Up-Sizing: The Tale of The Polyploid Giants

By Céline Caseys

Once upon a time, there was a land where food was not guaranteed everyday. Children were searching woods to pick up berries. When they were lucky they would find strawberries. The fruits were small but really tasty. Around the fire at night, they would hear legends about magical beans and giant plants.

Centuries later, for most, the reality is quite different. While ‘Jack and the Beanstalk’ is now a children book derived into a few movies, giant veggies and fruits are real and abundant. And you probably don’t even realize it because it is so normal.

Ploidy, The Up-Sizer

Fruit size of wild and cultivated strawberries. Photo credit: Maine Agriculture in the Classroom

Think about strawberries. Did you ever wonder why they are so giant? Is it just good genes and nutrition, mostly healthy plants that produce larger fruit?

Happy plants definitely helps, but the answer lies elsewhere: ploidy.

What’s ploidy? Is it a sort of monster?

No, not at all! It’s a natural phenomenon.

The ploidy is the number of sets of chromosomes in a cell. Humans are diploid: we have two sets of each chromosome, one from mom, one from dad. Most animals maintain that arrangement.

Plants are far less conservative about chromosome number and tolerate extra sets of chromosomes quite well. A quarter of the plants on this planet are polyploid (>2 sets of chromosomes) including 30% of crops [1].

Polyploidy has some advantages, and both nature and plant breeders have played with it quite extensively. For example, polyploidy makes strawberry giant, banana seedless, cotton fibers more abundant and lily flowers larger and brighter.

More, More & More!

While scientists understood the effects of ploidy, how it increased plant organs size remained a partial mystery. Dana Robinson and colleagues from Cornell University studied the plant model Arabidopsis thaliana to quantify many cell components in chromosome-heavy plants. Their findings were published in The Plant Cell [2].

Flower size and trichome branching for Arabidopsis plants with two to eight sets of chromosomes. Reproduced from Figure 3AB from Robinson et al., 2018., Plant Cell.

There are ways to double chromosome numbers in the laboratory with some common chemicals. Arabidopsis is a diploid (2C; C=number of set of chromosomes) plant but the researchers generated some plants that are tetra- (4C) and octoploid (8C). They confirmed that the flower size increased with ploidy. They also noticed morphological differences on leaf surface with microscopic hairs (trichome) that get more branches when ploidy increases. In this case, it is possible to approximate the number of set of chromosomes by looking at the plants with a magnifying glass.

Drawing of plants at different ploidy and real trace of cells (grey lines) on sepal surface. White dots represent guard cells. Reproduced from Figure 1D and 3D from Robinson et al., 2018., Plant Cell.

To quantify how plant organs change in size, the researchers looked closely at the sepals (the green leaf-like structures under a flower) with a 20x zoom, took images and traced all of the cells on the surface. They repeated their measurements on at least 50 sepals to get reliable estimates.

The researchers found that the cell area increases, but does not exactly double (~1.7x increase) when ploidy doubles. More importantly, the plant compensates for the increase in size by decreasing the number of cells.

The Tale Of An Expansion

Imagine a book. Now let’s imagine the printing of this book goes wrong and duplicates every page. It will generate a book much larger. Well, it’s the same idea with the genome and chromosome number. If it doubles, it takes a lot more space.

Ploidy has a proportional relationship with the nuclear, cell and organ size. KR for Karyoplasmic ratio. Reproduced from Figure 1A from Robinson et al., 2018., Plant Cell.

The nucleus that contains the DNA has to expand to accommodate the extra material, just as the book cover would to accommodate more words. This study shows that having more sets of chromosomes also generates more gene activity. It means a lot more genetic material and its attendant proteins in each cell. All together it’s not just the nucleus but the cell size that increases following a well-known biological law: the karyoplasmic ratio.

If the doubling happens to all the books on the bookshelf, the bookshelf itself will also require a size update. That’s what happens to the organs. If it happened to you, at this point you would probably start throwing out the books you like the least in a space compromise. The plant has the same idea and decreases the cell number in an attempt to compensate the expansion.

Enough! Says the plant.

Scientists have pushed the limits of ploidy in some crops. For example, they have constructed 32-ploid strawberries, and while this fill the nucleus with more chromosome sets, the ultimate size does not increase much anymore at such high levels of ploidy. For comparison, the commercial strawberry you just craved is octoploid (8C).

Scientists are still puzzled about how plants regulate their size. It is amazing that a plant understands how big it is supposed to be, and makes adjustments to ensure it gets there. It opens questions on how do plants sense their ploidy, and when and how do they activate size compensation. The current hypothesis is that because the organs grow larger faster, organ development may stop earlier. The plant must have a way to check on its organ size and then pull the plug on cell division. If you have bigger cells, you don’t need as many of them to fit your genetically predetermined size.

If you were dreaming of an exponentially large strawberry fruit, know that plant cells know their limits and maintain strong mechanisms to enforce them.

Celine Caseys

Department of plant sciences

University of California, Davis

celcaseys@ucdavis.edu

Orcid: 0000–0003–4187–9018

Twitter: C_Caseys

References:

[1] Salman-Minkov A, Sabath, N, Mayrose I. (2016). Whole-genome duplication as a key factor in crop domestication. Nature plants 2: 16115.

[2] Robinson D, Coate J, Singh A, Hong L, Bush M, Doyle J, Roeder A. (2018). Ploidy and size at multiple scales in the Arabidopsis sepal. Plant Cell 30: 2308–2329.