Nir Sade

Improving plant stress tolerance and yield production: is the tonoplast aquaporin SlTIP2;2 a key to isohydric to anisohydric conversion?

Anisohydric plants are thought to be more drought tolerant than isohydric plants. However, the molecular mechanism determining whether the plant water potential during the day remains constant or not regardless of the evaporative demand (isohydric vs anisohydric plant) is not known. Here, it was hypothesized that aquaporins take part in this molecular mechanism determining the plant isohydric threshold. Using computational mining a key tonoplast aquaporin, tonoplast intrinsic protein 2;2 (SlTIP2;2), was selected within the large multifunctional gene family of tomato (Solanum lycopersicum) aquaporins based on its induction in response to abiotic stresses. SlTIP2;2-transformed plants (TOM-SlTIP2;2) were compared with controls in physiological assays at cellular and whole-plant levels. Constitutive expression of SlTIP2;2 increased the osmotic water permeability of the cell and whole-plant transpiration. Under drought, these plants transpired more and for longer periods than control plants, reaching a lower relative water content, a behavior characterizing anisohydric plants. In 3-yr consecutive commercial glasshouse trials, TOM-SlTIP2;2 showed significant increases in fruit yield, harvest index and plant mass relative to the control under both normal and water-stress conditions. In conclusion, it is proposed that the regulation mechanism controlling tonoplast water permeability might have a role in determining the whole-plant ishohydric threshold, and thus its abiotic stress tolerance.

The Role of Tobacco Aquaporin1 in Improving Water Use Efficiency, Hydraulic Conductivity, and Yield Production Under Salt Stress

Tobacco (Nicotiana tabacum; C3) plants increase their water use efficiency (WUE) under abiotic stress and are suggested to show characteristics of C4 photosynthesis in stems, petioles, and transmitting tract cells. The tobacco stress-induced Aquaporin1 (NtAQP1) functions as both water and CO2 channel. In tobacco plants, overexpression of NtAQP1 increases leaf net photosynthesis (AN), mesophyll CO2 conductance, and stomatal conductance, whereas its silencing reduces root hydraulic conductivity (Lp). Nevertheless, interaction between NtAQP1 leaf and root activities and its impact on plant WUE and productivity under normal and stress conditions have never been suggested. Thus, the aim of this study was to suggest a role for NtAQP1 in plant WUE, stress resistance, and productivity. Expressing NtAQP1 in tomato (Solanum lycopersicum) plants (TOM-NtAQP1) resulted in higher stomatal conductance, whole-plant transpiration, and AN under all conditions tested. In contrast to controls, where, under salt stress, Lp decreased more than 3-fold, TOM-NtAQP1 plants, similar to maize (Zea mays; C4) plants, did not reduce Lp dramatically (only by approximately 40%). Reciprocal grafting provided novel evidence for NtAQP1s role in preventing hydraulic failure and maintaining the whole-plant transpiration rate. Our results revealed independent, albeit closely related, NtAQP1 activities in roots and leaves. This dual activity, which increases the plants water use and AN under optimal and stress conditions, resulted in improved WUE. Consequently, it contributed to the plants stress resistance in terms of yield production under all tested conditions, as demonstrated in both tomato and Arabidopsis (Arabidopsis thaliana) plants constitutively expressing NtAQP1. The putative involvement of NtAQP1 in tobaccos C4-like photosynthesis characteristics is discussed.

Risk-taking plants: anisohydric behavior as a stress-resistance trait.

Water scarcity is a critical limitation for agricultural systems. Two different water management strategies have evolved in plants: an isohydric strategy and an anisohydric strategy. Isohydric plants maintain a constant midday leaf water potential (Ψleaf) when water is abundant, as well as under drought conditions, by reducing stomatal conductance as necessary to limit transpiration. Anisohydric plants have more variable Ψleaf and keep their stomata open and photosynthetic rates high for longer periods, even in the presence of decreasing leaf water potential. This risk-taking behavior of anisohydric plants might be beneficial when water is abundant, as well as under moderately stressful conditions. However, under conditions of intense drought, this behavior might endanger the plant. We will discuss the advantages and disadvantages of these two water-usage strategies and their effects on the plant’s ability to tolerate abiotic and biotic stress. The involvement of plant tonoplast AQPs in this process will also be discussed.

