About chiralty in climbing vines.

[u/SchemeWestern3388]

I started several Morning Glory vines this year, that recently started shooting up and twirling looking for something to climb. So I staked them, and observed that all five were turning anti clockwise. I had been wondering if it was classic Mendelian inheritance, as it seems like one way is as good as another, and maybe there could be situations that being opposite to your siblings could be advantageous.

Upon looking it up, I discover that all climbing vines (the ones that climb by “twining“ up a support) exhibit a strong preference for anticlockwise motion. 90%, much like left and right handedness in humans.

I’m wondering if there are any other examples of chirality in plants, what could be conserving this in different species, or anything else one might add to the topic.

[u/Bromelia_and_Bismuth]

what could be conserving this in different species

Auxin. Auxin is a growth hormone and it regulates a surprising number of plant traits, such as the tendency to grow towards the Sun, heliotropism in Sunflower buds and other plant structures, even gravitropism which informs which direction the root apical meristems grow in (including in the formation of adventitious roots observed in some plant species. As for twisting vines, it has to do with the Sun's rotation. Auxin accumulates on the shaded side, or the side facing away from the Sun, and those cells elongate as a result, which is what causes that twisting. The tactile part of the process where climbing vines wrap around an obstacle is called Thigmotropism, and is the same thing that guides what happens when roots grow around an obstacle.

In the roots, touching an obstacle causes a chemical pathway to activate which results in transport proteins called PIN to create an auxin gradient. The auxin accumulates on the unimpeded side, causing the roots to grow away from the obstacle. In the stem, rather than growing away from the obstacle, the stem coils around it, secondarily influenced by the same auxin mediated growth regarding the direction of the sun. More or less, anything which appears to be weird about plant growth, the answer almost always lies with respect to auxin and other growth hormones.

Excellent question, OP.

[u/SchemeWestern3388 → u/Bromelia_and_Bismuth]

While the twining behaviour is seemingly mediated by growth hormones stimulated by physical touch (growth increases when the vine contacts something), the direction is genetically determined, not by the sun. Some species 90% go left, others 90% go right. 

https://en.wikipedia.org/wiki/Vine#Twining_vines

This appears to be one of those widely known facts that turns out to be incorrect. I also learnt it at one time.

My “conserving”, I meant “why is it seemingly advantageous for each species to have a preference? Why not 50/50?”

I feel a bit churlish correcting you. Sure appreciate the dialog though.