Overexpression of PIP2;5 aquaporin alleviates effects of low root temperature on cell hydraulic conductivity and growth in Arabidopsis

Differential responses of plasma membrane aquaporins in mediating water transport of cucumber seedlings under osmotic and salt stresses

It has long been recognized that inhibition of plant water transport by either osmotic stress or salinity is mediated by aquaporins (AQPs), but the function and regulation of AQPs are highly variable among distinct isoforms and across different species. In this study, cucumber seedlings were subjected to polyethylene glycol (PEG) or NaCl stress for duration of 2 h or 24 h. The 2 h treatment with PEG or NaCl had non-significant effect on the expression of plasma membrane AQP (CsPIPs) in roots, indicating the decrease in hydraulic conductivity of roots (Lpr ) and root cells (Lprc ) measured in these conditions were due to changes in AQP activity. After both 2 h and 24 h PEG or NaCl exposure, the decrease in hydraulic conductivity of leaves (Kleaf ) and leaf cells (Lplc ) could be attributed to a down-regulation of the two most highly expressed isoforms, CsPIP1;2 and CsPIP2;4. In roots, both Lpr and Lprc were further reduced after 24 h PEG exposure, but partially recovered after 24 h NaCl treatment, which were consistent with changes in the expression of CsPIP genes. Overall, the results demonstrated differential responses of CsPIPs in mediating water transport of cucumber seedlings, and the regulatory mechanisms differed according to applied stresses, stress durations and specific organs.

Biological significance and topological basis of aquaporin-partnering protein-protein interactions

Aquaporins (AQPs) are intramolecular channels essential for transport of H2O, CO2, and other small substrates across membranes. Through this function, AQPs can modulate CO2 uptake and assimilation in plants and regulate water relations and many other physiological processes in all living organisms. To execute their physiological roles, AQPs may experience 3 types of hetero-molecular interaction, between AQPs and their kinases; between AQP isoforms; and between AQPs and other proteins that are neither AQPs nor kinases. Interacting with non-AQP non-kinase proteins may enable AQPs to extend their functions beyond substrate transport, and most fascinatingly, to serve as a gateway control for translocation of virulence effectors from pathogenic bacteria into the cytosol of eukaryotic cells. In this mini review, we will summarize the latter 2 types of interaction and discuss the physiological and/or pathological significance. We will also discuss a research angle to elucidate the structural basis of AQP-partnering protein interactions.

Overexpression of Laccaria bicolor aquaporin JQ585595 alters root water transport properties in ectomycorrhizal white spruce (Picea glauca) seedlings

The contribution of hyphae to water transport in ectomycorrhizal (ECM) white spruce (Picea glauca) seedlings was examined by altering expression of a major water-transporting aquaporin in Laccaria bicolor. Picea glauca was inoculated with wild-type (WT), mock transgenic or L. bicolor aquaporin JQ585595-overexpressing (OE) strains and exposed to root temperatures ranging from 5 to 20°C to examine the root water transport properties, physiological responses and plasma membrane intrinsic protein (PIP) expression in colonized plants. Mycorrhization increased shoot water potential, transpiration, net photosynthetic rates, root hydraulic conductivity and root cortical cell hydraulic conductivity in seedlings. At 20°C, OE plants had higher root hydraulic conductivity compared with WT plants and the increases were accompanied by higher expression of P. glauca PIP GQ03401_M18.1 in roots. In contrast to WT L. bicolor, the effects of OE fungi on root and root cortical cell hydraulic conductivities were abolished at 10 and 5°C in the absence of major changes in the examined transcript levels of P. glauca root PIPs. The results provide evidence for the importance of fungal aquaporins in root water transport of mycorrhizal plants. They also demonstrate links between hyphal water transport, root aquaporin expression and root water transport in ECM plants.

Hpa1 harpin needs nitroxyl terminus to promote vegetative growth and leaf photosynthesis in Arabidopsis

Hpa1 is a harpin protein produced by Xanthomonas oryzae, an important bacterial pathogen of rice, and has the growth-promoting activity in plants. To understand the molecular basis for the function of Hpa1, we generated an inactive variant protein, Hpa1 delta NT, by deleting the nitroxyl-terminal region of the Hpa1 sequence and compared Hpa1 delta NT with the full-length protein in terms of the effects on vegetative growth and related physiological responses in Arabidopsis. When Hpa1 was applied to plants, it acted to enhance the vegetative growth but did not affect the floral development. Enhanced plant growth was accompanied by induced expression of growth-promoting genes in plant leaves. The growth-promoting activity of Hpa1 was further correlated with a physiological consequence shown as promoted leaf photosynthesis as a result of facilitated CO2 conduction through leaf stomata and mesophyll cells. On the contrary, plant growth, growth-promoting gene expression, and the physiological consequence changed little in response to the Hpa1 delta NT treatment. These analyses suggest that Hpa1 requires the nitroxyl-terminus to facilitate CO2 transport inside leaf cells and promote leaf photosynthesis and vegetative growth of the plant.

