Species Spotlight: Crocus

This is a post from my old PhD blog. It was originally published 3/12/16

We are barreling towards spring here in the Northern Hemisphere. One of the early signs of spring are crocus flowers. The Crocus genus contains about 100 species. The flowers are cone shaped and live for about 3-4 days while they produce a massive amount of pollen. It's estimated that 1 flower can produce 1.9 milligrams (that's almost 7 ounces) of pollen!

Crocus from grandparents yard - personal image

Crocus often start to bloom in late winter/early spring. They can break through the snow. The flowers are, on average, 3 degrees Celsius higher than surrounding ambient temperature. The bowl shape of the flowers helps keep the heat in the interior floral area. Because they flower early, produce a lot of pollen, and generate heat in the flowers they are crucial for bee survival.

Embed from Getty Images

If you have any saffron in your house, you have a container of Crocus sativus stamen (flower male sex organs). Saffron is the most expensive spice in the world. It takes 70,000 hand-picked flowers stripped of their stamens to make 0.45 kilogram (about 1 pound). The scent, flavor, and color of saffron comes from unique carotenoids that are produced only by C. sativus. Technically, these compounds are apocarotenoids as they are shorter chains of carbon cut from zeaxanthin (which is also important in the xanthophyll cycle). The 3 apocarotenoids produced in the stamens of C. sativus are crocin, responsible for the red color, picrocrocin, providing the bitter taste, and safranal resulting in the characteristic aroma.

Researchers have been trying to elucidate the pathway used to produce saffron in the hopes of being able to synthesize it. Recently, the enzyme responsible for the initial step in this process was discovered to be a member of the carotenoid cleavage dioxygenase enzyme family.  All CCD are rigid 7-bladed propeller shaped with a central iron active site surrounded by 4 histidine residues which facilitate oxygen induced cleavage (cuts) of double bonds in carotenoids. CCD2 is the first enzyme to be described that is unique to saffron synthesis. CCD2 cuts zeaxanthin at the 7,8 double bond on both ends of zeaxanthin. Each bond is cut individually in a two step process to produce crocetin dialdehyde which is further processed to produce crocin, picrocrocin, and safranal.

It's amazing how one little flower can be so important and potent!

References

Frusciante, et al (2014) Novel carotenoid cleavage dioxygenase catalyzes the first dedicated steps in saffron crocin biosynthesis. Proceedings of the National Academy of Sciences of the United States of America 111 (33):12246-12251. doi:10.1073/pnas.1404629111 

McKee and Richards 1998 Effect of flower structure and flower colour on intrafloral warming and pollen germination and pollen-tube growth in winter flowering Crocus L. (Iridaceae) Botanical Journal of the Linnean Society 128: 369-384

Weryszko-Chmielewska and Chwil, 2011. Structure of the Floral Parts of Crocus vernus (L.) Hill Acta Agrobotanica 64(4):35-46 https://pbsociety.org.pl/journals/index.php/aa/article/viewFile/aa.2011.044/1133
Whitney and Chittka 2007 Warm flowers, happy pollinators Biologist 54(3) 154-160. http://chittkalab.sbcs.qmul.ac.uk/2007/Biol_54%283%29_Chittka.pdf