Changes in ultrastructure and transcription induced by elevated temperature in Zea mays embryonic root cells

A study has been made of the changes occurring in Zea mays kernels exposed to thermal shock at the 48th h of soaking at 16°C, i.e., just before the protrusion of the root. Heat shock of 5 h at 46°C temporarily inhibits the resumption of root growth and consequently retards the protrusion of the root. On the ultrastructural level, it is the nucleolus which undergoes the most dramatic changes. Total loss of the granular component occurs and new electron opaque corpuscles with diameters ranging from 80 to 140 nm appear. These corpuscles contain RNA and proteins. Microstereology shows that the vacuolation of the nucleolus is increased whereas its volume is decreased. Autoradiographical and biochemical studies of RNA synthesis show that the heat shock induces an inhibition of pre-rRNA synthesis. Only RNAs of low molecular weight are still synthesized. After 19 h of a return to 16°C, the ultrastructural changes of the nucleolus are reversed. Nevertheless, increased nucleolar vacuolation persists up to that time. Exposure of maize kernels to 46°C also produces the appearance of corpuscles in some areas of the cytoplasm and in the matrix of mitochondria. Disappearance of these structures is complete within 19 h, except in some mitochondria.