Thermomorphogenesis of the Arabidopsis thaliana root: Flexible cell division, constrained elongation, and the role of cryptochrome.

Understanding how plants respond to temperature is relevant for agriculture in a warming world. Responses to temperature of the shoot have been characterized more fully than those of the root. Previous work on thermomorphogenesis in roots established that for Arabidopsis thaliana (Columbia) seedlings grown continuously at a given temperature, the root meristem produces cells at the same rate at 15 as at 25ºC and the root's growth zone is the same length. To uncover the pathway(s) underlying this constancy, we screened 34 A. thaliana genotypes for parameters related to growth and division. No line failed to respond to temperature. Behavior was little affected by mutations in phytochrome or other genes that underly thermomorphogenesis in shoots. However, a mutant in cryptochrome2 was disrupted substantially in both cell division and elongation, specifically at 15ºC. Among the 34 lines, cell production rate varied extensively and was associated only weakly with root growth rate; in contrast, parameters relating to elongation were stable. Our data are consistent with models of root growth that invoke cell non-autonomous regulation for establishing boundaries between meristem, elongation zone, and mature zone.