Citrus: A Family Affair
Citrus are some of the most widely recognized and consumed fruits in the world. Citrus, while thought to be native to China and Southeast Asia, has been grown for centuries throughout the world and is even referenced in the Bible.
The modern citrus is the result of the hybridization between the parents of the Citrus genus. In fact the family tree of Citrus reads like one of the major houses from Game of Thrones. In plants, hybridization refers to the mating of dissimilar genotypes. In other words, closely related species which have minor differences in their DNA sequence can be cross-bred to produce a hybrid of the two parents.
In the family tree below, parents of the Citrus genus were crossed to make some of the mainstream crops we eat today. These hybrids were then hybridized again, either with other parents or other hybrids, to produce even more varieties of Citrus. I wasn’t kidding with that Game of Thrones reference.
Hybridization is beneficial in a natural setting as it increases genetic diversity and therefore, the likelihood that the offspring of the parents survive. In cultivation, hybridization is used to combine desirable genes from two different varieties into one.
For thousands of years, Citrus has been continually hybridized and interbred to produce the citrus we know today. For example, it is speculated that the sweet oranges we eat today resulted from the hybridization between primitive Citrus species closely related to the Mandarin and Pummelo.
Citrus family tree. (Velasco & Licciardello, 2014)
Given the longevity of the Citrus life cycle, hybridization within the family is not an ideal practice as it takes several years to produce fruit. This means it takes decades to produce a single new family member. The introduction of transgenes is also a very time consuming process for Citrus so you won’t be seeing any transgenic Citrus on the market in the near future.
However, improving Citrus is not a lost cause and the solution has been around for a millennia. Grafting has been used for centuries to make a desirable plant and involves the physical joining of the root system (rootstock) of one plant to the shoot (scion) of another.
Typically, a rootstock will have advantageous traits, such as the ability to resist certain pathogens, withstand droughts and low-quality soil, or change the shape of the plant to be more accessible during harvest. However, these plants used for their roots can have terrible fruit. Bitter or sour fruits with many seeds don’t sell well, but their rootstocks are what we value.
Fruit of the Troyer citrange, a common rootstock that produces sour fruits with many large seeds. Photo credit: UC Riverside Citrus Variety Collection
In contrast, certain varieties are prized for their scions which produce the fruit we are familiar with today. Unfortunately, they may be highly susceptible to a particular pathogen or lack the ability to withstand a drought. Therefore, they can be grafted to a rootstock that is tolerant of a particular stressor. The scion and rootstock form a physical relationship, meaning that their cells connect to one another and exchange information. The information shared between the two come in the form of genetic material, such as certain RNAs (similar to the stuff inside your Moderna or Pfizer Covid vaccine) and various other molecules which undergo long distance transport to reach their final destination. The information exchanged between scion and rootstock ultimately turns on or off specific genes and allow the hybrid plant to thrive in an otherwise hostile environment.
It will take more than the traditional and popular methods to improve our crops in the ever-changing world. Citrus provides us with a novel challenge given the length of time it takes to reach maturity. Luckily, the solution has been around for thousands of years through grafting. Coupling grafting with hybridization will generate newer, tastier varieties that can survive whatever our world throws at them.