Water uptake and hydraulic properties of elongating cells in hydrotropically bending roots of Pisum sativum L

Versatile Roles of Aquaporins in Plant Growth and Development

Aquaporins (AQPs) are universal membrane integrated water channel proteins that selectively and reversibly facilitate the movement of water, gases, metalloids, and other small neutral solutes across cellular membranes in living organisms. Compared with other organisms, plants have the largest number of AQP members with diverse characteristics, subcellular localizations and substrate permeabilities. AQPs play important roles in plant water relations, cell turgor pressure maintenance, the hydraulic regulation of roots and leaves, and in leaf transpiration, root water uptake, and plant responses to multiple biotic and abiotic stresses. They are also required for plant growth and development. In this review, we comprehensively summarize the expression and roles of diverse AQPs in the growth and development of various vegetative and reproductive organs in plants. The functions of AQPs in the intracellular translocation of hydrogen peroxide are also discussed.

Hydrotropism: how roots search for water

Fresh water is an increasingly scarce resource for agriculture. Plant roots mediate water uptake from the soil and have developed a number of adaptive traits such as hydrotropism to aid water foraging. Hydrotropism modifies root growth to respond to a water potential gradient in soil and grow towards areas with a higher moisture content. Abscisic acid (ABA) and a small number of genes, including those encoding ABA signal transducers, MIZ2/GNOM, and the hydrotropism-specific MIZ1, are known to be necessary for the response in Arabidopsis thaliana, whereas the role of auxin in hydrotropism appears to vary depending on the plant species. This review will describe recent progress characterizing the hormonal regulation of hydrotropism. Recent advances in identifying the sites of hydrotropic perception and response, together with its interaction with gravitropism, will also be discussed. Finally, I will describe putative mechanisms for perception of the water potential gradient and a potential role for hydrotropism in acclimatizing plants to drought conditions.

Root hydrotropism is controlled via a cortex-specific growth mechanism

Plants can acclimate by using tropisms to link the direction of growth to environmental conditions. Hydrotropism allows roots to forage for water, a process known to depend on abscisic acid (ABA) but whose molecular and cellular basis remains unclear. Here we show that hydrotropism still occurs in roots after laser ablation removed the meristem and root cap. Additionally, targeted expression studies reveal that hydrotropism depends on the ABA signalling kinase SnRK2.2 and the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing differential cell-length increases in the cortex, but not in other cell types. We conclude that root tropic responses to gravity and water are driven by distinct tissue-based mechanisms. In addition, unlike its role in root gravitropism, the elongation zone performs a dual function during a hydrotropic response, both sensing a water potential gradient and subsequently undergoing differential growth.

A pump/leak model of growth: the biophysics of cell elongation in higher plants revisited

Current concepts of growth hydraulics in higher plants are critically revisited, and it is concluded that they partly fail to interpret the experimental data adequately, particularly in the case of hydroponics-grown roots. Theoretical considerations indicate that the growth rate in roots is controlled by the extensibility of the cell wall, excluding water availability (i.e. hydraulic conductance) as a major constraint. This is supported by the findings that the growth rate does not scale with turgor, and that no radial nor axial water potential gradients have been observed in the root elongation zone. Nevertheless, a water potential deficit ranging from -0.2 to -0.6MPa has repeatedly been reported for growing cells that by far exceeds the shallow trans-membrane water potential difference required for the uptake of growth water. Unexpectedly, growth was also shown to depend on the hydraulic conductance (LP) of the plasma membrane of root cells, even though LP should generally be too large to have an impact on growth. For leaves, similar observations have been reported, but the interpretation of the data is less straightforward. Inconsistencies associated with the current model of growth hydraulics prompt the author to suggest a revised model that comprises, in addition to a passive mechanism of water transport across the plasma membrane of growing cells mediated by aquaporins ('leak') a secondary active water transport ('pump'), in analogy to a mechanism previously demonstrated for mammalian epithelia and postulated for xylem parenchyma cells in roots. Water is hypothesised to be secreted against a trans-membrane water potential difference by cotransport with solutes (salts, sugars, and/or amino acids), taking advantage of the free energy released by this transport step. The solute concentration gradient is supposed to be maintained by a subsequent retrieval of the solutes from the apoplast and back-transport at the expense of metabolic energy. Water secretion tends to reduce the turgor pressure and retards growth, but turgor and, in turn, growth can be upregulated very rapidly independent from any adjustment in the osmolyte deposition rate by increasing LP and/or reducing secondary active water transport, e.g. when the root is exposed to mild osmotic stress, as confirmed by experimental studies.

Hormonal interactions during root tropic growth: hydrotropism versus gravitropism

Terrestrial plants have evolved remarkable morphological plasticity that enables them to adapt to their surroundings. One of the most important traits that plants have acquired is the ability to sense environmental cues and use them as a basis for governing their growth orientation. The directional growth of plant organs relative to the direction of environmental stimuli is a tropism. The Cholodny-Went theory proposes that auxin plays a key role in several tropisms. Recent molecular genetic studies have strongly supported this hypothesis for gravitropism. However, the molecular mechanisms of other tropisms are far less clear. Hydrotropism is the response of roots to a moisture gradient. Since its re-discovery in 1985, root hydrotropism has been shown to be common among higher plant species. Additionally, in some species, gravitropism interferes with hydrotropism, suggesting that both shared and divergent mechanisms mediating the two tropisms exist. This hypothesis has been supported by recent studies, which provide an understanding of how roots sense multiple environmental cues and exhibit different tropic responses. In this review, we focus on the overlapping and unique mechanisms of the hormonal regulation underlying gravitropism and hydrotropism in roots.

The root tip and accelerating region suppress elongation of the decelerating region without any effects on cell turgor in primary roots of maize under water stress

To identify the region in which a root perceives a decrease in the ambient water potential and changes its elongation rate, we applied two agar blocks (1 x 1 x 1 mm(3)) with low water potential bilaterally to primary roots of maize (Zea mays) at various positions along the root. When agar blocks with a water potential of -1.60 MPa (-1.60-MPa blocks) or lower were attached to a root tip, the rate of elongation decreased. This decrease did not result from any changes in the water status of elongating cells and was not reversed when the -1.60-MPa blocks were replaced by -0.03-MPa blocks. The rate decreased slightly and was unaffected, respectively, when -1.60-MPa blocks were applied to the so-called decelerating region of the elongating zone and the mature region. However, the rate decreased markedly and did not recover for several hours at least when such blocks were attached to the accelerating region. In this case, the turgor pressure of the elongating cells decreased immediately after the application of the blocks and recovered thereafter. The decrease in elongation rate caused by -1.60-MPa blocks applied to the root tip was unaffected by additional -0.03-MPa blocks applied to the accelerating region and vice versa. We concluded that a significant reduction in root growth could be induced by water stress at the root tip, as well as in the accelerating region of the elongating zone, and that transmission of some signal from these regions to the decelerating region might contribute to the suppression of cell elongation in the elongation region.