Volume 4 Special Issue 1 2010
PLANT NUTRITION and ABIOTIC STRESS TOLERANCE I

ISBN 978-4-903313-59-7

How to reference: Tuteja N, Gill SS, Trivedi PK, Asif MH, Nath P (2010) Plant Growth Regulators and their Role in Stress Tolerance. In: Anjum NA (Ed) Plant Nutrition and Abiotic Stress Tolerance I. Plant Stress 4 (Special Issue 1), 1-18

Guest Editor

Naser A. Anjum

Aligarh Muslim University, India

www.amu.ac.in

Centre for Environmental and Marine Studies (CESAM) & Department of Chemistry, University of Aveiro, Portugal

www.cesam.ua.pt/

CONTENTS AND ABSTRACTS

Narendra Tuteja, Sarvajeet Singh Gill, Prabodh Kumar Trivedi, Mehar Hasan Asif, Pravendra Nath (India) Plant Growth Regulators and their Role in Stress Tolerance (pp 1-18)

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Invited Review: Abiotic and biotic stresses affect the growt and yield adversely in agronomically important crop plants. Management through avoidance is not possible in plants, which have developed various kinds of signaling processes which sense stress signal, transduce it to desired places and activate/recruit molecules that are capable of protecting them against that stress. Signal molecules can be various phytohormones including ethylene, jasmonic acid, auxin, abscissic acid, brassionosteroids, etc. or simple ions like Ca²⁺ or molecules like salicylic acid. These signaling molecules activate a cascade of events either on their own or by evolving a cross-talk with other signaling systems or metabolites. As a result various transcription factors are recruited which activate or suppress a variety of genes. The gene products or their absence trigger the production of various protectant molecules which may include chaperons, osmolytes, gases, ions or other molecules which are capable of protecting cell constituents. Various mechanisms which underlie these processes have provided some very useful leads in understanding how plants resist stress. This has also helped in identifying various factors and genes that are responsible for stress management. With the advent of modern tools of recombinant DNA technology it seems possible to utilize this knowledge in developing varieties that are resistant to stress. This will not only help in reducing losses in agricultural crops but also protect and save elite germplasm when conditions are not conducive. This review deals with various studies that have been done with respect to different kind of stresses in the past two decades or so. It also brings out various molecules and genes that are associated with stress management in plants.

Yankun Zhao, Tao Wang, Wensheng Zhang, Xia Li (China) The Role of Auxin in Plastic Development of Plant Root System Architecture in Response to Abiotic Stimuli (pp 19-31)

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Invited Review: Plants have evolved abilities to cope with changes in their external environment because of their sessile nature. This regulatory plasticity leads to modifications in physiology, morphology and/or development, which improve the chances of survival and reproductive success under adverse environmental conditions. Hormones are involved in the link between the genome and the environment, and directly affect phenotypic characteristics. The growth hormone auxin has been considered as a master regulator controlling almost every aspect of plant growth; however, the role of auxin in plant developmental plasticity in response to biotic and abiotic stimuli has not drawn much attention. Recently, genetic and biochemical analyses have revealed that changes in auxin biosynthesis, polar transport and auxin sensitivity in response to various abiotic stresses can produce changes in gene transcription and modification of metabolic rates in stressed plants. Dynamic localization of auxin and inhibitory, stimulatory and/or synergistic interaction with other hormones both influence phenotypic plasticity. These results have provided novel insights into a molecular basis for auxin regulation of developmental plasticity in coupling extrinsic and intrinsic cues. In this review, we summarize recent advances in plant root morphological adaptation with a particular focus on understanding the interaction between the auxin and environmental cues in plant root plastic morphogenesis.

