Progress in understanding the network of mechanisms involved in maize primary root growth maintenance under water deficits is reviewed. These include the adjustment of growth zone dimensions, turgor maintenance by osmotic adjustment, and enhanced cell wall loosening. The role of the hormone abscisic acid (ABA) in maintaining root growth under water deficits is also addressed. The research has taken advantage of kinematic analysis, i.e. characterization of spatial and temporal patterns of cell expansion within the root growth zone. This approach revealed different growth responses to water deficits and ABA deficiency in distinct regions of the root tip. In the apical 3 mm region, elongation is maintained at well-watered rates under severe water deficit, although only in ABA-sufficient roots, whereas the region from 3–7 mm from the apex exhibits maximum elongation in well-watered roots, but progressive inhibition of elongation in roots under water deficit. This knowledge has greatly facilitated discovery of the mechanisms involved in regulating the responses. The spatial resolution with which this system has been characterized and the physiological knowledge gained to date provide a unique and powerful underpinning for functional genomics studies. Characterization of water deficit-induced changes in transcript populations and cell wall protein profiles within the growth zone of the maize primary root is in progress. Initial results from EST and unigene analyses in the tips of well-watered and water-stressed roots highlight the strength of the kinematic approach to transcript profiling.

Previous work showed that primary root elongation in maize (Zea mays L.) seedlings at low water potentials (psi(w)) requires the accumulation of abscisic acid (ABA) (R.E. Sharp, Y. Wu, G.S. Voetberg, I.N. Saab, M.E. LeNoble [1994] J Exp Bot 45: 1743-1751). The objective of the present study was to determine whether the inhibition of elongation in ABA-deficient roots is attributable to ethylene. At a psi(w) of -1.6 MPa, inhibition of root elongation in dark-grown seedlings treated with fluridone to impose ABA deficiency was largely prevented with two inhibitors of ethylene synthesis (aminooxyacetic acid and aminoethoxyvinylglycine) and one inhibitor of ethylene action (silver thiosulfate). The fluridone treatment caused an increase in the rate of ethylene evolution from intact seedlings. This effect was completely prevented with aminooxyacetic acid and also when ABA was supplied at a concentration that restored the ABA content of the root elongation zone and the root elongation rate. Consistent results were obtained when ABA deficiency was imposed using the vp5 mutant. Both fluridone-treated and vp5 roots exhibited additional morphological symptoms of excess ethylene. The results demonstrate that an important role of ABA accumulation in the maintenance of root elongation at low psi(w) is to restrict ethylene production.

To examine whether the reduced shoot growth of abscisic acid (ABA)‐deficient mutants of tomato is independent of effects on plant water balance, flacca and notabilis were grown under controlled‐humidity conditions so that their leaf water potentials were equal to or higher than those of well‐watered wild‐type plants throughout development. Most parameters of shoot growth remained markedly impaired and root growth was also greatly reduced. Additional experiments with flacca showed that shoot growth substantially recovered when wild‐type levels of ABA were restored by treatment with exogenous ABA, even though improvement in leaf water potential was prevented. The ability of applied ABA to increase growth was greatest for leaf expansion, which was restored by 75%. The ethylene evolution rate of growing leaves was doubled in flacca compared to the wild type and treatment with silver thiosulphate to inhibit ethylene action partially restored shoot growth. The results demonstrate that normal levels of endogenous ABA are required to maintain shoot development, particularly leaf expansion, in well‐watered tomato plants, independently of effects on plant water balance. The impairment of shoot growth caused by ABA deficiency is at least partly attributable to ethylene.

Previous work on the adaptation of maize (Zea mays) primary roots to water deficit showed that cell elongation is maintained preferentially toward the apex, and that this response involves modification of cell wall extension properties. To gain a comprehensive understanding of how cell wall protein (CWP) composition changes in association with the differential growth responses to water deficit in different regions of the elongation zone, a proteomics approach was used to examine water soluble and loosely ionically bound CWPs. The results revealed major and predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. In total, 152 water deficit-responsive proteins were identified and categorized into five groups based on their potential function in the cell wall: reactive oxygen species (ROS) metabolism, defense and detoxification, hydrolases, carbohydrate metabolism, and other/unknown. The results indicate that stress-induced changes in CWPs involve multiple processes that are likely to regulate the response of cell elongation. In particular, the changes in protein abundance related to ROS metabolism predicted an increase in apoplastic ROS production in the apical region of the elongation zone of water-stressed roots. This was verified by quantification of hydrogen peroxide content in extracted apoplastic fluid and by in situ imaging of apoplastic ROS levels. This response could contribute directly to the enhancement of wall loosening in this region. This large-scale proteomic analysis provides novel insights into the complexity of mechanisms that regulate root growth under water deficit conditions and highlights the spatial differences in CWP composition in the root elongation zone.

Previous work demonstrated that normal levels of endogenous abscisic acid (ABA) are required to maintain shoot growth in well‐watered tomato plants independently of effects of hormone status on plant water balance. The results suggested that the impairment of shoot growth in ABA‐deficient mutants is at least partly attributable to increased ethylene production. To assess the extent to which ABA maintains shoot growth by ethylene suppression, the growth of ABA‐deficient (aba2‐1) and ethylene‐insensitive (etr1‐1) single‐ and double‐mutants of Arabidopsis was examined. To ensure that the results were independent of effects of hormone status on plant water balance, differential relative humidity regimes were used to achieve similar leaf water potentials in all genotypes and treatments. In aba2‐1, shoot growth was substantially inhibited and ethylene evolution was doubled compared with the wild type, consistent with the results for tomato. In the aba2‐1etr1‐1 double mutant, in which ABA was equally as deficient as in aba2‐1 and shoot growth was shown to be insensitive to ethylene, shoot growth was substantially, although incompletely, restored relative to etr1‐1. Treatment with ABA resulted in the complete recovery of shoot growth in aba2‐1 relative to the wild type, and also significantly increased the growth of aba2‐1 etr1‐1 such that total leaf area and shoot fresh weight were not significantly lower than in etr1‐1. In addition, ABA treatment of aba2‐1 etr1‐1 restored the wider leaf morphology phenotype exhibited by etr1‐1. The results demonstrate that normal levels of endogenous ABA maintain shoot development, particularly leaf expansion, in well‐watered Arabidopsis plants, partly by suppressing ethylene synthesis and partly by another mechanism that is independent of ethylene.