Light perception by the root is essential for gibberellin-induced primary-root elongation in tomato

Abstract

AbstractPrevious studies in Arabidopsis show that gibberellin (GA) has an important role in the regulation of primary root elongation. However, the significance of the hormone in the regulation of root elongation in some other plant species is less clear, with numerous conflicting reports. Here we have studied the role of GA in primary-root elongation in tomato. Primary-roots of the tomato GA-deficient mutantgib-1are shorter than those of WT and exhibited slower growth rate. Application of GA togib-1right after germination on vermiculite, promoted primary-root elongation, but when the roots were deeper in the growth medium, in the dark, the hormone had no effect on their elongation. Although the global GA transcriptional response was normal in dark grown roots, the output of the GA signal toward cell-expansion genes was blocked. When the primary roots were exposed to light deep in the vermiculite, using LED light source in the growth medium, GA induced their elongation. Moreover, when the GA-deficient mutantsgib-1orga 20-oxidase1seedlings were grown on agar plates, GA promoted primary-root elongation in the light but not in the dark. Using the D-root (shoot in light, root in dark) and D-shoot (shoot in dark, root in light) agar plates, we demonstrated that perception of red light, but not blue or far-red, by the primary roots is essential for the effect of GA on their elongation. Similar light dependent GA-elongation response was found in wheat primary roots, but not in Arabidopsis. We suggest that in tomato and wheat GA promotes root elongation right after germination, when the root is still exposed to low light intensity underground, and this contributes to rapid seedling establishment. Later, dipper in the soil, in the dark, the insensitivity to GA may be important for the continues root elongation under fluctuating water availability (xerotropism), as water deficiency inhibits GA accumulation.