Farida M. Shakirova, Azamat M. Avalbaev, Marina V. Bezrukova, Guzel R. Kudoyarova (Russia) Role of Endogenous Hormonal System in the Realization of the Antistress Action of Plant Growth Regulators on Plants (pp 32-38)

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Invited Mini-Review: The endogenous hormonal system plays a leading role in the regulation of growth and development of plants. This regulatory system responds sensitively to even slight changes in the plant environment, which is manifested in reorganization of the hormonal status. Detrimental factors lead to an imbalance in the content of different groups of phytohormones, as a rule reflected in the accumulation of abscisic acid (ABA) and in a parallel decline in the content of metabolism-stimulating hormones indole-3-acetic acid (IAA) and cytokinins. In total, these changes in hormone content in plants are related to inducing an effective stress response of plants accompanied however with inhibition of growth processes and plant productivity. This makes actual application of natural growth regulators characterized by growth-stimulating and anti-stress activity to increase both plant resistance and productivity as well. The present mini review mainly concentrates on the shifts in endogenous phytohormones (ABA, IAA and cytokinins) level caused by application of hormones such as, brassinosteroids, cytokinins and salicylic acid (SA). The main attention is focused on the role of these changes in endogenous hormones in the realization of growth-stimulating and protective function of applied plant growth regulators (PGRs). It is necessary to underline that simultaneous analysis of different groups of phytohomones allowed us to reveal a complex pattern of changes in plant hormonal system in response to treatments with exogenous growth regulators and to evaluate their contribution to the control of resistance to stress factors.

Shabina Syeed, Nafees A. Khan (India) Physiological Aspects of Salicylic Acid-Mediated Salinity Tolerance in Plants (pp 39-46)

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Invited Mini-Review: Salicylic acid (SA) has been known to present in plant tissues for quite some time, but has only recently been recognized as a potential plant growth regulator. Studies indicate the significant role of SA in the signal transduction pathway of biotic stresses in plants. The application of exogenous SA could provide protection against several types of abiotic stresses such as, salinity, high or low temperature, heavy metals, etc. Although SA may also cause oxidative stress to plants, partially through the accumulation of hydrogen peroxide, the results published so far show that the preliminary treatment of plants with low concentrations of SA might have acclimation-like effects. Such acclimation effect is associated with several physiological processes. The present mini-review explores the available information in the literature on the physiological aspects of SA-mediated salinity tolerance.

P. K. Singh, Shruti Gautam (India) Biochemical and Molecular Basis of Salinity Stress Tolerance in Plants: Role of Salicylic Acid (pp 47-54)

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Invited Review: Environmental stresses come in many forms and the ability to withstand these stresses is one of the most important factors that decide the distribution of plant species in an area. These stresses affect several physiological, biochemical and molecular processes in plants and limit plant productivity. The study of abiotic stresses especially salinity in plants has advanced significantly in recent years, however the majority of experiments testing the response of plants to salt stress have focused on chemical treatment applied to plants under controlled conditions. Thus development of methods for inducing stress tolerance in plants is vital and would be a major focus of research in adverse environmental conditions. More recently, some of the endogenous growth regulators like abscisic acid, jasmonic acid and salicylic acid (SA) have been implicated in imparting stress tolerance in plants. SA has appeared as a signaling molecule modulating plant response to biotic and abiotic stresses. The established effects of SA on photosynthetic activity, nitrate assimilation and regulation of ROS in plants, give an indication that SA might possess defensive function probably induction of some stress responsive proteins in plants under saline conditions . It also counteracts the deleterious effects of salt on biochemical changes like decreasing MDA content (a product of lipid peroxidation), and free proline content. Thus studies suggest that exogenous application of salicylic acid in plants may bring some potential practical utilization. For example, manipulating the tissue level of SA in plants may be a promising area for the importance of biotechnology to crop protection and high yield. The main objective of this article was to develop new concept about the role of salicylic acid in growth characteristics, metabolic homeostasis, osmoregulation, ROS regulation, gene induction and signaling of various metabolic pathways in plants.

