Transcriptional and post‐transcriptional processes regulate gene expression in oxygen‐deprived roots of maize

Abstract

SummaryTo investigate the regulation of gene expression in maize (Zea mays L.) in response to oxygen deprivation (flooding), we quantitated run-on transcription in isolated nuclei, steady-state mRNA accumulation and mRNA loading with ribosomes for seven genes that encode proteins synthesized predominantly in oxygen-deprived roots (anaerobic polypeptides) and seven genes that encode proteins synthesized in aerobic roots (aerobic polypeptides). Run-on transcription of anaerobic polypeptide genes was induced in response to oxygen deprivation and run-on transcription of the aerobic polypeptide genes continued during the stress treatment. The increased accumulation of mRNAs that encode anaerobic polypeptides occurred concomitant with the induction of gene transcription and efficient association of these mRNAs with large polysomes. The steady-state accumulation of aerobic polypeptide mRNAs was within twofold of aerobic levels and in a number of cases fewer ribosomes were loaded per transcript. These results demonstrate that selected synthesis of anaerobic polypeptides involves transcriptional as well as significant post-transcriptional regulation of gene expression. The repression of synthesis of many aerobic polypeptides occurs without interruption of gene transcription and is due to translational regulation and possibly the sequestration of mRNAs on mRNPs. Ribosome loading patterns indicated that this translational control occurs at both initiation and post-initiation phases in a messagespecific manner.

Evaluation of Translational Control Mechanisms in Response to Oxygen Deprivation in Maize

“…There is also evidence of decreased elongation of translation of mRNAs under anoxia. In oxygendeprived maize roots SH1 mRNA was associated with polysomes but inefficiently translated [13][14][15]. Similarly, in oxygen-deprived potato tubers wound-induced mRNAs were associated with polysomes but poorly translated [16].…”

“…4). In aerobic roots both normal cellular protein and stress protein genes (i.e., Adh1) are transcribed [12,13]. Under aerobic conditions both classes of transcripts appear to be translated.…”

“…The translation of an individual mRNA can be examined by determination of the density of polysomes formed on that transcript. When maize roots were deprived of oxygen, mRNAs that encode anaerobic polypeptides (ANPs, also referred to as stress proteins) (e.g., alcohol dehydrogenase-1 (ADH1), alcohol dehydrogenase-2 (ADH2), sucrose synthase-1 (SH1), sucrose synthase-2 (SH2), aldolase, and enolase) were associated with large polysomes, whereas mRNAs that encode nonstress cellular proteins (e.g., actin, mitochondrial adenine nucleotide translocator, eukaryotic initiation factor 4A (eIF4A), and ribosomal protein P2a) were maintained but show dramatically reduced association with polysomes [12,13]. There is also evidence of decreased elongation of translation of mRNAs under anoxia.…”

Pursuing the identification of O2deprivation survival mechanisms in plants related to selective mRNA translation, hormone-independent cellular elongation and preparation for the arrival of oxygen

“…4 Core anaerobic and aerobic responses have been proposed based on transcriptional changes that occur during switch experiments, 5 which suggests that selective transcription of genes under anoxia is likely to be linked with specific promoter responses. 6,7 There is concern however over the degree of predictive power that these measurements offer since transcription does not necessarily result in translation and the accumulation of a functional protein product. 8 When we compared published transcript fold differences 4 with our protein abundance ratios, we found the correlation to be r = 0.62 (Fig.…”

Transcriptional and post-transcriptional regulation of gene expression in submerged root cells of maize

“…Although a large number of aerobic transcripts are also induced in response to oxygen deprivation, fewer aerobic transcripts are loaded with ribosomes. 24,25 Comparison of total mRNA populations with polysomal RNA populations reveal that low oxygen reduces the average proportion of individual mRNA species in large polysome complexes, including not only aerobic transcripts, but also a subset of known anaerobic-induced gene transcripts. Moreover, a large number of mRNAs displayed a significant decrease in polysome association without a concomitant decrease in steady-state accumulation.…”