Plant water regime Water stress Development of water

Plant water regime • Water stress – – Development of water stress Adaptation to drought Signals and signalling pathways Basic processes affected by water stress

Water stress • Water stress is induced when transpiration rate (E) is higher than absorption rate (A) • High E – low air humidity, high temperature, high irradiance, strong wind • Low A – low soil moisture, high concentration of salts, low soil temperature • Gradient of water potential between substrate and shoot is prerequisite for water transport • Transport of water from storage equilibrates small differences between E and A • Transient and permanent water deficit • Rate of development • Recovery after rehydration

Adaptations of plants to water stress • 1) drought avoidance – whole growth cycle in a wet season, leaf fall under drought • 2) drought tolerance (resistance) • a) drought tolerance at low cell water potential – survive with minimum metabolism: seeds, pollen grains, resurrection plants • b) drought tolerance at high cell water potential – regulation of water loss (stomata, cuticle, trichomes, leaf movements, leaf shape, leaf area, C 3 CAM), regulation of absorption (amount and morphology of roots, osmotic adjustment), efficient water transport with low water potential at cavitation, water storages (stems, trunks, fruits), low damage under mild water stress, low sensitivity of basic metabolic processes, production of protection compounds (e. g. carotenoids, osmolytes, stress proteins, antioxidants)

Water stress signalling • Signal of water stress can be decreased cell water content, decreased water potential and its components osmotic and pressure potentials, increased concentration of solutes, decreased cell volume, change in membrane tension, changes in structure of macromolecules due to changes in their hydration, changes in interaction between cell wall and plasmalemma • Water stress receptors are not sufficiently known, probably different for different signals • For root-shoot communication hydraulic and chemical signals are used • Direct and indirect effects of water stress

Water stress affects almost all processes in plants • Inhibition of elongation growth, cell division, changes in cell wall synthesis • Inhibition of shoot growth and stimulation of root growth and thus increase of root/shoot ratio • Acceleration of ageing • Production of stress proteins • Accumulation of osmotically active compounds (proline, glycinebetaine, sugars, sugar alcohols) • ROS production and development of antioxidative systems • Inhibition of photosynthesis, transport of assimilates, respiration • Changes in enzyme activities (decrease in activity of Rubisco, PEPC, nitratereductase, but increase in activity of hydrolases or dehydrogenases) • Changes in biosynthesis and catabolism of phytohormones, especially ABA • Changes in absorption and transport of ions

Multiple effects of water stress

Different water stress sensitivity of different processes

Signalling pathway leading to stress induced changes in gene expression (Xiong et al. 2002)

Changes in gene expression induced by drought, salinity or cold (Seki et al. 2002)

Role of ABA in plant response to stress (Nakashima and Yamaguchi-Shinozaki 2006)

Water stress and protein synthesis 1) Inhibition of synthesis of some proteins 2) Stimulation of synthesis of other proteins 3) Synthesis of specific stress proteins • A) proteins taking part in signal transduction and gene expression, e. g. transcription factors (MYC, MYB), protein kinases (MAPK), enzymes of phospholipid metabolism (phospholipase C, D) • B) proteins participating in stress tolerance, e. g. membrane proteins, proteins of water and ion channels, protection factors (chaperones, LEA proteins), syntases of osmoprotectants, stress proteins localized in chloroplasts, specific inhibitors of proteolytic activity, antioxidants, antioxidative enzymes, proteins taking part in recovery after stress

Oono et al. 2003

Osmotic and elastic adjustment • Osmotic and elastic adjustment (growth, drought, salinity, low temperature, etc. ), induction by decrease in soil water potential, air humidity, etc. • Osmotic adjustment – ion uptake, production and accumulation of osmotically active substances such as sugars (glucose, trehalose, saccharose), sugar alcohols (mannitol, sorbitol, glycerol), polyamines, amino acids (proline), betaines (glycinebetaine) • Membrane protection, source of C or N, defence against reactive oxygen species (ROS) • Dehydrines – ripening of seeds or pollen grains, in plant vegetative parts during stresses, induced also by abscisic acid (ABA) • Elastic adjustment – expansin, endoglucanase, transglycosylase, peroxidase • At different plant species are different adaptations, amount of osmoticum is not always in correlation with water stress tolerance

Ogawa and Yamauchi 2006

Synthesis and degration of proline L-Glu - L-glutamate, GSA - glutamate semialdehyde, P 5 C - -pyrroline-5 -carboxylate, P 5 CS - P 5 C syntetase, P 5 CR - P 5 C reductase, Pro. DH - proline dehydrogenase, L-Pro - proline

Water stress induced accumulation of proline and glycine betaine as affected by calcium Fig. 1. Effect of water stress, 1 m. M calcium chloride (Ca) and 0. 5 m. M calcium channel blocker verapamil (VP) on proline and glycine beatine content in shoots and roots at 1 st and 7 th day of stress in C 306 (C) and HD 2329 (H) wheat genotypes. 15 -d-old plants were subjected to PEG-6000 of -1. 0 MPa for 7 d and the observations were recorded during stress period. Means SE of three different samples are represented by vertical bars. ( Nayyar 2003)

Synthesis and accumulation of other osmoprotectants • Glycine betaine synthesis in chloroplasts by oxidation of choline in two steps • Accumulation of mono- and di-saccharides by inhibition of starch synthesis from new photosynthates, degradation of starch, inhibiton of respiration • Sugars serve not only as osmotica but also in signalling pathways, or gene expression regulation

Stress and reactive oxygen species (ROS) • ROS: singlet oxygen 1 O 2, superoxide radical O 2. -, hydroxyl radical (OH. ) and H 2 O 2 • ROS can be formed during photosynthesis (under stress is insufficient regeneration of NADP+ by Calvin cycle, O 2 is electron acceptor and in so called Mehler reaction 1 O 2 a O 2. - are formed), during respiration, photorespiration or degradation of lipids • Oxidative stress – lipid peroxidation, membrane damage, changes in nucleic acids, oxidative stress in chloroplasts can accelerate ageing • Under stress increased activity of different defence mechanisms is of vital importance • Carotenoids, xanthophylls • Non-enzymatic antioxidants (ascorbic acid, glutathione, tocopherols) • Antioxidative enzymes (superoxide dismutase, ascorbate peroxidase, catalase, glutathione reductase)

Water stress induced production of ABA, ROS, and antioxidants (Lei et al. 2006)

Antioxidative enzymes Fig. 3. Activity staining for SOD (A), POX (B) and PPO (C) in roots of two sesame cultivars at different levels of drought: Darab 14 subjected to 100, 75, 50 and 25 % FC (1 to 4), Yekta subjected to 100, 75, 50 and 25 % FC (5 to 8).