Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

https://hal.archives-ouvertes.fr/hal-00886451/document

Plant drought stress: effects, mechanisms and management

Zinc (Zn) is an essential component of thousands of proteins in plants, although it is toxic in excess. In this review, the dominant fluxes of Zn in the soil-root-shoot continuum are described, including Zn inputs to soils, the plant availability of soluble Zn(2+) at the root surface, and plant uptake and accumulation of Zn. Knowledge of these fluxes can inform agronomic and genetic strategies to address the widespread problem of Zn-limited crop growth. Substantial within-species genetic variation in Zn composition is being used to alleviate human dietary Zn deficiencies through biofortification. Intriguingly, a meta-analysis of data from an extensive literature survey indicates that a small proportion of the genetic variation in shoot Zn concentration can be attributed to evolutionary processes whose effects manifest above the family level. Remarkable insights into the evolutionary potential of plants to respond to elevated soil Zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous Zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri.

Zinc in plants

Summary
The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of ‘promoter’ substances, such as ascorbate, β-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of ‘antinutrients’, such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

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Biofortification of crops with seven mineral elements often lacking in human diets--iron, zinc, copper, calcium, magnesium, selenium and iodine.

https://www.cell.com/trends/plant-science/pdf/S1360-1385(06)00163-4.pdf

Silicon uptake and accumulation in higher plants

INTRODUCTION. Internal Organization of the Plant Body. Summary of Types of Cells and Tissues. General References. DEVELOPMENT OF THE SEED PLANT. The Embryo. From embryo to the Adult Plant. Apical Meristems and Their Derivatives. Differentiation, Specialization, and Morphogenesis. References. THE CELL. Cytoplasm. Nucleus. Plastids. Mitochondria. Microbodies. Vacuoles. Paramural Bodies. Ribosomes. Dictyosomes. Endoplasmic Reticulum. Lipid Globules. Microtubules. Ergastic Substances. References. CELL WALL. Macromolecular Components and Their Organization in the Wall. Cell Wall Layers. Intercellular Spaces. Pits, Primary Pit--Fields, and Plasmodesmata. Origin of Cell Wall During Cell Division. Growth of Cell Wall. References. PARENCHYMA AND COLLENCHYMA. Parenchyma. Collenchyma. References. SCLERENCHYMA. Sclereids. Fibers. Development of Sclereids and Fibers. References. EPIDERMIS. Composition. Developmental Aspects. Cell Wall. Stomata. Trichomes. References. XYLEM: GENERAL STRUCTURE AND CELL TYPES. Gross Structure of Secondary Xylem. Cell Types in the Secondary Xylem. Primary Xylem. Differentiation of Tracheary Elements. References. XYLEM: VARIATION IN WOOD STRUCTURE. Conifer Wood. Dicotyledon Wood. Some Factors in Development of Secondary Xylem. Identification of Wood. References. VASCULAR CAMBIUM. Organization of Cambium. Developmental Changes in the Initial Layer. Patterns and Causal Relations in Cambial Activity. References. PHLOEM. Cell Types. Primary Phloem. Secondary Phloem. References. PERIDERM. Structure of Periderm and Related Tissues. Development of Periderm. Outer Aspect of Bark in Relation to Structure. Lenticels. References. SECRETORY STRUCTURES. External Secretory Structures. Internal Secretory Structures. References. THE ROOT: PRIMARY STATE OF GROWTH. Types of Roots. Primary Structure. Development. References. THE ROOT: SECONDARY STATE OF GROWTH AND ADVENTITIOUS ROOTS. Common Types of Secondary Growth. Variations in Secondary Growths. Physiologic Aspects of Secondary Growth in Roots. Adventitious Roots. References. THE STEM: PRIMARY STATE OF GROWTH. External Morphology. Primary Structure. Development. References. THE STEM: SECONDARY GROWTH AND STRUCTURAL TYPES. Secondary Growth. Types of Stems. References. THE LEAF: BASIC STRUCTURE AND DEVELOPMENT. Morphology. Histology of Angiosperm Leaf. Development. Abscission. References. THE LEAF: VARIATIONS IN STRUCTURE. Leaf Structure and Environment. Dicotyledon Leaves. Monocotyledon Leaves. Gymnosperm Leaves. References. THE FLOWER: STRUCTURE AND DEVELOPMENT. Concept. Structure. Development. References. THE FLOWER: REPRODUCTIVE CYCLE. Microsporogenesis. Pollen. Male Gametophyte. Megasporogenesis. Female Gametophyte. Fertilization. References. THE FRUIT. Concept and Classification. The Fruit Wall. Fruit Types. Fruit Growths. Fruit Abscission. References. THE SEED. Concept and Morphology. Seed Development. Seed Coat. Nutrient Storage Tissues. References. EMBRYO AND SEEDLING. Mature Embryo. Development of Embryo. Classification of Embryos. Seedling. References. Glossary. Index.

Anatomy of Seed Plants

Asymmetrical development of root endodermis and exodermis in reaction to abiotic stresses.

