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Plant growth and development are adversely affected by salinity – a major environmental stress that limits agricultural production. This chapter provides an overview of the physiological mechanisms by which growth and development of crop plants are affected by salinity. The initial phase of growth reduction is due to an osmotic effect, is similar to the initial response to water stress and shows little genotypic differences. The second, slower effect is the result of salt toxicity in leaves. In the second phase a salt sensitive species or genotype differs from a more salt tolerant one by its inability to prevent salt accumulation in leaves to toxic levels. Most crop plants are salt tolerant at germination but salt sensitive during emergence and vegetative development. Root and shoot growth is inhibited by salinity; however, supplemental Ca partly alleviates the growth inhibition. The Ca effect appears related to the maintenance of plasma membrane selectivity for K over Na. Reproductive development is considered less sensitive to salt stress than vegetative growth, although in wheat salt stress can hasten reproductive growth, inhibit spike development and decrease the yield potential, whereas in the more salt sensitive rice, low yield is primarily associated with reduction in tillers, and by sterile spikelets in some cultivars.

Plants with improved salt tolerance must thrive under saline field conditions with numerous additional stresses. Salinity shows interactions with several stresses, among others with boron toxicity, but the mechanisms of salinity-boron interactions are still poorly known. To better understand crop tolerance under saline field conditions, future research should focus on tolerance of crops to a combination of stresses

The growth and function of roots are essential for crop productivity under water-limiting conditions, but direct improvement of roots by plant breeding has been slow. One difficulty is the observation and quantitative measurement of root systems under conditions that are relevant to field environments. Another challenge is the identification of and selection for specific loci that could improve the acquisition of water from the soil profile. However, advances are being made in the understanding of root growth regulation and development. We review the evidence for the maintenance of root growth by ABA during water deficit, and the interactions with ethylene and other hormones. A biophysical model of cell expansion serves to focus discussion of topics relating to regulation of growth and development. The power of kinematic growth analysis is demonstrated by highlighting changes in growth regulatory processes and associated patterns of gene expression and protein composition that occur specifically in regions of the root where cell expansion is maintained under water deficit conditions. Growth is a complex process; new information adds further insight and further complexity to our understanding of how roots sense and respond to changes in environmental conditions. It is important to unravel these adaptive mechanisms so that it is clear how the manipulation of one process will affect the function of the whole plant, and so that the effect on final yield and water use can be predicted. This complexity makes simple linear models inadequate as explanatory tools, and a systems approach is needed to incorporate the weave of interacting networks of signaling and response pathways. The real challenge is to discover how root growth can be improved, to supply breeders with the practical tools to identify or introduce superior alleles in crop species, and ultimately to ensure that discoveries lead to improvements in productivity in the field

In this chapter we consider the advantages and disadvantages of different root growth patterns and root functional characteristics in terms of water and nutrient uptake from soils depleted of these resources. Impacts are considered within a framework of analysis which considers crop yield to be a function of water available to the crop during its life cycle, the amount of biomass produced by the crop for every unit of water available and the proportion of the biomass produced going into reproductive yield. Root properties will impact on all of these variables and can therefore impact substantially on yield in conditions where water and nutrients are limiting. We suggest that regulation of this kind can form an effective basis for crop improvement programs focused on dryland environments

The regulation of gas exchange at the leaf level is a key factor for plant survival under a fluctuating environment (Buckley, 2005). In this context, control of stomatal opening and closure is the evolutionary solution to balance water loss with CO uptake and yield. A decrease in leaf/root water potential resulting from soil drought is typically accompanied by an elevated level of abscisic acid (ABA), which is well established as a stress hormone (Davies et al., 2005). ABA is a central component in drought-stress sensing leading to efficient stomatal control, thereby avoiding deleterious yield losses during stress conditions. Depending on the crop species, or its growing environment, different strategies for yield-optimization need to be chosen (Araus et al., 2002; Chaves and Oliveira, 2004). ABA effects are modulated by the levels of and sensitivity to other hormones, in an interdependent network. Unraveling the complex regulatory mechanisms of stomatal control between hormones, second messengers, ion channels and other classes of implicated proteins will lead to new insights in how to tailor plants to take maximum advantage of the available natural resources (Li et al., 2006). Possible strategies are either to trigger an earlier stress response without a negative impact on yield, or to attenuate the plant stress response so that assimilation will increase. These desired traits can be brought about by overexpressing or downregulating the expression of specific genes involved in the complex and possibly redundant signaling network of stomatal responses. This chapter provides an overview of the mechanisms behind the changes in stomatal movements under water-limiting conditions, including hormonal regulation and developmental influences

