Heat stress results in incomplete C-to-U editing of maize chloroplast mRNAs and correlates with changes in chloroplast transcription rate

The editing status rps14 and rpl20 mRNAs decreased rapidly from nearly 100% edited to about 30% edited when maize plants were shifted from 20 degreesC to 37 degreesC. A decrease in the extent of editing was easily detected wi thin 2 h and decreased to a steady-state level of about 40% C-to-U conversi on at 37 C. In contrast, the editing status of these chloroplast mRNAs incr eased relatively slowly after plants were shifted from 37 degreesC to 20 de greesC. Chloroplasts isolated from maize plants which were grown at 20 degr eesC and then shifted to 37 degreesC for 24 h were 5-10 times more transcri ptionally active than chloroplasts isolated from maize plants grown continu ously at 20 degreesC. Thus, the high transcription rate at 37 degreesC may establish a kinetic condition where the rate of transcription exceeds the c apacity of the editing apparatus and results in incomplete editing.