Watch the episode · 4:19Every auto you have ever grown carries the same tiny typo in its DNA. Regular plants wait for long nights before they flower; autos don't. For decades nobody knew exactly why. Now researchers have found the gene: a timer with a broken brake. And the strange part is that rice and sorghum farmers ended up with the very same broken gene, completely independently.
How plants read the night ▶ 0:32
How does a plant even know what season it is? It has a clock, a real one. Inside every leaf, a set of clock genes switches on and off in a 24-hour rhythm, like an alarm clock that resets every morning. The plant compares that internal clock with the light outside, and that is how it measures the night.
Most cannabis is a short-day plant. Indoors, under 18 hours of light, most photoperiod plants will stay in their growing phase indefinitely. Cut the light to 12 hours and they flower.
Source · Leckie et al., bioRxiv preprint (2023)
The brake ▶ 1:06
One of those clock genes is called PRR37. Under long summer days it acts as a brake, holding back the plant's flowering signal, a protein called florigen. When the nights grow long the brake eases off, florigen builds up, and the plant flowers.
- 01Long days
the clock reads summer
- 02Brake on
PRR37 holds back the flower signal
- 03Long nights
the brake eases off
- 04Flower
florigen builds up
Now imagine that brake simply doesn't work. Florigen is free to build up from the start, and the plant flowers as soon as it is old enough, whatever the light. That is an autoflower.
The typo ▶ 1:36
In 2023, a team at a Canadian breeding lab compared photoperiod and autoflowering plants and shared their results as a preprint (not yet peer reviewed). In autos they found a single mutation in PRR37, at what is called a splice site.
Source · Leckie et al., bioRxiv preprint (2023)
Before a gene is used, its message is cut and pasted together, like editing a recipe before you cook. The mutation spoils one of the cut marks. The edit goes wrong, the recipe is scrambled, and the protein comes out cut short: no working brake.
It also explains why the trait is recessive, as a Cornell team found when they mapped it in 2022. One working copy is enough to keep the brake on. Cross two plants that each carry one broken copy and about one in four seeds is an auto.
Source · Toth et al., Front. Plant Sci. (2022)
The same fix, twice ▶ 2:29
Now, that strange part. Sorghum and rice have their own versions of PRR37. In sorghum it is the classic maturity gene Ma1; in rice it is called Hd2. In both crops, weakened versions helped farmers grow them further north, where they flower through long summer days.
Source · Murphy et al., PNAS (2011) · Koo et al., Mol. Plant (2013)
Cannabis seems to have found the same fix. Wild plants in northern China have been found flowering under long days, and the ruderalis from episode 3 comes from short Siberian summers. Up north, a plant that waits for long nights risks being caught by the frost.
| Photoperiod | Autoflower | |
|---|---|---|
| PRR37 | working | broken by one mutation |
| Under long days | brake on: keeps growing | no brake: flowers by age |
| Inheritance | one working copy is enough | recessive: needs two copies |
Why autos can't pause ▶ 3:21
With a photoperiod plant, you decide when flowering starts by changing the light. An auto's clock starts the day it sprouts. Stress, a late transplant, a slow start: none of it buys extra time. That is why the 5 autoflower mistakes cost so much.
PRR37 is a clock gene that brakes flowering. Autos carry a typo in it, so with no brake they flower by age. The trait is recessive, and rice and sorghum broke the same gene all on their own.
Follow us on Google
Add Seedscanner as a preferred source and Google shows you more Seed Science in Top Stories.
Sources
- Leckie, K.M. et al. (2023). "Loss of daylength sensitivity by splice site mutation in Cannabis." bioRxiv preprint, doi:10.1101/2023.03.10.532103 (not yet peer reviewed).
- Toth, J.A., Stack, G.M., Carlson, C.H. & Smart, L.B. (2022). "Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L." Frontiers in Plant Science 13: 991680.
- Murphy, R.L. et al. (2011). "Coincident light and clock regulation of pseudoresponse regulator protein 37 (PRR37) controls photoperiodic flowering in sorghum." PNAS 108(39): 16469–16474.
- Koo, B.-H. et al. (2013). "Natural variation in OsPRR37 regulates heading date and contributes to rice cultivation at a wide range of latitudes." Molecular Plant 6(6): 1877–1888.
Every claim in this article comes from the episode script, which is checked against these papers. Prices on Seedscanner are dated observations; this page is not growing or legal advice.
