Sunflower, an important oil-yielding crop of tremendous economic importance worldwide, is sensitive to salt stress like many other agriculturally important crops. Different varieties of sunflower exhibit notable variations in their sensitivity/tolerance to salt-stress. Sensing of salt stress in sunflower is evident as early as at the seedling stage. Oil bodies, the major storehouse of fatty acids, are encased in a phospholipid monolayer containing intrinsic and extrinsic proteins. Any changes expected in the fatty acid composition of oil bodies as a response to salt stress are first perceived through alterations in the expression of oil body membrane proteins (OBMPs). The present investigations provide an in-depth proteomic analysis of OBMPs in the seedling cotyledons of three sunflower varieties exhibiting variations in their salt sensitivity. The exhaustive data from the LC‒MS/MS analysis of OBMPs highlight the differences in the levels of expression of a number of intrinsic and transiently expressed protein constituents of oil body membranes. The present proteomic analysis, thus, provides an insight into proteins capable of sensing salt stress as an early signaling response in sunflower seedlings.

Nitric oxide– mediated tyrosine nitration of proteins influences cellular responses to internal and external stimuli. Identification of melatonin receptor in plants strongly suggests its significant role as a plant hormone. In animals, melatonin primarily functions through receptor-mediated pathways, whereas in plants, it involves a complex interaction with other plant hormones and signaling molecules. Present work aims to examine the effects of NaCl stress and heme oxygenase-1 (HO-1) activity inhibition using its potent inhibitor, zinc protoporphyrin IX (ZnPPIX) on melatonin biosynthesis, its spatial distribution, and tyrosine nitration of proteins in the cotyledons of etiolated sunflower seedlings. The findings highlight possible role/s of tyrosine nitration of cytosolic proteins and its correlation with HO-1 action and melatonin accumulation. Salt stress upregulated the activities of HO-1 and the key regulatory enzyme involved in melatonin biosynthesis [N-acetyl serotonin methyl transferase (ASMT)], leading to enhanced melatonin accumulation and its spatial distribution. This suggests a possible crosstalk between melatonin and HO-1 in mitigating salt stress in sunflower seedlings. These observations further highlight the role of melatonin as a long-distance salt-sensing molecule that regulates HO-1 function in salt stress mitigation. Accumulation of melatonin inside the nuclei of cotyledon cells further suggests the presence of probable melatonin receptors on the nuclear membrane of plant cells. These findings indicate a complex mechanism involved in salt stress tolerance in plants.

Modulation of melatonin signaling pathway occurs either by direct interaction with calmodulin (CaM) or through regulation of intracellular Ca 2+ concentration. Intracellular CaM and CaM-dependent phosphodiesterase (PDE) activity are also possibly modulated by melatonin in vitro through melatonin-CaM interaction. Melatonin acts as a CaM antagonist and modulates CaM-dependent protein phosphorylation, thereby rhythmically regulating cellular functions. Melatonin also acts as a nitric oxide synthase (NOS) inhibitor by interfering with CaM binding to NOS peptide, thereby inhibiting NO biosynthesis. CaM also inhibits the effects of melatonin on cyclic GMP biosynthesis. Melatonin can cause increase in reactive oxygen species (ROS) generation through CaM–mediated activation of phospholipase A 2 (PLA 2 ). Thus, the prooxidant action of melatonin is mediated through CaM. Melatonin can directly scavenge ROS species and regulate the activities of various antioxidant enzymes though interaction with CaM. Melatonin causes microtubule depolymerization, thereby affecting cytoskeleton organization in the cells. CaM also inhibits tubulin polymerization. Melatonin is known to regulate photoperiodism and circadian rhythmicity in plants. Both salt stress and melatonin seem to modulate seedling growth by inhibiting CaM-mediated calcium signaling. Upregulation of genes encoding CaM-like protein, calcium-binding protein, calcium-dependent phosphodiesterase and CBL (calcineurin B-like proteins)-interacting protein kinase indicates possible calcium-mediated melatonin signaling in plants.

