Abstract
BackgroundPrevious work showed that the maize primary root adapts to low Ψw (-1.6 MPa) by maintaining longitudinal expansion in the apical 3 mm (region 1), whereas in the adjacent 4 mm (region 2) longitudinal expansion reaches a maximum in well-watered roots but is progressively inhibited at low Ψw. To identify mechanisms that determine these responses to low Ψw, transcript expression was profiled in these regions of water-stressed and well-watered roots. In addition, comparison between region 2 of water-stressed roots and the zone of growth deceleration in well-watered roots (region 3) distinguished stress-responsive genes in region 2 from those involved in cell maturation.ResultsResponses of gene expression to water stress in regions 1 and 2 were largely distinct. The largest functional categories of differentially expressed transcripts were reactive oxygen species and carbon metabolism in region 1, and membrane transport in region 2. Transcripts controlling sucrose hydrolysis distinguished well-watered and water-stressed states (invertase vs. sucrose synthase), and changes in expression of transcripts for starch synthesis indicated further alteration in carbon metabolism under water deficit. A role for inositols in the stress response was suggested, as was control of proline metabolism. Increased expression of transcripts for wall-loosening proteins in region 1, and for elements of ABA and ethylene signaling were also indicated in the response to water deficit.ConclusionThe analysis indicates that fundamentally different signaling and metabolic response mechanisms are involved in the response to water stress in different regions of the maize primary root elongation zone.
Highlights
Previous work showed that the maize primary root adapts to low Ψw (-1.6 MPa) by maintaining longitudinal expansion in the apical 3 mm, whereas in the adjacent 4 mm longitudinal expansion reaches a maximum in well-watered roots but is progressively inhibited at low Ψw
To prioritize the differentially expressed genes revealed in this comparison, a distinction was made between those genes that are associated with growth inhibition in region 2 as a response to water stress, and those genes that are involved in root cell maturation whether under stress or control conditions
A hypothetical example of the former might be genes involved in auxin response since water stress can increase maize root auxin content [8] and application of exogenous auxin can shorten the root growth zone [9]
Summary
Previous work showed that the maize primary root adapts to low Ψw (-1.6 MPa) by maintaining longitudinal expansion in the apical 3 mm (region 1), whereas in the adjacent 4 mm (region 2) longitudinal expansion reaches a maximum in well-watered roots but is progressively inhibited at low Ψw. To identify mechanisms that determine these responses to low Ψw, transcript expression was profiled in these regions of water-stressed and well-watered roots. Progress in understanding the mechanisms that determine root growth at low Ψw has been made using a maize seedling system involving precise and reproducible imposition of water deficits [4,5]. Kinematic analyses detected distinct responses of longitudinal expansion rate to low Ψw in different regions of the root growth zone 48 h after stress imposition when the root elongation rate was at steady state [4,6]. Most striking was the complete maintenance of longitudinal expansion rate in the apical 3-mm region of roots growing at low compared to high Ψw. The adjacent, older, tissue of water-stressed roots decreased expansion rate compared to well-watered roots leading to a shortening of the growth zone
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