Mapping the citrus genome

Mapping the citrus genome

Understanding the citrus species’ past to unlock a more disease-tolerant future 

Citrus was first domesticated in Southeast Asia, then spread to Europe and the Americas via trade routes, and has ancestral roots that can be traced back to over five million years ago. Interestingly, the orange you may be eating today is thought to have originated from two wild citrus species, citrus maxima and citrus reticulata. For the last ten years, Dr. Fred G. Gmitter at the UF/IFAS Citrus Research and Education Center in Lake Alfred has been working collaboratively with citrus scientists from Brazil, France, Spain, Italy, and the United States to map the entire citrus genome.

Through traditional breeding methods like grafting, in which the scion (fruit-bearing part of the tree) is propagated onto the rootstock, the new tree will produce fruit quickly. However, the fruit will all be genetically identical, and unfortunately, that includes identical disease susceptibility.

“Citrus has incestuous genes— nothing is pure,” explains Dr. Gmitter. Since the modern cultivated citrus trees have such a “narrow genetic diversity,” Dr. Gmitter and his team of scientists hope to be able to use their new understanding of the citrus genome to identify sequences that will deploy genes for resistance to citrus greening, the devastating disease caused by bacterial infection via the Asian citrus psyllid. Dr. Gmitter’s genetic analysis of sweet and sour oranges was published recently online in the journal Nature Biotechnology. He is the chairman of the International Citrus Genome Consortium and has employed the efforts of US GENOSCOPE France and IGA Italy to sequence a full catalog of all genes in those varieties, as well as the DNA structure of the genome.

The wild species citrus maxima gave rise to the modern pummelo, the largest citrus fruit, which can weigh up to four pounds or even more. Today’s modern mandarins are genetic mixtures of Citrus reticulata and pummelo; sweet oranges are a complex hybrid, constituted by parts of the pummelo and mandarin genomes. Because sweet orange is the world’s most commonly grown citrus species, the benefits of researching its ancestral roots are endless. Dr. Gmitter explains, “Now that we understand the genetic structure of sweet orange, for example, we can imagine reproducing early citrus domestication using modern breeding techniques that could draw from a broader pool of natural variation and resistance.”

Dr. Gmitter’s tireless work to map the entire citrus genome has enabled him and his team to now use that information to work on finding the genetic sequence in sweet orange that can be manipulated to improve the tree’s disease resistance, response to environmental stress, fruit flavor, and even health-promoting benefits. Genetic modifications that could potentially eradicate citrus greening are now on the horizon!

Dr. Gmitter’s full publication can be viewed here: http://www.nature.com/nbt/journal/v32/n7/full/nbt.2906.html