Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

/pdf/salt-and-drought-stress-signal-transduction-in-plants-4bj87ki2ww.pdf

Salt and drought stress signal transduction in plants

Cold, salinity and drought stresses: an overview.

Seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. It is determined by genetics with a substantial environmental influence which is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Not only is the dormancy status influenced by the seed maturation environment, it is also continuously changing with time following shedding in a manner determined by the ambient environment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In this review, we present an integrated view of the evolution, molecular genetics, physiology, biochemistry, ecology and modelling of seed dormancy mechanisms and their control of germination. We argue that adaptation has taken place on a theme rather than via fundamentally different paths and identify similarities underlying the extensive diversity in the dormancy response to the environment that controls germination.

/pdf/seed-dormancy-and-the-control-of-germination-4tdhm2985s.pdf

Seed dormancy and the control of germination

Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.

/pdf/abscisic-acid-emergence-of-a-core-signaling-network-1o69szbfnn.pdf

Abscisic Acid: Emergence of a Core Signaling Network

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

/pdf/abscisic-acid-signaling-in-seeds-and-seedlings-21b3w9tc2t.pdf

Abscisic Acid Signaling in Seeds and Seedlings

The three mutant alleles of the ABA locus of Arabidopsis thaliana result in plants that are deficient in the plant growth regulator abscisic acid (ABA). We have used 18O2 to label ABA in water-stressed leaves of mutant and wild-type Arabidopsis. Analysis by selected ion monitoring and tandem mass spectrometry of [18O]ABA and its catabolites, phaseic acid and ABA-glucose ester (beta-D-glucopyranosyl abscisate), indicates that the aba genotypes are impaired in ABA biosynthesis and have a small ABA precursor pool of compounds that contain oxygens on the ring, presumably oxygenated carotenoids (xanthophylls). Quantitation of the carotenoids from mutant and wild-type leaves establishes that the aba alleles cause a deficiency of the epoxy-carotenoids violaxanthin and neoxanthin and an accumulation of their biosynthetic precursor, zeaxanthin. These results provide evidence that ABA is synthesized by oxidative cleavage of epoxy-carotenoids (the "indirect pathway"). Furthermore the carotenoid mutant we describe undergoes normal greening. Thus the aba alleles provide an opportunity to study the physiological roles of epoxy-carotenoids in photosynthesis in a higher plant.

https://www.pnas.org/content/pnas/88/17/7496.full.pdf

The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis

http://www.faculty.ucr.edu/~jkzhu/articles/2001/Devcell.pdf

Modulation of Abscisic Acid Signal Transduction and Biosynthesis by an Sm-like Protein in Arabidopsis

The formation of a seed in the life cycle of higher plants is a unique adaptation. It incorporates embryo development with various physiological processes that insure the survival of the plant in the next generation. These adaptations include the accumulation of nutritive reserves, an arrest of tissue growth and development, and the ability to withstand desiccation, all of which are of considerable agronomic importance (e.g., nutritive value, yield, germination). The extent of these adaptations are quite spectacular. For example, the embryo must acquire the ability to withstand a reduction in water content from about 85% to 10%; in other plant tissues, such a severe desiccation is lethal. To survive long periods of time in this dry state until environmental conditions are favorable to resume development into a seedling, numerous plants have acquired different mechanisms of seed dormancy. The term “dormancy” is not entirely appropriate for many higher plants; this term can be defined as the absence of germination during environmental conditions which otherwise promote germination. Typically some external stimulus such as light or chilling (stratification) is required. However, many angiosperms undergo the developmental program of maturation, developmental arrest, and desiccation without true dormancy.

The Role of Hormones During Seed Development

Recent progress in ABA signalling is summarized from the perspectives gained by genetic (mutant) analysis, 'reverse genetics' (starting from unknown ABA-inducible sequences and working backwards) and biochemical studies. What emerges is a cell-biological model of overlapping tissue-specific stress (e.g. drought, salt and cold) and developmental (e.g. sugars and other hormones) response pathways that integrate into responses mediated by ABA, including but not limited to seed maturation, dormancy, inhibition of cell division and germination, stomatal closure and changes in gene expression leading to stress adaptation. ABA signalling involves putative ABA receptors (extracellular or intracellular), cell-surface membrane proteins including ion channels, glycoproteins and membrane trafficking components, secondary messengers such as phosphatidic acid, inositol 1,4,5-trisphosphate, cyclic ADP-ribose and calcium, and protein phosphorylation/dephosphorylation cascades leading to chromatin remodelling and binding of transcriptional complexes to ABA-responsive promoter elements. The large gaps in our understanding of complex regulatory networks such as ABA signalling can be best addressed by multidisciplinary, integrated approaches such as those discussed. Contents Summary I. INTRODUCTION 358 II. GENETIC ANALYSIS OF ABA RESPONSES 359 III. 'REVERSE GENETIC' ANALYSIS OF ABA-REGULATED GENE EXPRESSION 371 IV. BIOCHEMICAL AND CELLULAR ANALYSES OF ABA SIGNALLING 378 V. CONCLUSIONS AND PERSPECTIVES 387 Acknowledgements 387 References 387.

https://www.depts.ttu.edu/biology/people/Faculty/Rock/publications/NewPhytol148357.pdf

Christopher D. Rock

Papers

Abscisic Acid Signaling in Seeds and Seedlings

The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis

Modulation of Abscisic Acid Signal Transduction and Biosynthesis by an Sm-like Protein in Arabidopsis

The Role of Hormones During Seed Development

Tansley review no. 120: pathways to abscisic acid-regulated gene expression.