Hydraulic adjustments in aspen (Populus tremuloides) seedlings following defoliation involve root and leaf aquaporins

Changes in root and leaf hydraulic properties and stimulation of transpiration rates that were initially triggered by defoliation were accompanied by corresponding changes in leaf and root aquaporin expression. Aspen (Populus tremuloides) seedlings were subjected to defoliation treatments by removing 50, 75 % or all of the leaves. Root hydraulic conductivity (Lpr) was sharply reduced in plants defoliated for 1 day and 1 week. The decrease in L pr could not be prevented by stem girdling and it was accompanied in one-day-defoliated plants by a large decrease in the root expression of PIP1,2 aquaporin and an over twofold decrease in hydraulic conductivity of root cortical cells (L pc). Contrary to L pr and L pc, 50 and 75 % defoliation treatments profoundly increased leaf lamina conductance (K lam) after 1 day and this increase was similar in magnitude for both defoliation treatments. Transpiration rates (E) rapidly declined after the removal of 75 % of leaves. However, E increased by over twofold in defoliated plants after 1 day and the increases in E and K lam were accompanied by five- and tenfold increases in the leaf expression of PIP2;4 in 50 and 75 % defoliation treatments, respectively. Defoliation treatments also stimulated net photosynthesis after 1 day and 3 weeks, although the increase was not as high as E. Leaf water potentials remained relatively stable following defoliation with the exception of a small decrease 1 day after defoliation which suggests that root water transport did not initially keep pace with the increased transpirational water loss. The results demonstrate the importance of root and leaf hydraulic properties in plant responses to defoliation and point to the involvement of PIP aquaporins in the early events following the loss of leaves.

The Thellungiella salsuginea tonoplast aquaporin TsTIP1;2 functions in protection against multiple abiotic stresses

Examination of aquaporin (AQP) membrane channels in extremophile plants may increase our understanding of plant tolerance to high salt, drought or other conditions. Here, we cloned a tonoplast AQP gene (TsTIP1;2) from the halophyte Thellungiella salsuginea and characterized its biological functions. TsTIP1;2 transcripts accumulate to high levels in several organs, increasing in response to multiple external stimuli. Ectopic overexpression of TsTIP1;2 in Arabidopsis significantly increased plant tolerance to drought, salt and oxidative stresses. TsTIP1;2 had water channel activity when expressed in Xenopus oocytes. TsTIP1;2 was also able to conduct H₂O₂ molecules into yeast cells in response to oxidative stress. TsTIP1;2 was not permeable to Na(+) in Xenopus oocytes, but it could facilitate the entry of Na(+) ions into plant cell vacuoles by an indirect process under high-salinity conditions. Collectively, these data showed that TsTIP1;2 could mediate the conduction of both H₂O and H₂O₂ across membranes, and may act as a multifunctional contributor to survival of T. salsuginea in highly stressful habitats.

Frost hardiness in walnut trees (Juglans regia L.): how to link physiology and modelling?

In the literature, frost hardiness (FH) studies in trees have often been restricted to one organ (buds, leaves, needles or twigs). To extend our knowledge and gain a unified view, FH differences between organs and tissues or throughout the life of the tree have to be characterized in relation to physiological changes. In this study, different organs and tissues of young potted and mature orchard walnut trees (Juglans regia L.) were compared for seasonal changes in FH during different years. FH was assessed using the electrolyte leakage method. Physiological parameters were concomitantly monitored focusing on two significant traits: water content (WC) and carbohydrate content (glucose + fructose + sucrose, GFS). No seasonal variation in FH was observed in the root system, but acclimation and deacclimation were observed aboveground. Among organs and tissues, cold sensitivity levels were different in deep winter, with buds most sensitive and bark most resistant, but acclimation/deacclimation dynamics followed similar patterns. Physiological variation was also similar among organs: FH increased when WC decreased and/or soluble carbohydrates increased. Based on these results, relations between soluble carbohydrate content, WC and FH were calculated independently or in interaction. The key results were that: (i) the relationship between FH and physiological parameters (GFS and WC), which had previously been shown for branches only, could be generalized to all aboveground organs; (ii) lower WC increased the cryoprotective effect of GFS, showing a synergic effect of the two factors; (iii) the best fit was a non-linear function of WC and GFS, yielding a predictive model with an root mean square error of 5.07 °C on an independent dataset and 2.59 °C for the most sensitive stages; and (iv) the same parameters used for all organs yielded a unified model of FH depending on physiology, although the variability of GFS or WC was wide. The model should be of value for predicting FH in walnut independently of previous growing conditions (i.e., after sublethal stress accumulation).

