Brassicaceae, the mustard family Economically important plants in the Brassicaceae include:
Vegetable crops, leafy greens Brassica oleracea: cabbage, broccoli, Brussels sprouts, cauliﬂower, kale Brassica rapa: chinese cabbage, bok choy Nastur/um oﬃcianale, N. microphyllum: watercress Eruca vesicaria: arugula Root crops Brassica rapa: turnips Brassica napus: rutabaga Raphanus sa/vus: radish, daikon Flavoring agents Brassica alba, B. nigra: white & black mustard, Armoracia rus/cana: horseradish Eutrema (Wasabia) japonica: wasabi Oil crops Brassica napus: canola (globally the most economically important brassica crop) Dye plants Isa/s /nctoria: woad GeneFc model organism Arabidopsis thaliana: rock cress, complete genome sequenced 2001 Ornamental garden plants (several genera and species)
CharacterisFcs of Brassicacaeae Herbaceous, non-‐woody Fruits are siliques (long and narrow, >3x longer than wide) or silicle (< 3x longer than wide). Siliques and silicles are modiﬁed capsules. Siliques resemble the pods of legumes, but there is a central parFFon, the replum, in a silique.
CharacterisFcs of Brassicacaeae Glucosinolates
Secondary metabolite glycoside molecules (a glucose molecule aUached to a small non-‐ carbohydrate molecule). In glucosinolates the non-‐carbohydrate molecule is a sulfur and nitrogen containing backbone
Glucosinolates are a common aUribute of Brassicaceae. Their natural funcFon is to deter herbivory and aUack by parasiFc fungi, nematodes etc. They are the source of disFnct
glucose molecule pungent ﬂavors in many Brassicaceae. When the glucose molecule is cleaved oﬀ by the enzyme myrosinase, the resulFng sulfur-‐nitrogen molecule is unstable and spontaneously converts to isothiocyanate, the acFve ingredient in some commercial fumigants and mustard gas. Glucosinolates and myrosinase are kept in separate compartments within cells and only come into contact if the cell is damaged. When a cell is damaged the myrosinase acts quickly on the glucosinolate to release the cytotoxic isothiocyanates. While the isothiocyanates are toxic to many insects, cabbage moths use the glucosinolates to speciﬁcally idenFfy Brassica species that are suitable food plants for larvae.
Arabidopsis, a model for plant geneFcs and genomics One species of Brassicaceae that has become economically very important, but is not a food plant, is Arabidopsis thaliana. Arabidopsis is important because it has become a research model organism for understanding the geneFcs and development of plants, much like Drosophila and E. coli. Arabidopsis thaliana (n=5) is a wild Brassicaceae, a weed. But it has become a model for plant genomic research because of its small genome size ~125 Mb), small size, and because of its rapid growth and ﬂowering cycle; it can grow from seed to seed in about six weeks (48 days). A. thaliana was the ﬁrst plant to have its complete genome sequenced (completed in 2001) and has since become a reference model for genome organizaFon in plants and geneFc regulaFon of plant development. A. thaliana is a geneFc/genomic model for all plants but research on the A. thaliana genome has parFcularly helped reveal interesFng facts about the evoluFon of culFvated Brassica species. There is a great deal of informaFon about Arabidopsis and related research available on the web. A good place to start is The Arabidopsis InformaFon Resource (TAIR): hUp://www.arabidopsis.org/index.jsp
EvoluFon of Brassica species Naga-‐hara U proposed in 1935 that hybridizaFon between three diploid Brassica species, B. nigra (black mustard, n=8), B. rapa (turnip, n=10) and B. oleracea (cabbage, n=9) had resulted in three hybrid species: B. juncea (Indian mustard, Chinese mustard, n=18), B. carinata (Ethiopian mustard, n=17), and B. napus (oilseed mustard, canola, n= 19). RelaFonships between the diploid and hybrid species is shown in “U’s triangle” diploid
EvoluFon of Brassica species The hybrid origins of species suggested by U’s triangle has been used by plant breeders to combine new traits, such as disease resistance from wild species by producing new hybrid crop species. Genome research with Arabidopsis has now revealed that the “diploid” genomes of the Brassica species in U’s triangle actually appear to be three copies of an ancient genome very similar to that of Arabidopsis. The diploid Brassicas actually represent ancient hexaploids that have been reshuﬄed over the past 10-‐20 million years. That means that the hybrid species in U’s triangle are not really tetraploids (allopolyploids between two diploid parents) but are actually dodecaploids (12 copies of the genome, allopolyploid hybrids between two hexaploid parents. However it gets worse! Further research on the Arabidopsis genome showed that it is actually an ancient tetraploid. That means that the “diploid” Brassicas in U’ triangle have 12 copies of the original genome (dodecaploid) and the “tetraploid” species are 24-‐ploids.
years before present
EvoluFon of Brassica species Arabidopsis diploid ancestor
ploidy and genome size
TRUE DIPLOID, 2n 60Mb
ﬁrst hybridizaFon or spontaneous autopolyploid
Ancestor of present-‐day Arabidopsis
TETRAPLOID 120 Mb
second hybridizaFon or spontaneous autopolyploid
First Brassica ancestor hybridizaFon or spontaneous autopolyploid third Second Brassica ancestor
OCTOPLOID 220 Mb
DODECAPLOID 440 Mb
genome rearrangement, speciaFon
5 million 4 million
Modern ‘diploid’ Brassica species
DODECAPLOID 500-‐600 Mb
Modern ‘tetraploid’ Brassicas
24-‐ploid 1200 Mb
EvoluFon of Brassica species What do Brassica species do with all the ‘extra’ genes? If a 60 Mb genome was suﬃcient for the existence of a precursor species 40 million years ago, why do modern day Brassica species need genomes of 1200 Mb? One answer is that the addiFonal genes have allowed Brassicas to develop more metabolic diversity/versaFlity. Brassica species produce a wide range of secondary metabolites, including the glucosinolates, that help them to deter herbivores and parasites. The cabbage moth has evoloved resistance and adapted to use the glucosinolates in Brassica species as an aUractant. So in response the Brassicas have evolved to produce volaFle chemicals that are aUractants to insect predators and parasites of the cabbage moth. VolaFle chemicals released from Brassica plants are aUracFve to parasiFc wasps that deposit their eggs in the eggs and larvae of the cabbage moth.
parasiFc wasp pupae next to cabbage moth larva
cabbage moth adult
DomesFcaFon of Brassica species The ﬁrst culFvated Brassica species was probably B. rapa (turnip) grown for their seed oil ca. 4000 BP Brassica rapa was domesFcated repeatedly from wild populaFons from the eastern Mediterranean to India. Wild B. rapa likely occurred as a common weed with culFvated cereals (wheat and barley). Brassica oleracea was probably ﬁrst used as leafy kale in the Mediterranean as well as China. Its use was menFoned by ancient Greeks from at least 2500 BP. Oilseed rape, B. napus, was the most recently culFvated species, not being widely culFvated unFl around 1000 BP. The main use of culFvated oilseed rape was not for cooking oil but rather for fuel for lighFng. The development of low erucic acid oils (canola) from B. napus is even more recent, daFng to the 1970s. Erucic acid is biUer and poisonous, and occurs at high
concentraFons in most wild B. napus, which had prevented its use as a food oil. “Canola” is a patented trade name and stands for ‘Canadian oil low acid’. B. napus is now considered the most valuable of the culFvated Brassica species, and is the third most important oilseed crop worldwide, ater palm and soybean.