Claudio Lovisolo

The tobacco aquaporin NtAQP1 is a membrane CO2 pore with physiological functions.

Aquaporins, found in virtually all living organisms, are membrane-intrinsic proteins that form water-permeable complexes. The mammalian aquaporin AQP1 has also shown CO2 permeability when expressed heterologously in Xenopus oocytes, although whether this is a biochemical curiosity or of physiological significance is a matter of debate. Here we report that, in the same expression system, a CO2 permeability comparable to that of the human AQP1 is observed for the tobacco plasma membrane aquaporin NtAQP1. NtAQP1 facilitates CO2 membrane transport in the homologous plant system at the cellular level, and has a significant function in photosynthesis and in stomatal opening. NtAQP1 overexpression heightens membrane permeability for CO2 and water, and increases leaf growth. The results indicate that NtAQP1-related CO2 permeability is of physiological importance under conditions where the CO2 gradient across a membrane is small, as is the case between the atmosphere and the inside of a plant cell.

PIP1 Plasma Membrane Aquaporins in Tobacco: From Cellular Effects to Function in Plants

The molecular functions of several aquaporins are well characterized (e.g., by analysis of aquaporin-expressing Xenopus oocytes). However, their significance in the physiology of water transport in multicellular organisms remains uncertain. The tobacco plasma membrane aquaporin NtAQP1 was used to elucidate this issue. By comparing antisense plants that were inhibited in NtAQP1 expression with control plants, we found evidence for NtAQP1 function in cellular and whole-plant water relations. The consequences of a decrease in cellular water permeability were determined by measurement of transpiration rate and stem and leaf water potential as well as growth experiments under extreme soil water depletion. Plants impaired in NtAQP1 expression showed reduced root hydraulic conductivity and lower water stress resistance. In conclusion, our results emphasize the importance of symplastic aquaporin-mediated water transport in whole-plant water relations.

Aquaporins and plant water balance

The impact of aquaporin function on plant water balance is discussed. The significance of these proteins for root water uptake, water conductance in the xylem, including embolism refilling and the role of plant aquaporins in leaf physiology, is described. Emphasis is placed on certain aspects of water stress reactions and the correlation of aquaporins to abscisic acid as well as on the relation of water and CO2 permeability in leaves.

Drought-induced changes in development and function of grapevine (Vitis spp.) organs and in their hydraulic and non-hydraulic interactions at the whole-plant level: a physiological and molecular update

This review deals with grapevine responses to water stress by examining perturbations to physiological and molecular processes at the root, shoot, leaf and berry levels. Long-distance signalling among organs is also considered. Isohydric or anisohydric Vitis genotypes are described in relation to their response to drought, which is linked to stomatal behaviour. Stomatal regulation of grapevine under abscisic acid and hydraulic control (the latter being linked to embolism formation and recovery in water pathways upstream the stomata) is reviewed and linked to impairments of photosynthetic assimilation. We define three stages of photosynthesis regulation in grapevines that are subjected to progressive water stress on the basis of the main causes of assimilation decline. Early and late contributions of aquaporins, which play a fundamental role in water stress control, are discussed. Metabolic mechanisms of dehydration tolerance are rewieved, and variation linked to differences in transcript abundance of genes involved in osmoregulation, photosynthesis, photorespiration, detoxification of free radicals and coping with photoinhibition. Results of these defence strategies accumulated in berries are reviewed, together with perturbations of their molecular pathways. Features observed in different organs show that grapevine fits well as a complex model plant for molecular and physiological studies on plant drought avoidance/tolerance.

