Expression of the genes encoding hsp73, hsp18, and ubiquitin in radicles of heat-shocked maize seedlings

Abstract

Radicles of intact 5-day-old maize (cv. OH43) seedlings exposed to a rapid, short-term elevation (2 h) in environmental (25 to 42.5 °C) temperature exhibit new and (or) enhanced synthesis of a specific set of proteins, the so-called heat shock proteins (hsps), with molecular masses of 108 000, 89 000, 84 000, 76 000, 73 000, 30 000, 23 000, and 18 000. Continuous exposure of intact seedlings to this elevated temperature results in a depression in the synthesis of these hsps after 8 – 12 h and a shift in the pattern of the proteins synthesized to one that resembles those proteins synthesized by radicles from seedlings grown at 25 °C. The transient synthesis of hsp73 and the hsp18 family in radicles from seedlings exposed to, and maintained at, an elevated temperature correlates with the levels of hsp73 and hsp18 mRNAs associated with their polyribosomes. The heat shock induced accumulation of these hsp mRNAs occurs concomitantly with a fourfold increase in polyribosome-associated ubiquitin mRNAs. However, unlike the transient association of hsp73 and hsp18 mRNAs with polyribosomes in radicles from seedlings maintained at 42.5 °C (8 – 12 h), the level of uniquitin mRNAs associated with polyribosomes does not return to a steady-state control (25 °C) level for at least 24 h. The marked, rapid increase in the level of ubiquitin mRNAs associated with polyribosomes in radicles from heat-shocked seedlings provides the first evidence implicating ubiquitin as a heat shock protein of plants.Key words: heat Shock, heat shock proteins, ubiquitin, maize.