Plant root growth and development undergo significant alterations as an adaptation to environmental stressful conditions. Remodeling of roots exposed to salinity is coordinated by complex interactions among various signaling pathways involving phytohormones, nitric oxide (NO), and reactive oxygen species (ROS). Polyamines (PAs), small, cationic amine molecules with diverse roles in plant growth and stress responses, are also known to influence root morphology. Studies reported that treatment of sunflower seedlings with PA biosynthesis inhibitors [DFMA (DL-α-difluoromethylarginine) or DFMO (DL-α-difluoromethylornithine)], promotes extension growth of primary roots both in the absence or presence of NaCl. This work explores the possible role of PAs and their crosstalk with auxin signaling in the modeling of the root morphology in etiolated sunflower seedlings. We observed that inhibition of root growth by salinity is possibly governed by a disruption of the polar localization of PIN1 (auxin efflux protein) leading to IAA deficiency in the root apex. Application of PA inhibitors (DFMA or DFMO), in contrast, brings about an enhancement in IAA accumulation in the root apices of seedlings relative to control (−NaCl), albeit to a lesser degree in seedlings also exposed simultaneously to 120 mM NaCl. These alterations in IAA accumulation coincide with changes in primary root extension previously reported in sunflower seedlings in response to treatment with PA inhibitors, both in the absence or presence of NaCl. We found that the enhancement in root extension observed in seedlings subjected to a combined treatment of PA biosynthesis inhibitors and NaCl possibly involves the maintenance of polar distribution of PIN1 in the root cells which, in turn, may be responsible for the restoration of IAA distribution in the root apex to further support the observed extension growth of primary root. On the other hand, the role of nitric oxide and hydrogen peroxide in the observed PA inhibitor-triggered response on root extension remains uncertain at present. Therefore, a possible role of PAs and their crosstalk with auxin is evident in root architecture remodeling in sunflower seedlings exposed to salinity.

Fruit formation is a characteristic feature of angiosperms. Fruits types exhibit a great deal of diversity. Nutritional quality of the fruit is determined by various factors including those affecting the ripening stage. A fruit must have an optimum degree of ripeness so as to be consumed. Fruit ripening is a complex process, accomplished through several physiological, biochemical, and molecular mechanisms. These mechanisms also bring about changes in pigmentation due to loss of chlorophylls and a substantial increase in nonphotosynthetic pigments, such as anthocyanins and carotenoids. Increase in the activity of cell wall hydrolases leads to fruit softening which is reflected in the texture of mature fruits. Ripe fruits possess characteristic taste (due to elevated levels of sugars and depletion of organic acids) and aroma (due to biosynthesis of volatile compounds). Fruit ripening is regulated by several genes that play significant role in the accumulation of antioxidants, sugars, bioactive compounds, release of volatile compounds, and softening of cell wall.

Plants have developed efficient defense responses to counterbiotic stress factors that cause major losses in productivity. These defense responses are triggered upon the recognition of specific factors released by the invading pathogens by the corresponding elements of the host plants. They are encoded by the avirulence (Avr) and resistance (R) genes, respectively. R genes are crucial for providing resistance to pathogens in plant hosts by recognizing the cognate pathogenic Avr-dependent effectors, either by direct or indirect means. Following recognition of a pathogen, R gene-encoded products or R proteins mediate signaling cascade(s) to coordinate specific defense responses that terminate in hypersensitive responses. Nucleotide-binding leucine-rich repeat proteins (NLRs) form the largest class of R proteins. Effector recognition is followed by the activation of NLRs and their oligomerization into homo- and heteromeric complexes. While some of the NLRs are capable of inducing signaling by themselves, others work in complementary pairs with other NLRs (NLR–NLR pairs) and some others rely on 'helper' NLRs. Downstream NLR signaling requires other non-NLR proteins, such as Enhanced Disease Sensitivity 1 (EDS1) and Non-race-specific Disease Resistance 1 (NDR1), and may also involve MAPKs and transcription factors (TFs) for positive regulation of defense-related genes, and salicylic acid signaling for the activation of systemic acquired resistance (SAR). Recently, it has been shown that some NLRs function through the formation of higher-order structures termed as 'resistosomes', which get inserted into the plasma membrane and trigger Ca2+ influx by serving as Ca2+-permeable channels. The Ca2+ influx, in turn, induces an oxidative burst through ROS production required for cell death. Despite considerable advancement in our understanding of R gene-mediated signaling, many missing links still exist which need to be investigated. As further progress is made in the area of R gene-mediated signaling, many promising opportunities will be unraveled for resistance introduction in the host plants to facilitate effective disease management.

