Abstract
A mechanistic model for circumnutation must explain the salient features of seedlings' circumnutational behaivior. Gravity is not required but can influence circumnutation. Mechanical perturbations often have an immediate effect on the oscillations. Circumnutations are absolutely growth dependent. Because circumnutation is clearly advantageous to the plant only in a few of cases (such as in twining plants) but is universally found in the plant kingdom, some fundamental growth process must underlie the behavior.
Our studies of Arabidopsis twisting mutants suggest that defects in cortical microtubule (cMT) organization underlies the helical growth. When cMT arrays are arranged in right- or left-handed helices, nascent cellulose microfibrils may adopt the same skewed orientation parallel to the cMT arrays. Wild-type Arabidopsis roots tend to grow askew to the left; "wild-type" cells thus might elongate slightly askew to the organ axis. I will discuss the dynamic oscillation of cMT arrangement as a possible cause of circumnutation.