B. Vidya Vardhini, S. Anuradha, E. Sujatha, S. Seeta Ram Rao (India) Role of Brassinosteroids in Alleviating Various Abiotic and Biotic Stresses - A Review (pp 55-61)

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Invited Mini-Review: Brassinosteroids (BRs) are a new class of plant hormones with significant growth-promoting influence. BRs were first isolated and characterized from the pollen of Brassica napus L. Subsequently they were reported from 44 plants (9 monocots, 28 dicots, 5 gymnosperms, 1 pteridophyte and 1 alga) so far and are regarded probably ubiquitous in plant kingdom. BRs are considered as hormones with pleiotropic effects as they influence various developmental processes like growth, germination of seeds, rhizogenesis, flowering and senescence. BRs also confer resistance to plants against various abiotic stresses. The article reviews the work relating to their effect to alleviate various abiotic and biotic stresses.

Shamsul Hayat, Syed Aiman Hasan, Qaiser Hayat, Mohd Irfan (India), Aqil Ahmad (Sultanate of Oman) Effect of Salicylic Acid on Net Photosynthetic Rate, Chlorophyll Fluorescence, and Antioxidant Enzymes in Vigna radiata Plants Exposed to Temperature and Salinity Stresses (pp 62-71)

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Original Research Paper: Ten-days-old seedlings of mung bean (Vigna radiata cv. ‘T-44’) were exposed to salicylic acid (SA) and/or temperature and/or NaCl stresses. The treated seedlings were sampled at 18 days after sowing (DAS) to assess the change in growth pattern, photosynthetic attributes, quantum yield of PSII (Fv/Fm), activity of antioxidative enzymes i.e. peroxidase (POX), catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR) and the activity of nitrate reductase and carbonic anhydrase. The plants exposed to temperature and/or saline stress exhibited a reduction in growth, photosynthesis, and the activity of nitrate reductase and carbonic anhydrase. However, treatment with SA both in the presence or absence of stresses significantly improved the values for the above mentioned parameters. Moreover, the activities of antioxidative enzymes and proline content increased in response to both SA and the stress(s). The interaction of salinity and temperature stress with SA treatment had an additive effect enhancing significantly the values by 107.3% (POX), 37.2% (CAT), 37.5% (SOD), 55.3% (GR) and 65.3% (proline content) over control. SA treatment significantly affected the membrane stability index (MSI) under stressed as well as unstressed conditions resulting in a significant enhancement, 27% more than the MSI of plants exposed to a combined temperature and salinity stress. It may therefore be concluded that SA plays an important role in signaling pathways of plants leading to resistance against temperature and/or salinity stresses.

Masoomeh Etehadnia, Jeff Schoenau, Doug Waterer, Tanino Karen (Canada) The Effect of CaCl₂ and NaCl Salt Acclimation in Stress Tolerance and its Potential Role in ABA and Scion/Rootstock-Mediated Salt Stress Responses (pp 72-81)

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Original Research Paper: Both NaCl and CaCl₂ salt acclimation pre-treatment responses were examined in contrasting salt stress resistant potato genotypes, ABA-, ABA+, 9506, ‘Norland’ and reciprocally grafted sensitive and resistant potato genotypes. Plants were grown in a hydroponic nutrient sand culture under greenhouse conditions and were subjected to: (a) no pre-treatment; (b) NaCl pre-treatment; (c) CaCl₂ pre-treatment; and (d) combined CaCl₂ + NaCl pre-treatment and stressed with higher levels of NaCl salt for two weeks. Pre-treatment with NaCl-based acclimation generally enhanced salt stress resistance to a greater extent than CaCl₂. This was associated with a specific Na⁺ ion effect rather than a non-specific EC-dependent response. That the ABA(-) genotype was unable to exclude Na⁺ from the shoot relative to the ABA(+) and other genotypes. That the CaCl₂ pre-treatment also enhanced shoot K⁺ except in the ABA(-) genotype suggests that both ABA and CaCl₂ are requirements for this mode of salt stress defence. The salt resistant rootstock positively influenced water content in the salt stressed sensitive scions as reflected through stomatal conductivity, osmotic potential, shoot growth and water content of the scions. Grafting salt stress resistant scions onto the ABA(-) rootstock increased in root growth. Grafting of salt- resistant scions under salt acclimation treatments may be an alternative approach to increasing stress resistance in commercially important potato cultivars. While the CaCl₂ acclimation pre-treatment was not as effective as NaCl pre-treatment, the importance of calcium in stress acclimation cannot be excluded, particularly since low levels of Ca²⁺ existed in the background nutrient and NaCl salt stress solutions. Calcium and ABA appear to be involved in increasing the ratio of K⁺/ Na⁺ in the shoots, associated with elevated salt stress resistance.