Differential tissue‑specific expression of NtAQP1 in Arabidopsis thaliana reveals a role for this protein in stomatal and mesophyll conductance of CO 2 under standard and salt‑stress conditions

The regulation of plant hydraulic conductance and gas conductance involves a number of different morphological, physiological and molecular mechanisms working in harmony. At the molecular level, aquaporins play a key role in the transport of water, as well as CO2, through cell membranes. Yet, their tissue-related function, which controls whole-plant gas exchange and water relations, is less understood. In this study, we examined the tissue-specific effects of the stress-induced tobacco Aquaporin1 (NtAQP1), which functions as both a water and CO2 channel, on whole-plant behavior. In tobacco and tomato plants, constitutive overexpression of NtAQP1 increased net photosynthesis (AN), mesophyll CO2 conductance (gm) and stomatal conductance (gs) and, under stress, increased root hydraulic conductivity (Lpr) as well. Our results revealed that NtAQP1 that is specifically expressed in the mesophyll tissue plays an important role in increasing both AN and gm. Moreover, targeting NtAQP1 expression to the cells of the vascular envelope significantly improved the plants’ stress response. Surprisingly, NtAQP1 expression in the guard cells did not have a significant effect under any of the tested conditions. The tissue-specific involvement of NtAQP1 in hydraulic and gas conductance via the interaction between the vasculature and the stomata is discussed.

The Pitfalls of Transgenic Selection and New Roles of AtHXK1 : A High Level of AtHXK1 Expression Uncouples Hexokinase1-Dependent Sugar Signaling from Exogenous Sugar

Arabidopsis ( Arabidopsis thaliana ) hexokinase ( AtHXK1 ) encodes a dual-function enzyme that mediates sugar sensing in addition to its involvement in hexose phosphorylation activity, thereby coordinating sugar availability with plant physiology and development ([Moore et al., 2003][1]; [Rolland et

Relationship between Hexokinase and the Aquaporin PIP1 in the Regulation of Photosynthesis and Plant Growth

Increased expression of the aquaporin NtAQP1, which is known to function as a plasmalemma channel for CO2 and water, increases the rate of both photosynthesis and transpiration. In contrast, increased expression of Arabidopsis hexokinase1 (AtHXK1), a dual-function enzyme that mediates sugar sensing, decreases the expression of photosynthetic genes and the rate of transpiration and inhibits growth. Here, we show that AtHXK1 also decreases root and stem hydraulic conductivity and leaf mesophyll CO2 conductance (g m). Due to their opposite effects on plant development and physiology, we examined the relationship between NtAQP1 and AtHXK1 at the whole-plant level using transgenic tomato plants expressing both genes simultaneously. NtAQP1 significantly improved growth and increased the transpiration rates of AtHXK1-expressing plants. Reciprocal grafting experiments indicated that this complementation occurs when both genes are expressed simultaneously in the shoot. Yet, NtAQP1 had only a marginal effect on the hydraulic conductivity of the double-transgenic plants, suggesting that the complementary effect of NtAQP1 is unrelated to shoot water transport. Rather, NtAQP1 significantly increased leaf mesophyll CO2 conductance and enhanced the rate of photosynthesis, suggesting that NtAQP1 facilitated the growth of the double-transgenic plants by enhancing mesophyll conductance of CO2.