High salt content in soil restricts the growth and development of agricultural crops, limiting their yield. Aquaporins (AQP) are specialized organelles responsible for precise regulation of water movement across the membranes. Experimental analysis of multigenic defense approaches for acclimatization against salt stress has shown that nitric oxide (NO) and AQP may play a crucial role in overcoming salt stress during early stages of seedling growth in oilseeds, such as sunflower. Present work demonstrates the variation in the abundance of AQPs [Plasma membrane Intrinsic Protein 2 (PIP2) and Tonoplast Intrinsic Protein 1 (TIP1)] in whole tissue homogenates of 2 d old, dark-grown, sunflower seedling cotyledons, in response to salt stress. Enhanced expression of PIP2 and TIP1 in cotyledons of seedlings raised in the presence of 120 mM NaCl suggests a link of these AQP isoforms with salt stress acclimatization, possibly through protection against osmotic stress. NO application leads to expression of dimeric forms of both PIP isoforms (PIP1 and PIP2). Present work also reports accumulation of four aquaporin isoforms (PIP1 and PIP2; TIP1 and TIP2) on the oil body (OB) membrane. A molecular crosstalk between NO and expression of specific AQP isoforms thus appears to regulate the hydration status of oilseed cotyledons in seedlings facing salt stress.

Life cycle of plants is fundamentally different from that of animals. It is characterized by the presence of two distinct multicellular generations, referred as sporophytic (diploid) and gametophytic (haploid) generation which alternate with each other during the life cycle. Male and female reproductive organs in plants are stamens (androecium) and carpels (gynoecium), respectively. Both the reproductive structures produce haploid spores as a result of meiosis, namely microspores (male) and megaspores (female). These spores undergo repeated mitotic divisions to produce male and female gametophytes, called microgametophyte and megagametophyte, respectively. Development of male gametophyte takes place inside the anther, whereas female gametophyte develops inside the ovule. Upon maturity, male and female gametophytes divide mitotically to produce male and female gametes, i.e., sperm and egg, which fuse to form zygote that develops to give rise to sporophytic plant (refer Fig. 25.1 ).

Seeds are highly dehydrated, quiescent or resting structures, and carriers of next generation in the life cycle of plants. Seeds have different shapes and sizes, ranging from the smallest orchid seed (10−6 g) to the huge seed of the double coconut palm (up to 25 kg). Some seeds are short-lived, e.g., willow seeds are viable for less than 1 week. Mimosa seeds can live up to ~200 years. Seeds of Canna compacta have been reported to remain viable up to 600 years, and seeds of lotus can survive up to 1000 years. To accomplish the remarkable feat of next generation, seed contain an embryo with reserve food. During seed germination, the embryo divides and differentiates into shoot–root axis. The tissue containing reserve food material (depending on the species) may persist and get reabsorbed later. Depending on species, the reserve food material is stored either in the embryo or in the endosperm and, in some cases, it is present in both embryo and endosperm. The degree of arrest of growth is variable among different species. It may be true in sleeping (dormancy) or quiescent stage, which only requires water for resuming the growth. A viable and nondormant seed is capable of germination after all the necessary environmental conditions are met, but dormant seeds do not germinate even when provided with favorable conditions. Different types of mechanisms impose seed dormancy in diverse climates and habitats. Abscisic acid (ABA) present in seeds induces dormancy, but dormancy induction also depends on the genetic make-up of the plant and the environment in which it grows. Temperature, relative humidity, and day length also interact to modulate dormancy, thereby making it a complex phenomenon. Germination in different seeds is not synchronous, and stimuli required to promote germination vary widely. Prior to germination, seeds need to undergo imbibition, i.e., uptake of water by dry seed, followed by reactivation of metabolic activity, and redifferentiation of embryonic tissue to mobilize the reserve food material stored in the seed and initiate meristematic activity. The transition from dry seed to seedling is highly sensitive to different environmental conditions, especially light, temperature, and availability of water. This response to environmental signals is mediated by one or more hormones whose signaling events lead to de novo synthesis and/or activation of hydrolytic enzymes. Emergence of radicle is the first visible step of seed germination, and it indicates that the seed is viable. If germination occurs in dark, then root growth is slow but shoot growth accelerates. This behavior increases chances of seedlings to obtain light so that it can turn green and start photosynthesizing. Once the seedling comes out of the soil, it turns green and starts producing new leaves.

