Agricultural productivity worldwide is subject to increasing environmental constraints, particularly to drought and salinity due to their high magnitude of impact and wide distribution. Traditional breeding programs trying to improve abiotic stress tolerance have had some success, but are limited by the multigenic nature of the trait. Tolerant plants such as Craterostigma plantagenium, Mesembryanthemum crystallinum, Thellungiella halophila and other hardy plants could be valuable tools to dissect the extreme tolerance nature. In the last decade, Arabidopsis thaliana, a genetic model plant, has been extensively used for unravelling the molecular basis of stress tolerance. Arabidopsis also proved to be extremely important for assessing functions for individual stress-associated genes due to the availability of knock-out mutants and its amenability for genetic transformation. In this review, the responses of plants to salt and water stress are described, the regulatory circuits which allow plants to cope wit...

Drought and Salt Tolerance in Plants

Abscisic acid (ABA) regulates many agronomically important aspects of plant development, including the synthesis of seed storage proteins and lipids, the promotion of seed desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from

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Abscisic Acid Signaling in Seeds and Seedlings

Stomata, the small pores on the surfaces of leaves and stalks, regulate the flow of gases in and out of leaves and thus plants as a whole. They adapt to local and global changes on all timescales from minutes to millennia. Recent data from diverse fields are establishing their central importance to plant physiology, evolution and global ecology. Stomatal morphology, distribution and behaviour respond to a spectrum of signals, from intracellular signalling to global climatic change. Such concerted adaptation results from a web of control systems, reminiscent of a 'scale-free' network, whose untangling requires integrated approaches beyond those currently used.

http://www.bio.bristol.ac.uk/research/HetheringtonLab/pdfs/nature01843.pdf

The role of stomata in sensing and driving environmental change.

This review summarizes current understanding of the mechanisms that underlie the response of photosynthesis and stomatal conductance to elevated carbon dioxide concentration ([CO2]), and examines how downstream processes and environmental constraints modulate these two fundamental responses. The results from free-air CO2 enrichment (FACE) experiments were summarized via meta-analysis to quantify the mean responses of stomatal and photosynthetic parameters to elevated [CO2]. Elevation of [CO2] in FACE experiments reduced stomatal conductance by 22%, yet, this reduction was not associated with a similar change in stomatal density. Elevated [CO2] stimulated light-saturated photosynthesis (Asat) in C3 plants grown in FACE by an average of 31%. However, the magnitude of the increase in Asat varied with functional group and environment. Functional groups with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis at elevated [CO2] had greater potential for increases in Asat than those where photosynthesis became ribulose-1,5-bisphosphate (RubP)-limited at elevated [CO2]. Both nitrogen supply and sink capacity modulated the response of photosynthesis to elevated [CO2] through their impact on the acclimation of carboxylation capacity. Increased understanding of the molecular and biochemical mechanisms by which plants respond to elevated [CO2], and the feedback of environmental factors upon them, will improve our ability to predict ecosystem responses to rising [CO2] and increase our potential to adapt crops and managed ecosystems to future atmospheric [CO2].

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01641.x

The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions.

The light-dependent generation of active oxygen species is termed photooxidative stress. This can occur in two ways: (1) the donation of energy or electrons directly to oxygen as a result of photosynthetic activity; (2) exposure of tissues to ultraviolet irradiation. The light-dependent destruction of catalase compounds the problem. Although generally detrimental to metabolism, superoxide and hydrogen peroxide may serve useful functions if rigorously controlled and compartmentalised. During photosynthesis the formation of active oxygen species is minimised by a number of complex and refined regulatory mechanisms. When produced, active oxygen species are eliminated rapidly by efficient antioxidative systems. The chloroplast is able to use the production and destruction of hydrogen peroxide to regulate the thermal dissipation of excess excitation energy. This is an intrinsic feature of the regulation of photosynthetic electron transport. Photoinhibition and photooxidation only usually occur when plants are exposed to stress. Active oxygen species are part of the alarm-signalling processes in plants. These serve to modify metabolism and gene expression so that the plant can respond to adverse environmental conditions, invading organisms and ultraviolet irradiation. The capacity of the antioxidative defense system is often increased at such times but if the response is not sufficient, radical production will exceed scavenging and ultimately lead to the disruption of metabolism. Oxidative damage arises in high light principally when the latter is in synergy with additional stress factors such as chilling temperatures or pollution. Environmental stress can modify the photooxidative processes in various ways ranging from direct involvement in light-induced free radical formation to the inhibition of metabolism that renders previously optimal light levels excessive. It is in just such situations that the capacity for the production of active oxygen species can exceed that for scavenging by the antioxidative defense systems. The advent of plant transformation, however, may have placed within our grasp the possibility of engineering greater stress tolerance in plants by enhancement of the antioxidative defence system.