Growth parameters of six fast growing trees showed that the roots responded to Cd treatment more sensitively than the shoots. Cd-treatment suppressed rooting and root growth (length and biomass production) as well as its development in all tested species. Root systems of Salix cinerea, Salix alba, and Populus cv. Robusta were more tolerant to Cd stress than the root system of the other studied species. Shoot growth parameters of Salix species were significantly reduced unlike Populus species, which were not affected by Cd treatment.

Changes in the Rooting and Growth of Willows and Poplars Induced by Cadmium

Zinc (Zn) is an essential microelement involved in various plant physiological processes. However, in excess, Zn becomes toxic and represents serious problem for plants resulting in Zn toxicity symptoms and decreasing biomass production. The effect of high Zn and its combination with silicon (Si) on ionome and expression level of ZmLsi genes was investigated in maize (Zea mays, L; hybrid Novania). Plants were cultivated hydroponically in different treatments: control (C), Zn (800 μM ZnSO4 · 7H2O), Si5 (5 mM of sodium silicate solution), and Si5 + Zn (combination of Zn and Si treatments). Growth of plants cultivated for 10 days was significantly inhibited in the presence of high Zn concentration and also by Zn and Si interaction in plants. Based on principal component analysis (PCA) and mineral element concentration in tissues, root ionome was significantly altered in both Zn and Si5 + Zn treatments in comparison to control. Mineral elements Mn, Fe, Ca, P, Mg, Ni, Co, and K significantly decreased, and Se increased in Zn and Si5 + Zn treatments. Shoot ionome was less affected than root ionome. Concentration of shoot Cu, Mn, and P decreased, and Mo increased in Zn and Si5 + Zn treatments. The PCA also revealed that the responsibility for ionome changes is mainly due to Zn exposure and also, but less, by Si application to Zn stressed plants. Expression level of Lsi1 and Lsi2 genes for the Si influx and efflux transporters was downregulated in roots after Si supply and even more downregulated by Zinc alone and also by Zn and Si interaction. Expression level of shoot Lsi6 gene was differently regulated in the first and second leaf. These results indicate negative effect of high Zn alone and also in interaction with Si on Lsi gene expression level and together with ionomic data, it was shown that homeostatic network of mineral elements was disrupted and caused negative alterations in mineral nutrition of young maize plants.

Ionome and expression level of Si transporter genes ( Lsi1 , Lsi2 , and Lsi6 ) affected by Zn and Si interaction in maize

The aim of this study was to determine the effect of exogenously applied Si on the growth and physiological parameters of sorghum [Sorghum bicolor (L.) Moench] cultivated in hydroponics with elevated zinc concentrations (75 μM and 150 μM Zn). Increased concentrations of Zn inhibited root growth and biomass production of roots and shoots. Application of Si individually showed a positive effect on root growth but negatively affected production of fresh and dry biomass of roots and shoots. On the other hand, silicon in combination with Zn significantly reduced the inhibitory effect of Zn on root growth but did not positively affect biomass production of roots and shoots. Accumulation of Zn in plant tissues increased with increasing Zn concentration in nutrient solution, but application of Si in combination with Zn did not significantly influence Zn accumulation in roots. Completely opposite results were found in Si accumulation in roots treated with Si in combination with Zn. Interaction of these ions resulted in considerable increase of Si accumulation in roots which almost doubled in comparison to individal Si treatment. Impact of Zn on the activity of some antioxidant enzymes was equivocal and differences were observed also between two Zn concentrations. Individual application of Si resulted in significant increase in the activity of all studied antioxidant enzymes but Si in combination with Zn mostly negatively affected their activity except the activity of catalase (CAT) which was the highest in roots grown in solution containing both Si and Zn ions. Comparing all obtained data we can assume that Si applied in combination with Zn did not significantly alleviate Zn toxicity in young sorghum except the growth of primary seminal root and further experiments are required for better understanding of their interaction.

Effect of silicon application on Sorghum bicolor exposed to toxic concentration of zinc

The primary root structure of Zn/Cd hyperaccumulator Thlaspi caerulescens was compared with the root anatomy of closely related non-accumulator Thlaspi arvense. The most striking structure formed close to the root tip in T. caerulescens, but not in T. arvense, is the peri-endodermal layer of cells with irregularly thickened inner tangential walls. This layer observed in some species of Brassicaceae by early anatomists and described as 'reseau sus-endodermique' is composed of secondary cell walls impregnated by lignin, and it forms a compact cylinder encircling the endodermis. The differences in the proportions of individual tissues ? rhizodermis, cortex, stele and endodermis ? of the two species are statistically insignificant. However, there are differences in the cell arrangement. The precise function of the lignified cell wall thickening of the peri-endodermal layer of T. caerulescens is unclear at present. Some possible roles are discussed and its potential relation with metal hyperaccumulation and/or tolerance should be investigated.

Alexander Lux

Papers

Asymmetrical development of root endodermis and exodermis in reaction to abiotic stresses.

Changes in the Rooting and Growth of Willows and Poplars Induced by Cadmium

Ionome and expression level of Si transporter genes ( Lsi1 , Lsi2 , and Lsi6 ) affected by Zn and Si interaction in maize

Effect of silicon application on Sorghum bicolor exposed to toxic concentration of zinc

Difference in the root structure of hyperaccumulator Thlaspi caerulescens and non-hyperaccumulator Thlaspi arvense