The Inhibition of Growth by Gravitropic Stimulation in Darkness in Agravitropic Primary Roots ofZeaand the Role of Calcium

摘要

The primary roots of the“Golden Cross Bantam 70”cultivar ofZea maysare agravitropic in darkness and their orthogravitropism is light-dependent. Analysis of the agravitropic roots provides important information about the mechanism of orthogravitropism. However, the underlying mechanism of the agravitropic response in darkness is unknown. We found that the growth of intact primary roots was inhibited by gravitropic stimulation (i.e., changing the orientation of the roots from vertical to horizontal) in darkness, but that of detipped roots was not. The role of calcium in this gravistimulation-dependent inhibition of growth was investigated using apical 5-mm segments of the primary roots. The gravistimulation-dependent inhibition of growth was prevented by applying 10 mMMES-KOH buffer at pH 6.0 to the root cap. By contrast, the application of 0.1–1 mMbuffer at pH 6.0 and 10 mMbuffer at pH 4.5–5.0 allowed the gravistimulation-dependent inhibition of growth. Furthermore, when the buffer of 10 mM(pH 6.0) contained 1–5 mMCaCl2, the gravistimulation-dependent inhibition of growth was apparent. By contrast, when weak (1 mM) buffer at pH 6.0 or 10 mMbuffer at pH 4.5 contained 5 mMEGTA, no gravistimulation-dependent inhibition of growth was observed. Thus, the gravistimulation-dependent inhibition of growth in darkness seemed to be mediated by an increase in the level of free Ca2+in the root tip. These results suggest that free Ca2+in the apoplast of the root tip plays an important role in the agravitropic response in darkness as well as in orthogravitropism under light of the roots of this cultivar o