Aquaporin Activity to Improve Crop Drought Tolerance

Abstract

In plants, aquaporins (AQP) occur in multiple isoforms in both plasmalemma and tonoplast membranes resulting in regulation of water flow in and out of cells, and ultimately, water transfer through a series of cells in leaves and roots. Consequently, it is not surprising that physiological and molecular studies have identified AQPs as playing key roles in regulating hydraulic conductance in roots and leaves. As a result, the activity of AQPs influences a range of physiological processes including phloem loading, xylem water exit, stomatal aperture and gas exchange. The influence of AQPs on hydraulic conductance in plants is particularly important in regulating plant transpiration rate, particularly under conditions of developing soil water-deficit stress and elevated atmospheric vapor pressure deficit (VPD). In this review, we examine the impact of AQP activity and hydraulic conductance on crop water use and the identification of genotypes that express soil water conservation as a result of these traits. An important outcome of this research has been the identification and commercialization of cultivars of peanut (Arachis hypogaea L.), maize (Zea mays L.), and soybean (Glycine max (Merr) L.) for dry land production systems.

Keywords: aquaporins (AQPs); high vapor pressure deficit (VPD); limited-transpiration (TRlim) trait; water deficit stress.

Figure 1

Graph of normalized transpiration rate relative to the transpiration rate of well-watered plants for each genotype plotted against fraction of transpirable soil water for 6 week old peanut plants grown on mineral soil in 2.4 L pots [32]. These plants were subjected to controlled soil drying over about 2 weeks. The normalized was done to allow convenient comparison between genotypes ICGV 8699 (a) and TAG 24 (b).

Figure 2

Plot of transpiration rate (TR) vs. vapor pressure deficit (VPD) of 5 week old plants of (a) Tifrunner and (b) NC 3033 [37]. Entire plants were enclosed in individual mini-chambers (21 L) and TR was measured gravimetric for 1-exposure at each VPD. Regression analysis showed a linear increase in TR for Tifrunner over the entire range of tested VPD while a two-segment response was found for NC 3033 with a breakpoint (BP) between segments at 2.05 kPa.

Figure 3

Twelve maize hybrids that were phenotyped for expressing a TR_lim breakpoint in response to increasing VPD were also measured for decrease in transpiration rate (DTR) when exposed to silver ions (500 μM) [43]. These results showed a high correlation the breakpoint and DTR reflecting apparent variation in aquaporins (AQP) sensitivity to silver. *** a significant correlation at P < 0.001.

Figure 4

RNA abundance response relative to untreated tissues 15 min and 180 min after treatment with 200 μM AQPs inhibitor [silver nitrate (AgNO3)] of eight AQPs: (a) PIP 1;7; (b) PIP 2;7; (c) PIP 2;8; (d) PIP 2;9; (e) GmTIP 2;7; (f) GmTIP 3;1; (g) GmNIP 7;2; and (h) PesudoSIP#2 in soybean roots and leaves [42]. Bars indicate transcript abundance ±S.E. Those noted with * are significantly different at P < 0.05, NS are non-significant and ND are not detectable.