Besides playing other roles, carbohydrates are also the major source of energy for all living beings. Almost 30% of the carbohydrates in plants are utilized for cell wall biosynthesis by each cell. The carbon skeleton also needs to be diverted for synthesis of defense chemicals (secondary metabolites) in order to deter herbivory. This requires a continuous flow of carbohydrates from the source to the sink. In autotrophic plants, carbon dioxide (CO2) is fixed in the green parts, resulting in the production of simple sugars (monosaccharides). Sugars also act as signaling molecules in carbohydrate metabolism, for example, by stimulating the genes encoding the key enzymes of carbohydrate metabolism (adenosine diphosphate (ADP)-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), UGP (uridine diphosphate (UDP)-glucose pyrophosphorylase)). Surplus photosynthates, stored as transitory starch in chloroplasts during day time, are transported to other parts of the plant in soluble form at night (Fig. 9.1). These are transported in the least reactive soluble form, primarily as sucrose from the source (site of its synthesis) to the sink (site of its utilization), though carbohydrates other than sucrose are also translocated, which include raffinose, verbascose, and stachyose (in members of the family Cucurbitaceae) and sorbitol (in many plants of the family Rosaceae). Although starch is the primary storage form of carbohydrates, there are instances of sucrose also being stored, e.g., in sugarcane and beetroot. In some plants, fructans are the storage carbohydrates. In members of grass families, starch is stored in the grains. Although carbohydrate metabolism is similar in living organisms, there are certain unique features that make it distinct in plants. These features include their autotrophic nature and the presence of a specialized class of organelles, i.e., plastids. Besides, the cytosol and vacuoles are also involved in the metabolism of carbohydrates (Fig. 9.2). Because of their inability to move to a safer place under unfavorable environmental conditions, plants have flexible metabolism and many a times exhibit alternate metabolic pathways. In this chapter, the main focus will be metabolism of the storage form of carbohydrates in plants.

Oil body (OB) mobilization, a crucial event associated with early seedling growth, is delayed in response to salt stress. Previous reports suggest that careful regulation of polyamine (PA) metabolism is essential for salt stress tolerance in plants. Many aspects of PA- mediated regulation of metabolism have been uncovered. However, their role in the process of OB mobilization remains unexplored. Interestingly, the present investigations reveal a possible influence of PA homeostasis on OB mobilization, while implicating complex regulation of oleosin degradation and aquaporin abundance in OB membranes in the process. Application of PA inhibitors resulted in the accumulation of smaller OBs when compared to control (-NaCl) and the salt-stressed counterparts, suggesting a faster rate of mobilization. PA deficit also resulted in reduced retention of some larger oleosins under controlled conditions but enhanced retention of all oleosins under salt stress. Additionally, with respect to aquaporins, a higher abundance of PIP2 under PA deficit both under control and saline conditions, is correlated with a faster mobilization of OBs. Contrarily, TIP1s, and TIP2s remained almost undetectable in response to PA depletion and were differentially regulated by salt stress. The present work, thus, provides novel insights into PA homeostasis-mediated regulation of OB mobilization, oleosin degradation, and aquaporin abundance on OB membranes.

Water is one of the most important constituents of life. Understanding the mechanisms underlying water uptake, transport, and balance in plants requires a clarity of several concepts, such as diffusion, osmosis, plasmolysis, and various components of water potential. The resistance faced by water molecules during their transport from soil across the root cells and along their transcellular migration over short and long distance in the plant also requires attention. The photosynthetic efficiency of vascular plants is dependent on their ability to absorb water from the roots and its rapid transport over long distance through the continuum of xylem vessels, up to the canopy, for photolysis of water in the grana of chloroplasts. A major fraction of water reaching the leaves is continuously lost to the atmosphere by transpiration. Water transport from roots to the canopy poses challenges among terrestrial plants since it requires energy to transport water through the resistive vascular system. Water transport across xylem vessels occurs under “negative pressure,” and water molecules are pulled up to the stomatal cavities in the leaves by the tension of countless tiny menisci at the evaporative surface, capillary forces, and hydrogen bonds. The entire plant body, starting from roots to the stem, and up to the canopy, serves as a pump to create and harness the water potential gradient between the soil, canopy, and the atmosphere in order to transport massive amount of water. Energy is required not only to lift water up to the canopy against gravity but also to overcome the resistance produced by the plant’s roots, stem, branches, and the leaves in order to meet the transpirational requirements of plants. This energy is drawn from the water potential gradient from soil to the plant canopy, which is due to the evaporation of water molecules from the transpiring leaves. Evaporation is possible due to the positive energy flux into the canopy, which is mainly caused by solar radiation and, to some extent, by long wave radiation. Water transport across the plant’s hydraulic structure (xylem vessels) is passive although metabolic energy is required to operate stomatal opening and closing. Thus, transpiration is one of the most important components, which determine the land surface energy balance, land surface temperature, and biosphere–atmosphere interaction. This chapter shall focus on various mechanisms of short and long distance transport of water in plants (Box 1.1).

