ANALYSIS OF CHANGES IN RELATIVE ELEMENTAL GROWTH-RATE PATTERNS IN THEELONGATION ZONE OF ARABIDOPSIS ROOTS UPON GRAVISTIMULATION

Although Arabidopsis is an important system for studying root physiolo gy, the localized growth patterns of its roots have not been well defi ned, particularly during tropic responses. In order to characterize gr owth rate profiles along the apex of primary roots of Arabidopsis thal iana (L.) Heynh (ecotype Columbia) we applied small charcoal particles to the root surface and analyzed their displacement during growth usi ng an automated video digitizer system with custom software for tracki ng the markers. When growing vertically, the maximum elongation rate o ccurred 481 +/- 50 mu m back from the extreme tip of the root (tip of root cap), and the elongation zone extended back to 912 +/- 137 mu m T he distal elongation zone (DEZ) has previously been described as the a pical region of the elongation zone in which the relative elemental gr owth rate (REGR) is less than or equal to 30% of the peak rate in the central elongation zone. By this definition, our data indicate that th e basal limit of the DEZ was located 248 +/- 30 pm from the root tip. However, after gravi-stimulation stimulation, the growth patterns of t he root changed. Within the first hour of graviresponse, the basal lim it of the DEZ and the position of peak RECR shifted epically on the up per flank of the root. This was due to a combination of increased grow th in the DEZ and growth inhibition in the central elongation zone. On the lower flank, the basal limit of the DEZ shifted basipetally as th e REGR decreased. These factors set up the gradient of growth rate acr oss the root, which drives curvature.