Comparing regional transcript profiles from maize primary roots under well-watered and low water potential conditions.

Regionally distinct elongation responses to water stress in the maize primary root tip have been observed in the past. A genetic basis for such differential responses has been demonstrated. Normalized bar-coded cDNA libraries were generated for four regions of the root tip, 0-3 mm (R1), 3-7 mm (R2), 7-12 mm (R3), and 12-20 mm (R4) from the root apex, and transcript profiles for these regions were sampled. This permitted a correlation between transcript nature and regional location for 15 726 expressed sequence tags (ESTs) that, in approximately equal numbers, derived from three conditions of the root: water stress (water potential: -1.6 MPa) for 5 h and for 48 h, respectively, and well watered (5 h and 48 h combined). These normalized cDNA libraries provided 6553 unigenes. An analysis of the regional representation of transcripts showed that populations were largely unaffected by water stress in R1, correlating with the maintenance of elongation rates under water stress known for R1. In contrast, transcript profiles in regions 2 and 3 diverged in well-watered and water-stressed roots. In R1, transcripts for translation and cell cycle control were prevalent. R2 was characterized by transcripts for cell wall biogenesis and cytoskeleton formation. R3 and R4 shared prevalent groups of transcripts responsible for defence mechanisms, ion transport, and biogenesis of secondary metabolites. Transcripts which were followed for 1, 6, and 48 h of water stress showed distinct region-specific changes in absolute expression and changes in regulated functions.

The maize root transcriptome by serial analysis of gene expression.

Serial Analysis of Gene Expression was used to define number and relative abundance of transcripts in the root tip of well-watered maize seedlings (Zea mays cv FR697). In total, 161,320 tags represented a minimum of 14,850 genes, based on at least two tags detected per transcript. The root transcriptome has been sampled to an estimated copy number of approximately five transcripts per cell. An extrapolation from the data and testing of single-tag identifiers by reverse transcription-PCR indicated that the maize root transcriptome should amount to at least 22,000 expressed genes. Frequency ranged from low copy number (2-5, 68.8%) to highly abundant transcripts (100-->1,200; 1%). Quantitative reverse transcription-PCR for selected transcripts indicated high correlation with tag frequency. Computational analysis compared this set with known maize transcripts and other root transcriptome models. Among the 14,850 tags, 7,010 (47%) were found for which no maize cDNA or gene model existed. Comparing the maize root transcriptome with that in other plants indicated that highly expressed transcripts differed substantially; less than 5% of the most abundant transcripts were shared between maize and Arabidopsis (Arabidopsis thaliana). Transcript categories highlight functions of the maize root tip. Significant variation in abundance characterizes transcripts derived from isoforms of individual enzymes in biochemical pathways.