Transgenic banana plants overexpressing a native plasma membrane aquaporin MusaPIP1;2 display high tolerance levels to different abiotic stresses

Water transport across cellular membranes is regulated by a family of water channel proteins known as aquaporins (AQPs). As most abiotic stresses like suboptimal temperatures, drought or salinity result in cellular dehydration, it is imperative to study the cause-effect relationship between AQPs and the cellular consequences of abiotic stress stimuli. Although plant cells have a high isoform diversity of AQPs, the individual and integrated roles of individual AQPs in optimal and suboptimal physiological conditions remain unclear. Herein, we have identified a plasma membrane intrinsic protein gene (MusaPIP1;2) from banana and characterized it by overexpression in transgenic banana plants. Cellular localization assay performed using MusaPIP1;2::GFP fusion protein indicated that MusaPIP1;2 translocated to plasma membrane in transformed banana cells. Transgenic banana plants overexpressing MusaPIP1;2 constitutively displayed better abiotic stress survival characteristics. The transgenic lines had lower malondialdehyde levels, elevated proline and relative water content and higher photosynthetic efficiency as compared to equivalent controls under different abiotic stress conditions. Greenhouse-maintained hardened transgenic plants showed faster recovery towards normal growth and development after cessation of abiotic stress stimuli, thereby underlining the importance of these plants in actual environmental conditions wherein the stress stimuli is often transient but severe. Further, transgenic plants where the overexpression of MusaPIP1;2 was made conditional by tagging it with a stress-inducible native dehydrin promoter also showed similar stress tolerance characteristics in in vitro and in vivo assays. Plants developed in this study could potentially enable banana cultivation in areas where adverse environmental conditions hitherto preclude commercial banana cultivation.

The root cortex cell hydraulic conductivity is enhanced with increasing chromosome ploidy in wheat

Wheat (Triticum spp.) root water uptake is enhanced with increasing chromosome ploidy, but the underlying mechanism is unclear. The leaf transpiration rate (E), individual root (Lp(r)) and cortical cell (Lp(c)) hydraulic conductivity, cortical cell volume (V(c)) and transcription levels of two putative plasma intrinsic aquaporin genes (PIPs) were investigated in wheat seedlings with different chromosome ploidy (Triticum monococcum (2X, AA); Triticum dicoccum (4X, BB); Triticum aestivum (6X, AABBDD)). The E, Lp(r) and Lp(c) of wheat increased with increasing ploidy, but the Vc was reduced. Osmotic stress significantly reduced the E, Lp(c), Lp(r), and the relative mRNA content of TaPIP1;2 and TaPIP2;5 in wheat. Under both well-watered and osmotic stress conditions, the Lp(r) was significantly and positively correlated with the E and Lp(c), and the relative mRNA content of TaPIP1;2 and TaPIP2;5 was significantly and positively correlated with Lp(c) and Lp(r), respectively. For well-watered or osmotically stressed wheat plants, the Lp(c) was reduced, but the L(p)(c)/Lp(r) increased with increasing V(c), suggesting that Vc affects root radical water transport. Thus, the increased Lp(c) and transcription levels of TaPIP1;2 and TaPIP2;5 in wheat roots provides insight into the mechanisms underlying enhanced root water uptake with increasing chromosome ploidy during wheat evolution.

Tuning growth to the environmental demands

When plants encounter adverse environmental conditions, they often respond by modifying their growth patterns. This growth response tunes morphogenesis with environmental demands and allows plants to prioritize stress response over growth. The underlying molecular mechanism involves an active reprogramming of cell proliferation and cell expansion. Recent studies are starting to shed light on how various environmental and developmental cues are integrated and how this integration affects growth regulatory processes. Environmental signals modulate developmental pathways at multiple entry points, by which they tune the outcome of developmental pathways. In addition, developmental regulators mediate universal stress signals to a proper local response.