Mudawi Elobeid (Germany/Sudan), Andrea Polle (Germany) Response of Grey Poplar (Populus x canescens) to Copper Stress (pp 82-86)

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Original Research Paper: Copper (Cu) is an essential element for plant growth and development, but it can be toxic when available in excessive amounts. This study aimed to determine Cu sensitivity (0-500 µM) in stem height and radial growth in relation to photosynthetic performance and stress enzymes in young Populus x canescens trees. Biomass and leaf formation rate were unaffected by Cu variation in the range from 0.128 µM (normal supply with the nutrient solution) to 5 µM. Higher Cu concentrations caused reductions in all growth parameters and severe leaf injury. The quantum yield of photosystem II was decreased at Cu concentrations above 5 µM, but recovered in darkness almost completely indicating high Cu tolerance of photosystem II despite foliar damage. The activities of stress enzymes such as guaiacol peroxidase, glutathione peroxidase and NADH oxidase showed no increase with growth reductions suggesting that H₂O₂ was not involved in stress symptoms. Cu deficiency stimulated root growth. Modulation of Cu supply in the optimum range affected the relation between elongation and radial growth indicating differences in the Cu demand of these processes.

Noushina Iqbal, Nafees A. Khan (India) Variation in Growth, Photosynthesis Functions and Yield of Five Mustard (Brassica juncea L.) Cultivars under High Cadmium Stress (pp 87-93)

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Original Research Paper: Five cultivars of mustard(Brassica juncea L. Czern and Coss.) namely ‘Alankar’, ‘Varuna’, ‘Pusa Jai Kisan’, ‘SS2’ and ‘Dhanuka Bold’ were tested for tolerance to cadmium (Cd). Plants were raised from seeds in earthen pots with treatment of 0, 25 and 50 µM Cd in nutrient solution. All levels of Cd decreased growth and yield of the tested cultivars with varying degrees. Cadmium tolerance (CdT), the ability of plant to maintain high yield at maximum level of Cd, was calculated as the ratio of yield at the untreated and Cd-treated soils. Among mustard cultivars ‘Pusa Jai Kisan’ was identified as the Cd tolerant, while ‘SS2’ as the Cd non-tolerant cultivar. To find out the physiological basis of these differences, we investigated the possible role of photosynthetic pigments [chlorophyll (Chl), carotenoid, pheophytin, anthocyanin], dry matter accumulation and leaf-Cd accumulation capacity of the cultivars. Among photosynthetic traits, Cd treatment decreased the content of Chl (Chl a, Chl b, total Chl), Chl fluorescence (F_v/F_m) and carotenoid. However, the content of pheophytin and anthocyanin increased significantly in all the cultivars. Cd accumulation in leaves also increased with increase in Cd level. However, the extent of Cd-induced decrease or increase characteristics was found greater in Cd-non-tolerant (‘SS2’) than Cd-tolerant (‘Pusa Jai Kisan’) cultivars. ‘Pusa Jai Kisan’ maintained a higher content of Chl, carotenoid and relative amount of anthocyanin although it had the least percent pheophytin and Cd-content in leaves and subsequently produced higher dry matter and seed yield than the other cultivars at all levels of Cd.