An abscisic acid-related reduced transpiration promotes gradual embolism repair when grapevines are rehydrated after drought

* Proposed mechanisms of embolism recovery are controversial for plants that are transpiring while undergoing cycles of dehydration and rehydration. * Here, water stress was imposed on grapevines (Vitis vinifera), and the course of embolism recovery, leaf water potential (Psi(leaf)), transpiration (E) and abscisic acid (ABA) concentration followed during the rehydration process. * As expected, Psi(leaf) and E decreased upon water stress, whereas xylem embolism and leaf ABA concentration increased. Upon rehydration, Psi(leaf) recovered in 5 h, whereas E fully recovered only after an additional 48 h. The ABA content of recovering leaves was higher than in droughted controls, both on the day of rewatering and the day after, suggesting that ABA accumulated in roots during drought was delivered to the rehydrated leaves. In recovering plants, xylem embolism in petioles, shoots, and roots decreased during the 24 h following rehydration. * A model is proposed to describe plant recovery after rehydration based on three main points: embolism repair occurs progressively in shoots and further in roots and in petioles, following an almost full recovery of Psi(leaf); hydraulic conductance recovers during diurnal transpiring hours, when formation and repair of embolisms occurs in all plant organs; an ABA residual signal in rehydrated leaves hinders stomatal opening even when water relations have recovered, suggesting that an ABA-induced transpiration control promotes gradual embolism repair in rehydrated grapevines.

Effect of drought stress on growth and water relations of the mycorrhizal association Helianthemum almeriense-Terfezia claveryi

Abstract Plants of Helianthemum almeriense were micropropagated on MS medium and inoculated in vitro with Terfezia claveryi mycelium on MH medium and vermiculite. Mycorrhizal (M) and non-mycorrhizal (NM) plants were subjected to a drought stress period of 3 weeks in greenhouse conditions with the soil matric potential maintained at –0.5 MPa. Drought stress did not affect the amount of mycorrhizal colonization. The survival rate of M plants at the end of the drought stress period was higher than that of NM plants. The water potential was higher in M plants than in NM plants by 14% in well-watered and 26% in drought-stressed plants. Transpiration, stomatal conductance and net photosynthesis were higher in M plants than in NM plants. Transpiration was 92% higher in M plants than in NM plants under drought-stress conditions and 40% when irrigated. Stomatal conductance was 45% and 14% higher and net photosynthesis 88% and 54% higher, respectively, in M than in NM plants. Drought-stressed M plants accumulated more N, P and K than drought-stressed NM plants. Reduced negative effects of drought stress on H. almeriense by the desert truffle T. claveryi could be ascribed to specific physiological and nutritional mechanisms, suggesting that this mycorrhizal symbiosis aids adaptation to arid climates.

Whole-plant hydraulic conductance and root-to-shoot flow of abscisic acid are independently affected by water stress in grapevines

In order to investigate whether plant hydraulic conductance (gplant) is reduced under drought conditions via an ABA-related mechanism, a water-stress experiment was carried out using split-rooted grapevines. In addition, inversion of shoot growth orientation was imposed to reduce gplant independently of soil water availability, and thus of the putative ABA root-generated stress message. As expected, water stress imposed on split-roots affected ABA accumulation. ABA drought-stress message negatively affected stomatal conductance (gs) and transpiration (E), but modified neither leaf or stem water potentials (Ψleaf and Ψstem, respectively), nor gplant. When gplant was reduced in split-rooted, shoot-inverted (s-r/s-i) grapevines, Ψleaf and Ψstem decreased, without changes in ABA accumulation, gs and E. The ABA drought-stress message did not modify gplant, nor did gplant (impaired by shoot-growth inversion) decrease ABA delivery to the leaves. However, leaf growth was depressed in s-r/s-i grapevines. The fact that no interaction between ABA stress messages (caused by split-root technique) and hydraulic constraints to sap flow (caused by shoot inversion) was necessary to impair leaf growth suggests that the targets of ABA and hydraulic-limitation effects on leaf expansion are not the same.

Adjustments of water use efficiency by stomatal regulation during drought and recovery in the drought-adapted Vitis hybrid Richter-110 (V. berlandieri x V. rupestris).