Photooxidative stress in plants

The stomatal pores of higher plants allow for gaseous exchange into and out of leaves. Situated in the epidermis, they are surrounded by a pair of guard cells which control their opening in response to many environmental stimuli, including blue light. Opening of the pores is mediated by K(+) accumulation in guard cells through a K(+) channel and driven by an inside-negative electrical potential. Blue light causes phosphorylation and activation of the plasma membrane H(+)-ATPase that creates this potential. Thus far, no blue light receptor mediating stomatal opening has been identified, although the carotenoid, zeaxanthin, has been proposed. Arabidopsis mutants deficient in specific blue-light-mediated responses have identified four blue light receptors, cryptochrome 1 (cry1), cryptochrome 2 (cry2), phot1 and phot2. Here we show that in a double mutant of phot1 and phot2 stomata do not respond to blue light although single mutants are phenotypically normal. These results demonstrate that phot1 and phot2 act redundantly as blue light receptors mediating stomatal opening.

phot1 and phot2 mediate blue light regulation of stomatal opening

Stomatal pores, each surrounded by a pair of guard cells, regulate CO2 uptake and water loss from leaves. Stomatal opening is driven by the accumulation of K+ salts and sugars in guard cells, which is mediated by electrogenic proton pumps in the plasma membrane and/or metabolic activity. Opening responses are achieved by coordination of light signaling, light-energy conversion, membrane ion transport, and metabolic activity in guard cells. In this review, we focus on recent progress in blue- and red-light-dependent stomatal opening. Because the blue-light response of stomata appears to be strongly affected by red light, we discuss underlying mechanisms in the interaction between blue-light signaling and guard cell chloroplasts.

Light Regulation of Stomatal Movement

The opening of stomata, which is driven by the accumulation of K(+) salt in guard cells, is induced by blue light (BL). The BL activates the H(+) pump; however, the mechanism by which the perception of BL is transduced into the pump activation remains unknown. We present evidence that the pump is the plasma membrane H(+)-ATPase and that BL activates the H(+)-ATPase via phosphorylation. A pulse of BL (30 s, 100 micromol/m(2)/s) increased ATP hydrolysis by the plasma membrane H(+)-ATPase and H(+) pumping in Vicia guard cell protoplasts with a similar time course. The H(+)-ATPase was phosphorylated reversibly by BL, and the phosphorylation levels paralleled the ATP hydrolytic activity. The phosphorylation occurred exclusively in the C-termini of H(+)-ATPases on both serine and threonine residues in two isoproteins of H(+)-ATPase in guard cells. An endogenous 14-3-3 protein was co-precipitated with H(+)-ATPase, and the recombinant 14-3-3 protein bound to the phosphorylated C-termini of H(+)-ATPases. These findings demonstrate that BL activates the plasma membrane H(+)-ATPase via phosphorylation of the C-terminus by a serine/threonine protein kinase, and that the 14-3-3 protein has a key role in the activation.

Blue light activates the plasma membrane H(+)-ATPase by phosphorylation of the C-terminus in stomatal guard cells.

In plant leaves, light regulation of stomatal movement, and hence of gas exchange1, involves a non-photosynthetic photosensory system sensitive to blue light2–6, which transduces many other plant movement responses. The exact nature of the primary photo-reactions of this system is still not known7. Stomatal opening is, in general, based on swelling of the guard cells driven by ion uptake1, and it has been suggested that energy for such uptake may be provided by a chemi-osmotic extrusion of protons across the plasmalemma1,8,9. We report here that guard-cell protoplasts from Vicia faba, irradiated continuously with photosynthetically saturating, high-fluence-rate red light, extrude H+ ions in response to a short pulse (30s) of blue light. The response is relatively long-lived, such that a high rate of H+ extrusion continues for several minutes after the pulse stimulation, indicating that continuous input of blue light energy is not required for a sustained response. The kinetic relation between pulse input and response output matched closely that found for the stomatal response in intact leaves6. In agreement with the chemi-osmotic hypothesis, activation of an electrogenic plasmalemma H+-ATPase is postulated as the cause of blue light-induced H+ extrusion.

Blue light-dependent proton extrusion by guard-cell protoplasts of Vicia faba

Microscopic stomatal pores in the epidermes of aerial plant organs allow the loss of water vapor to the atmosphere in a process known as transpiration and the entry of CO2 into the plant for photosynthetic carbon fixation. Stomatal apertures are rapidly and reversibly regulated by pairs of guard

Ken-ichiro Shimazaki

Papers

phot1 and phot2 mediate blue light regulation of stomatal opening

Light Regulation of Stomatal Movement

Blue light activates the plasma membrane H(+)-ATPase by phosphorylation of the C-terminus in stomatal guard cells.

Blue light-dependent proton extrusion by guard-cell protoplasts of Vicia faba

The Multisensory Guard Cell. Stomatal Responses to Blue Light and Abscisic Acid