Jai Prakash Srivastava, Pratap Singh, Vinay Pratap Singh, Ruchi Bansal (India) Effect of Waterlogging on Carbon Exchange Rate, Stomatal Conductance and Mineral Nutrient status in Maize and Pigeonpea (pp 94-99)

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Original Research Paper: An investigation was carried out on maize (Zea mays) (‘V-32’ and ‘CML-49’) and pigeonpea (Cajanus cajan) (‘ICPL-84023’ and ‘MAL-18’) genotypes to study the effect of waterlogging on physiological parameters. Waterlogging stress was imposed after 20 days of sowing (DOS) in maize and 40 DOS in pigeonpea by placing pots in water-filled containers in such a way that the water level remained 2.5 to 3.0 cm above the soil surface in pots. Plants were kept at optimal supply of soil moisture, and labelled as “normal”. Changes in photosynthetic parameters and mineral nutrient status were observed in the first fully expanded leaf from top in normal and waterlogged plants after 9 days of imposing waterlogging stress in maize and 12 days in pigeonpea, at these stages visible symptoms of waterlogging stress were evident in both the crops. Waterlogging caused significant reduction in carbon exchange rate in pigeonpea, but the reduction was not significant in maize genotypes. Under waterlogged condition stomatal conductance decreased significantly in maize genotypes, but in pigeonpea it followed a variable trend. Variations in the contents of leaf N, Mg, Ca, Fe, Cu, Mn and Al were observed under the influence of waterlogging stress. Maize plants experienced N, Ca, Mg, Fe, Mn, Zn and Cu deficiency, while pigeonpea genotypes faced deficiencies of N, Mg and Mn under waterlogged condition. Al toxicity was observed in a waterlogging-susceptible maize genotype, but no such effect was evident in pigeonpea. Genotypic differences in both maize as well as pigeonpea were evident and correlated with the relative performance of the genotypes under waterlogged condition.

Sabrina Zuchi, Maurizio Enea Picarella, Gian Piero Soressi, Stefania Astolfi (Italy) Salt Stress: Effects on Nitrogen Metabolism in Tomato Plants Differing in Salt Tolerance (pp 100-106)

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Original Research Paper: The cultivated tomato (Solanum lycopersicum L.) is a widely-grown crop plant and the focus of a large agricultural industry. Tomato is cultivated in almost every corner of the world, but a major portion of the world tomato production is concentrated in a rather limited number of warm and not humid areas. Although such areas generally provide optimal environmental conditions for tomato production, a high level of salinity frequently encountered in the soil or in the irrigation water poses serious constraints to tomato production. The aim of this work was to analyze the relationship between salinity and nitrogen (N) metabolism, in order to evaluate the effects of using sea water for tomato irrigation and to optimize N use efficiency and thus yield amount and quality. Three tomato genotypes differing in their relative level of salt tolerance were exposed to salinity stress: cv. ‘Edkawi’ (EDK), salt tolerant; cv. ‘Gimar’ (GIM), relatively salt sensitive and its near isogenic line for the nor gene (NOR) defective in ethylene synthesis. Tomato plants were grown hydroponically for 4 weeks in control (EC = 3 mS cm^-1) or in saline conditions (EC = 10 mS cm^-1). Plant growth, ethylene emission, Cl^-, NO₃^- and free amino acids (a.a.) contents and the activities of nitrate reductase (NR; EC 1.6.6.1), glutamine synthetase (GS; EC 6.3.1.2) and NAD-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2) were measured in roots to compare control and salt-treated plants. Results confirmed the NO₃^-/Cl^- antagonism and suggested nitrate content as a marker of plant salt tolerance. The relationship between root NR activity and NO₃^- content suggests a complex response of tomato plants to imposed salt stress.