ROOT HAIR-GROWTH IN ARABIDOPSIS-THALIANA IS DIRECTED BY CALCIUM AND AN ENDOGENOUS POLARITY

Tip growth of plant cells has been suggested to be regulated by a tip- focused gradient in cytosolic calcium concentration ([Ca2+](c)). Howev er, whether this gradient orients apical growth or follows the driving force for this process remains unknown. Using localized photoactivati on of the caged calcium ionophore BrA23187 we have been able to artifi cially generate an asymmetrical calcium influx across the root hair ti p. This led to a change in the direction of tip growth towards the hig h point of the new [Ca2+](c) gradient. Such reorientation of growth wa s transient and there was a return to the original direction within 15 min. Root hairs forced to change the direction of their growth by pla cing a mechanical obstacle in their path stopped, reoriented growth to the side, and grew past the mechanical blockage. However, as soon as the growing tip had cleared the obstacle, growth returned to the origi nal direction. Confocal ratio imaging revealed that a tip-focused [Ca2 +](c) gradient was always centered at, the site of active growth. When the root hair changed direction the gradient also reoriented, and whe n growth returned to the original direction, so did the [Ca2+](c) grad ient. This normal direction of apical growth of Arabidopsis thaliana ( L.) Heynh. root hails was found to be at a fixed angle from the root o f 85 +/- 6.7 degrees. In contrast, Tradescantia virginiana (L.) pollen tubes that were induced to reorient by touch or localized activation of the caged ionophore, did not return to the original growth directio n, but continued to elongate in their new orientation. These results s uggest that the tip-focused [Ca2+](c) gradient is an important factor in localizing growth of the elongating root hair and pollen tube to th e apex. However, it is not the primary determinant of the direction of elongation in root hairs, suggesting that other information from the root is acting to continuously reset the growth direction away from th e root surface.