Transpirational demand affects aquaporin expression in poplar roots

Abstract

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.

Rice Plants Sense Daily Weather and Regulate Aquaporin Gene Expressions in the Roots -Close correlation with potential evaporation-

“…Therefore, we consider that difference in the light intensity not be the most important factor to explain the day-to-day variation of root aquaporin expression levels in rice. This is also supported by the data of study by Laur and Hacke (2013), who showed that altering relative humidity without changing irradiance triggered upregulation of PIP expression (and root water flow) in hybrid popular sapling roots. Future investigation is required to clarify whether there are any contributions of increase in the light intensity to root aquaporin expressions.…”

Down-Regulation of Plasma Intrinsic Protein1 Aquaporin in Poplar Trees Is Detrimental to Recovery from Embolism

“…Expression levels of several PIP and TIP members change after the dynamic of increasing water stress and recovery in many woody plants, including walnut (Juglans regia), poplar (Populus trichocarpa. ), and grapevine Vitis vinifera; (Sakr et al, 2003;Perrone et al, 2012aPerrone et al, , 2012bLaur and Hacke, 2013;Pou et al, 2013). Furthermore, an increase in the expression of PIP2.1 and PIP2.2 genes was observed in vessel-associated parenchyma cells in walnuts at the same time that recovery from embolism was taking place (Sakr et al, 2003).…”

Role of aquaporin activity in regulating deep and shallow root hydraulic conductance during extreme drought

“…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.…”

“…Further, trees in central Texas display diurnal and seasonal variation in deep root (*20 m) hydraulic conductivity and aquaporin activity (McElrone et al 2007). Aquaporin (membrane-bound channels, see Maurel et al 2008 for review) activity can be upregulated in roots during periods of increased transpirational demand (Laur and Hacke 2013) and can either be upregulated (e.g., Vandeleur et al 2009) or downregulated during periods of water stress, depending on the species and type of aquaporin (Aroca et al 2012).…”