Transpirational demand affects aquaporin expression in poplar roots

University of Alberta, Department of Renewable Resources, 4-42 Earth Sciences Building, Edmonton, AB, Canada, T6G 2E3.
Journal of Experimental Botany (Impact Factor: 5.53). 04/2013; 64(8). DOI: 10.1093/jxb/ert096
Source: PubMed


Isohydric plants tend to maintain a water potential homeostasis primarily by controlling water loss via stomatal conductance. However, there is accumulating evidence that plants can also modulate water uptake in a dynamic manner. The dynamics of water uptake are influenced by aquaporin-mediated changes in root hydraulics. Most studies in this area have been conducted on herbaceous plants, and less is known about responses of woody plants. Here a study was conducted to determine how roots of hybrid poplar plants (Populus trichocarpa×deltoides) respond to a step change in transpirational demand. The main objective was to measure the expression of selected aquaporin genes and to assess how transcriptional responses correspond to changes in root water flow (Q R) and other parameters of water relations. A subset of plants was grown in shade and was subsequently exposed to a 5-fold increase in light level. Another group of plants was grown at ~95% relative humidity (RH) and was then subjected to lower RH while the light level remained unchanged. Both plant groups experienced a transient drop in stem water potentials. At 28h after the increase in transpirational demand, water potentials recovered. This recovery was associated with changes in the expression of PIP1 and PIP2 subfamily genes and an increase in Q R. Stomata of plants growing at high RH were larger and showed incomplete closure after application of abscisic acid. Since stomatal conductance remained high and unchanged in these plants, it is suggested that the recovery in water potential in these plants was largely driven by the increase in Q R.