Water Balance, Hormone Homeostasis, and Sugar Signaling Are All Involved in Tomato Resistance to Tomato Yellow Leaf Curl Virus

Tonoplast water channels contribute to Tomato yellow leaf curl virus resistance through hormone homeostasis and sugar signaling. Vacuolar water movement is largely controlled by membrane channels called tonoplast-intrinsic aquaporins (TIP-AQPs). Some TIP-AQP genes, such as TIP2;2 and TIP1;1, are up-regulated upon exposure to biotic stress. Moreover, TIP1;1 transcript levels are higher in leaves of a tomato (Solanum lycopersicum) line resistant to Tomato yellow leaf curl virus (TYLCV) than in those of a susceptible line with a similar genetic background. Virus-induced silencing of TIP1;1 in the tomato resistant line and the use of an Arabidopsis (Arabidopsis thaliana) tip1;1 null mutant showed that resistance to TYLCV is severely compromised in the absence of TIP1:1. Constitutive expression of tomato TIP2;2 in transgenic TYLCV-susceptible tomato and Arabidopsis plants was correlated with increased TYLCV resistance, increased transpiration, decreased abscisic acid levels, and increased salicylic acid levels at the early stages of infection. We propose that TIP-AQPs affect the induction of leaf abscisic acid, which leads to increased levels of transpiration and gas exchange, as well as better salicylic acid signaling.

The dynamic isohydric-anisohydric behavior of plants upon fruit development: taking a risk for the next generation

Maintaining plant water status above a critical threshold is a crucial necessity for plant growth and reproduction. However, the factors that determine this threshold level and the ways in which it is regulated vary greatly between species and even across the life cycle of individual species. Isohydric waterbalance behavior involves the maintenance of a constant leaf water potential at midday, which is similar in well-irrigated plants and under drought conditions. In contrast, plants exhibiting anisohydric behavior have markedly decreased water potentials following the evaporative demand experienced during the day. This permits lower leaf water potentials in the presence of drought stress (Tardieu and Simonneau 1998). It has been suggested that the core physiological parameter thresh

Bundle-sheath aquaporins play a role in controlling Arabidopsis leaf hydraulic conductivity

The role of molecular mechanisms in the regulation of leaf hydraulics (Kleaf) is still not well understood. We hypothesized that aquaporins (AQPs) in the bundle sheath may regulate Kleaf. To examine this hypothesis, AQP genes were constitutively silenced using artificial microRNAs and recovery was achieved by targeting the expression of the tobacco AQP (NtAQP1) to bundle-sheath cells in the silenced plants. Constitutively silenced PIP1 plants exhibited decreased PIP1 transcript levels and decreased Kleaf. However, once the plants were recovered with NtAQP1, their Kleaf values were almost the same as those of WT plants. We also demonstrate the important role of ABA, acting via AQP, in that recovery and Kleaf regulation. These results support our previously raised hypothesis concerning the role of bundle-sheath AQPs in the regulation of leaf hydraulics.

Effects of abiotic stress on physiological plasticity and water use of Setaria viridis (L.).

The emerging model Setaria viridis with its C4 photosynthesis and adaptation to hot and dry locations is a promising system to investigate water use and abiotic stress tolerance. We investigated the physiological plasticity of six S. viridis natural accessions that originated from different regions of the world under normal conditions and conditions of water-deficit stress and high temperatures. Accessions Zha-1, A10.1 and Ula-1 showed significantly higher leaf water potential (Ψleaf), photosynthesis (A), transpiration (E), and stomatal conductance (gs) rates compared to Ast-1, Aba-1 and Sha-1 when grown under stress conditions. Expression analysis of genes associated with C4 photosynthesis, aquaporins, ABA biosynthesis and signaling including genes involved in stress revealed an increased sensitivity of Ast-1, Aba-1 and Sha-1 to stresses. Correlation analysis of gene expression data with physiological and biochemical changes characterized A10.1 and Ast-1 as two extreme tolerant and sensitive accessions originated from United States and Azerbaijan under water-deficit and heat stress, respectively. Although preliminary, our study demonstrated the plasticity of S. viridis accessions under stress, and allows the identification of tolerant and sensitive accessions that could be use to study the mechanisms associated with stress tolerance and to characterize of the regulatory networks involved in C4 grasses.