The hybrid Richter-110 (Vitis berlandieri x Vitis rupestris) (R-110) has the reputation of being a genotype strongly adapted to drought. A study was performed with plants of R-110 subjected to water withholding followed by re-watering. The goal was to analyze how stomatal conductance (g(s)) is regulated with respect to different physiological variables under water stress and recovery, as well as how water stress affects adjustments of water use efficiency (WUE) at the leaf level. Water stress induced a substantial stomatal closure and an increase in WUE, which persisted many days after re-watering. The g(s) during water stress was mainly related to the content of ABA in the xylem and partly related to plant hydraulic conductivity but not to leaf water potential. By contrast, low g(s) during re-watering did not correlate with ABA contents and was only related to a sustained decreased hydraulic conductivity. In addition to a complex physiological regulation of stomatal closure, g(s) and rate of transpiration (E) were strongly affected by leaf-to-air vapor pressure deficit (VPD) in a way dependent of the treatment. Interestingly, E increased with increasing VPD in control plants, but decreased with increasing VPD in severely stressed plants. All together, the fine stomatal regulation in R-110 resulted in very high WUE at the leaf level. This genotype is revealed to be very interesting for further studies on the physiological mechanisms leading to regulation of stomatal responsiveness and WUE in response to drought.

Recovery from water stress affects grape leaf petiole transcriptome

Fast and efficient recovery from water stress is a key determinant of plant adaptation to changing meteorological conditions modulating transpiration, i.e. air temperature and humidity. We analysed transcriptomic responses during rehydration after water stress in grapevine leaf petioles, where embolism formation and repair commonly take place, and where metabolic changes related to embolism recovery are expected to be particularly important. We compared gene expression of recovering plants with irrigated controls, upon high and low transpiration conditions, using cDNA microarrays. In parallel, we assessed the daily dynamics of water relations, embolism formation and repair, and leaf abscisic acid concentration. In recovering plants, the most affected gene categories were secondary metabolism, including genes linked to flavonoid biosynthesis; sugar metabolism and transport, and several aquaporin genes. The physiological dynamics of recovery were lower and the number of differentially expressed probes was much lower upon low transpiration than found in actively transpiring grapevines, suggesting the existence of a more intense metabolic reorganization upon high transpiration conditions and of a signal eliciting these responses. In plants recovering under high transpiration, abscisic acid concentrations significantly increased, and, in parallel, transcripts linked to abscisic acid metabolism and signalling (ABA-8′-hydroxylase, serine-threonine kinases, RD22 proteins) were upregulated; a trend that was not observed upon low transpiration. Our results show that recovery from water stress elicits complex transcriptomic responses in grapevine. The increase observed in abscisic acid cellular levels could represent a signal triggering the activation of responses to rehydration after stress.

CAROTENOID CLEAVAGE DIOXYGENASE 7 modulates plant growth, reproduction, senescence, and determinate nodulation in the model legume Lotus japonicus

Strigolactones (SLs) are newly identified hormones that regulate multiple aspects of plant development, infection by parasitic weeds, and mutualistic symbiosis in the roots. In this study, the role of SLs was studied for the first time in the model plant Lotus japonicus using transgenic lines silenced for CAROTENOID CLEAVAGE DIOXYGENASE 7 (LjCCD7), the orthologue of Arabidopsis More Axillary Growth 3. Transgenic LjCCD7-silenced plants displayed reduced height due to shorter internodes, and more branched shoots and roots than the controls, and an increase in total plant biomass, while their root:shoot ratio remained unchanged. Moreover, these lines had longer primary roots, delayed senescence, and reduced flower/pod numbers from the third round of flower and pod setting onwards. Only a mild reduction in determinate nodule numbers and hardly any impact on the colonization by arbuscular mycorrhizal fungi were observed. The results show that the impairment of CCD7 activity in L. japonicus leads to a phenotype linked to SL functions, but with specific features possibly due to the peculiar developmental pattern of this plant species. It is believed that the data also link determinate nodulation, plant reproduction, and senescence to CCD7 function for the first time.