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Available from: Joan Laur, Oct 02, 2015
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    • "Recently, in poplar, it has been demonstrated that aquaporin expression is mainly affected by RH rather than light. The expression of PIP1- (PIP1;1, PIP1;2 and PIP1;3) and PIP2-type (PIP2;3, PIP2;4 and PIP2;5) genes and also the root water flow were increased as early as 4 h of decrease in RH, whereas increase in light intensity had similar effect only after 28 h (Laur & Hacke, 2013). In wild strawberry (Fragaria vesca), distinct diurnal regulation of aquaporin isoforms in relation to drought has been recently demonstrated (Surbanovski et al., 2013). "
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    ABSTRACT: Abstract Abiotic stress has become a challenge to food security due to occurrences of climate change and environmental degradation. Plants initiate molecular, cellular and physiological changes to respond and adapt to various types of abiotic stress. Understanding of plant response mechanisms will aid in strategies aimed at improving stress tolerance in crop plants. One of the most common and early symptoms associated with these stresses is the disturbance in plant-water homeostasis, which is regulated by a group of proteins called "aquaporins". Aquaporins constitute a small family of proteins which are classified further on the basis of their localization, such as plasma membrane intrinsic proteins, tonoplast intrinsic proteins, nodulin26-like intrinsic proteins (initially identified in symbiosomes of legumes but also found in the plasma membrane and endoplasmic reticulum), small basic intrinsic proteins localized in ER (endoplasmic reticulum) and X intrinsic proteins present in plasma membrane. Apart from water, aquaporins are also known to transport CO2, H2O2, urea, ammonia, silicic acid, arsenite and wide range of small uncharged solutes. Besides, aquaporins also function to modulate abiotic stress-induced signaling. Such kind of versatile functions has made aquaporins a suitable candidate for development of transgenic plants with increased tolerance toward different abiotic stress. Toward this endeavor, the present review describes the versatile functions of aquaporins in water uptake, nutrient balancing, long-distance signal transfer, nutrient/heavy metal acquisition and seed development. Various functional genomic studies showing the potential of specific aquaporin isoforms for enhancing plant abiotic stress tolerance are summarized and future research directions are given to design stress-tolerant crops.
    Critical Reviews in Biotechnology 11/2014; DOI:10.3109/07388551.2014.973367 · 7.18 Impact Factor
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    • "During the 2011 drought, Q. fusiformis deep root K R increased 2.6-fold (Fig. 3a) and aquaporin activity also increased between April and September (Fig. 3) as the drought became particularly severe. Upregulation of root aquaporin activity during drought or during periods of increased transpirational demand has been observed in other woody plants including grape vine (Vandeleur et al. 2009) and poplar (Laur and Hacke 2013). The upregulation of aquaporin activity is much more rapid than the production of new roots and is therefore a more effective method of increasing water transport over short time scales or if carbon is in short supply. "
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    ABSTRACT: Key message Deep root hydraulic conductance is upregulated during severe drought and is associated with upregulation in aquaporin activity. Abstract In 2011, Texas experienced the worst single-year drought in its recorded history and, based on tree-ring data, likely its worst in the past millennium. In the Edwards Plateau of Texas, rainfall was 58 % lower and the mean daily maximum temperatures were >5 °C higher than long-term means in June through September, resulting in extensive tree mortality. To better understand the balance of deep and shallow root functioning for water supply, we measured root hydraulic conductance (K R) in deep (~20 m) and shallow (5–10 cm) roots of Quercus fusiformis at four time points in the field in 2011. Deep roots of Q. fusiformis obtained water from a perennial underground (18–20 m) stream that was present even during the drought. As the drought progressed, deep root K R increased 2.6-fold from early season values and shallow root K R decreased by 50 % between April and September. Inhibitor studies revealed that aquaporin contribution to K R increased in deep roots and decreased in shallow roots as the drought progressed. Deep root aquaporin activity was upregulated during peak drought, likely driven by increased summer evaporative demand and the need to compensate for declining shallow root K R. A whole-tree hydraulic transport model predicted that trees with greater proportions of deep roots would have as much as five times greater transpiration during drought periods and could sustain transpiration during droughts without experiencing total hydraulic failure. Our results suggest that trees shift their dependence on deep roots versus shallow roots during drought periods, and that upregulation of aquaporin activity accounts for at least part of this increase.
    Trees 10/2014; 28(5). DOI:10.1007/s00468-014-1036-8 · 1.65 Impact Factor
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    • "If this hypothesis is correct, CA would be in an optimal position for regulation of g m , as it uses as C i as a substrate and influences its diffusion inside the cell (Terashima et al., 2011). Although gene expression may not necessarily reflect protein function because of the posttranscriptional regulation (Maurel, 2007; Heinen et al., 2009; Laur and Hacke, 2013), the good relations of g m with expression of OePIP2.1 and CA and the good fitting of the causal model proposed allow us to go beyond already published literature and to infer a direct role of AQPs and CA in g m regulation, although certainly the mechanistic basis for such a role remains to be elucidated. "
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    ABSTRACT: The hypothesis that aquaporins and carbonic anhydrase (CA) are involved in the regulation of stomatal (g s) and mesophyll (g m) conductance to CO2 was tested in a short-term water-stress and recovery experiment in 5-year-old olive plants (Olea europaea) growing outdoors. The evolution of leaf gas exchange, chlorophyll fluorescence, and plant water status, and a quantitative analysis of photosynthesis limitations, were followed during water stress and recovery. These variables were correlated with gene expression of the aquaporins OePIP1.1 and OePIP2.1, and stromal CA. At mild stress and at the beginning of the recovery period, stomatal limitations prevailed, while the decline in g m accounted for up to 60% of photosynthesis limitations under severe water stress. However, g m was restored to control values shortly after rewatering, facilitating the recovery of the photosynthetic rate. CA was downregulated during water stress and upregulated after recovery. The use of structural equation modelling allowed us to conclude that both OePIP1.1 and OePIP2.1 expression could explain most of the variations observed for g s and g m. CA expression also had a small but significant effect on g m in olive under water-stress conditions.
    Journal of Experimental Botany 05/2014; 65(12). DOI:10.1093/jxb/eru160 · 5.53 Impact Factor
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