An important feature of plants is the ability to adapt their growth towards or away from external stimuli such as light, water, temperature and gravity. The physiological process of root gravitropism comprises gravity perception, signal transmission, growth response and the reestablishment of normal growth. Following are some of the modern concepts explaining the mechanism of root gravitropism. Which of the following best explains root gravitropism?

Statoliths within columella cells of root cap sediment in the direction of gravity, resulting in the generation of a signal that causes asymmetric growth.

Auxin influx and efflux carriers facilitate creation of a differential auxin gradient between the upper and lower side of gravistimulated roots. This causes differential growth responses in the elongation zone, leading to root curvature.

Curvature in geostimulated roots is due to the lateral redistribution of an inhibitor formed in the root cap.

Proplastids in root cap containing carotenoids and protochlorophyll respond to gravity.