A waxy cuticle covers the aerial organs of plants that functions to prevent uncontrolled water loss. The cuticle has often been considered a non-responsive adaptation that acts simply as a barrier to water loss, when in fact cuticle metabolism is quite responsive to environmental stresses. The responsiveness of the cuticle has been demonstrated by changes in cuticle chemistry and cuticle gene expression of drought and salt exposed plants. Alteration of cuticle traits through breeding and biotechnology approaches may prove useful in improving crops for drought and salt tolerance. However, work is still needed to lay the foundation for the use of cuticle genes and traits for agronomic purposes

Soil-water-deficit stress causes many changes in the biology of the plant cell beginning with the perception of the stress followed by changes that promote the acclimation to the stress. The mechanism by which plant cells transduce the physical stress of loss of water to biochemical changes in the cell continues to elude plant biologists. Using modern techniques that allow measurements of thousands of changes in gene expression at one time, researchers have catalogued and are beginning to make progress in interpreting the function of the many changes in gene expression. Although, it still remains a challenge to understand the function and relevance of many of these responses. There are indications that laboratory stress conditions intended to mimic plant water-deficit stress do not cause a universal water stress response; only a small number of genes are commonly induced when plants are subjected to water-deficit stress in different laboratories. Researchers remain optimistic that lessons learned from the molecular response of Arabidopsis plants to stress can be used to improve crops for growth under non ideal field conditions and lessen the need for irrigation in areas of the world where water availabilty for agriculture is decreasing

Plants cope with environmental changes by activating signal transduction cascades that control and coordinate the physiological and biochemical responses necessary for adaptation. Numerous signaling pathways that function as an integrated network have been implicated in plant abiotic stress response. Amongst them, calcium signaling was found to be incorporated in different signaling pathways during abiotic stress response, e.g. to heat, cold, drought, and salt. A well-recognized model of calcium signaling is that calcium signals characteristic of either elevation or oscillation of cytosolic Ca is generated upon stimulation and then transduced through an array of Ca activated proteins and downstream components, including calmodulins (CaMs) and CaM-binding proteins (CaMBPs), calcineurin B-like proteins (CBLs), Ca-dependent protein kinases (CDPKs), Ca and CaM-binding transcription factors, and other Ca-binding proteins. Potential targeted effectors of calcium signaling include important enzymes/proteins involved in various cellular metabolism and physiological adjustment. This review begins with the generation of calcium signals followed by reviewing components decoding calcium signals. Implication of the signaling components in drought and salt stress response is emphasized and discussed

Many stresses trigger transient increases in minor phospholipids, such as phosphatidic acid (PA) and phosphoinositides (PIs), in plants. Such changes are early events in signaling plant stress response. Lipid mediators affect cellular functions through direct interaction with proteins and/or structural effects on cell membranes. The identified lipid targets in plants include protein phosphatases, kinases, and proteins involved in membrane trafficking and cytoskeleton. The effect of lipids on signaling, intracellular trafficking, and cytoskeletal organization plays important roles in plant coping with drought and salinity

The plant stress hormone abscisic acid (ABA) plays several critical roles in plant response to stress and stress tolerance. ABA is well studied for its roles in the activation of stress-responsive genes and the regulation of guard cell movement. More recently, ABA has also been demonstrated to regulate root adaptation to drought stress. To date, limited success has been achieved in regulating plant ABA action for increasing plant drought tolerance. Revealing the mechanisms of ABA action in stress adaptation will further help the development of hardy crop plants

Understanding gene regulation mechanisms is important for genetic improvement of abiotic stress resistance of crops. In response to developmental and environmental cues, plants employ a plethora of gene regulation mechanisms, one of which is posttranscriptional regulation of gene expression by non-protein coding small RNAs. Samll RNAs, namely, microRNAs (miRNAs) and short interfering RNAs (siRNAs), are ∼20 to 24-nucleotide single stranded RNAs. miRNAs are synthesized from MIR gene transcripts, while siRNAs are synthesized from dsRNA formed by transcripts of heterochromatin DNA repeats, mRNAs encoded by natural cis-antisense gene pairs and miRNA directed cleavage of ssRNA/mRNA. Small RNAs regulate the expression of complementary/partially complementary genes by directing mRNA cleavage, translational repression, chromatin remodeling and DNA methylation. Several stress responsive small RNAs have been identified in plants and their role in oxidative stress tolerance, osmolyte accumulation/osmoprotection and nutrient starvation response have been established. Under abiotic stresses, stress-upregulated miRNAs may down-regulate their target genes, which are likely negative regulators of stress tolerance, while stress down regulated miRNAs may result in accumulation of their target gene mRNAs, which may positively regulate stress tolerance. Overexpression of miRNA-resistant target genes will help overcome post-transcriptional gene silencing, and thus may lead to better expression of engineered trait in transgenic plants. Understanding the roles of small RNAs in transcriptome homeostasis, cellular tolerance, phenological and developmental plasticity of plants under abiotic stress and recovery will help genetic engineering of abiotic stress resistance in crop plants