“it seems the actual direction of winding is determined by the plant’s genes and gravity. Japanese researchers (Hashimoto) found that a slight difference in the structure of tubulin, a microtubule protein in cells, determines the winding direction. They chemically mutated a straight growing vine until some wound left (CCW), then looked at the molecular structure of twisting. It was published in Nature. Other Japanese researchers (Kitazawa,et.al) found that gravity sensing cells are indespensible for shoot circumnutation (bending and bowing of the tip) and winding response. It was published in PNAS.So the plant knows which way is up from the gravisensing cells, then the tubulin structure determines whether it winds CW or CCW in relation to up”.
Quinn Smithwick. Cambridge, MA, USA