetclover germplasm. The germplasm is distributed in  packets of 100 seeds worldwide free of charge for research and  development projects. The goal of the Center is to conserve the  germplasm with high viability and genetic integrity. Most of the seed  regenerations are done with caged bees for controlled pollination within  single accessions. 

  3. A number of symbiotic mutants have been generated in white  sweetclover using EMS and gamma-irradiation by Thomas  A. LaRue and colleagues at the Boyce Thompson Institute (see the Sweetclover Mutant List).  LaRue found that M. alba is easily mutable; it is second only to  pea in the number of mutants that have been obtained.

    The symbiotic mutants have been or are being intensively studied by Joanna H. Norris of the University of Rhode Island, Ann  M. Hirsch of the University of California, Los Angeles, Barry Rolfe and Michael Djordjevic of the Australian National University in Canberra,  Australia, and Rebecca Wais of the Long Lab of Stanford University. In addition, LaRue  and colleagues generated a number of anthocyanin-deficient mutants,  several of which are being examined by Nancy L. Paiva  of the Samuel Roberts Noble Foundation and Ann M. Hirsch of the  University of California, Los Angeles. John Markwell  of the University of Nebraska is studying sweetclover chlorophyll  mutants. 

  4. The Melilotus genome is relatively small; N = 8 and the DNA content of M. officinalis has been measured by Arumuganathan and Earle  (1991) as 1086 megabasepairs (MBP) per haploid genome; the genome size of M. alba is as yet unknown. 

  5. A partial linkage map has been generated for M. alba and M.  polonica using morphological, isozyme, and RAPD markers by Norman F. Weeden of Montana State University. The maps for each of  these species is relatively short: 252 centimorgans with 62 markers for M. alba and 314 centimorgans with 81 markers for M. polonica.

  6. The flowers are autogamous and self without having to be tripped. In  continuous light, the first open flowers appear after 4 weeks after  planting. With long day conditions (18 hours of light), flowering starts  later. Depending on the lighting conditions, ripe seed is present 8 to  10 weeks after planting. Thus, Melilotus goes from seed-to-seed  in 2.5 months or less depending on the growing conditions. 

  7. The plants have an erect habit and grow to approximately 12 inches.  However, a dwarf line, U390, is very short (less than 10 inches) and is  currently being used for genetic studies. 

  8. Special crossing techniques (see Methods for Studying Sweetclover) have been developed for spontaneously  self-pollinated lines of sweetclover. Crossing in ordinary sweetclover  is much easier than in the U389 and U390 lines. 

  9. Sweetclover can be transformed by Agrobacterium tumefaciens (see  Methods for Studying Sweetclover). 

  10. Sweetclover seeds are long-lived in part due to their hard seed  coats. They need to be scarified; otherwise germination is very low (see Methods for Studying Sweetclover). 

References