While investigating the effect of unilateral light on the bending response of canary grass (Phalaris canariensis) seedlings, Charles Darwin (1881) observed that although a light signal is perceived at the shoot tip, the bending of coleoptile due to differential growth occurs in the subapical region. This classic case of environmental signal perception and transduction resulting in growth response led to the concept of signal transduction. It is now known that plant growth and development are modulated by a variety of environmental (external) and physiological (internal) signals. Some of the major signals (stimuli) to which plant cells are sensitive include light, mineral nutrients, organic metabolites, gravity, water status, soil quality, turgor, mechanical tensions, heat, cold, wind, freezing, growth hormones, pH, gases (carbon dioxide (CO2), oxygen (O2), nitrous oxide (NO), and ethylene (C2H4)), volatile compounds (e.g., jasmonates), electrical fluxes, wounding, and diseases (Fig. 23.1). These signals can vary in quality and quantity over a period. Some signals penetrate the plasma membrane (PM), whereas others are carried over long transcellular distances through vessel elements (xylem) and sieve tubes (phloem). The plasmodesmata also facilitate symplastic migration of a number of signaling biomolecules. With the advent of molecular genetic studies on Arabidopsis thaliana in the current era of plant biology research, there has been a flood of information on the signal perception and transduction mechanisms in plants. A variety of receptors for various plant hormones have been identified and characterized. Mutant analysis has facilitated the identification of many new signal transduction components, which act downstream of receptors for various environmental and internal signals perceived by plants. Thus, an entirely new level of understanding of the complexities of the signaling mechanisms in plants has been unfolded. New signaling molecules continue to be discovered, and sophisticated signaling mechanisms are being explained through the development of models, which explains the interactions between signaling pathways and the modulation of various signaling networks (Fig. 23.2). The application of knowledge thus acquired on signaling mechanisms using model plant systems, such as Arabidopsis thaliana, to agriculturally significant species, is likely to provide practical benefits in understanding plant responses to varied environmental stresses.

Interaction between the serotonin and auxin signaling routes is crucial for the regulation of root system architecture in plants under normal and challenging environments. Current investigation deciphers the interrelations among serotonin accumulation, auxin transport, and actin distribution accompanying root growth inhibition in NaCl-stressed sunflower seedlings. Application of NaCl stress (120 mM) or 1-napthylphthalamic acid (NPA-5 µM; auxin transport inhibitor) leads to increased serotonin accumulation in the endodermis and pericycle cells in the differentiating zone of primary roots. Analysis by high-performance liquid chromatography shows significant increase in serotonin content in NaCl-stressed and NPA-treated seedling roots. Immunolocalization of auxin efflux proteins (PIN1) shows that NaCl stress results in disruption of the acropetal PIN 1 gradient in the vascular cells of the primary roots, thereby possibly altering auxin levels in the differentiating region of roots. Serotonin accumulation in the endodermal and pericycle cells of NaCl-treated primary roots also coincides with possible auxin deficiency caused by a reduction in acropetal auxin transport. Actin localization in the vascular cells of primary roots shows a disruption of ACT 8 (an isoform of vegetative actin) assembly in response to salt stress. The disruption of PIN 1 gradient in the vascular cells of primary roots, thus, accompanies the disorganization of ACT 8 isoforms, coinciding with the inhibition of primary root elongation and reduction in lateral root branching. Since reactive oxygen species (ROS) are key players of auxin–serotonin signaling, NaCl stress (present work) presumably causes changes in the ROS levels which might regulate auxin transport and serotonin accumulation in the vascular region of roots. Thus, a probable crosstalk between serotonin, auxin, and actin is evident to accompany NaCl-stress-induced restructuration of root system architecture in sunflower seedlings.

Sustainable agriculture faces major challenges under abiotic stress conditions owing to extensive application of chemical fertilizers which pollute water, soil and atmosphere. Biostimulants (BSs), comprising of naturally derived complex mixtures of uncharacterized biomolecules, pure biochemicals and nanomaterials, enhance nutrient use efficiency (NUE) and trigger crop's natural defense mechanisms. While it is difficult to specify the metabolic effects of uncharacterized natural mixtures (seaweed extract, protein hydrolyzates, etc.), exogenous application of pure biochemicals and nanomaterials offers an edge as BSs since their physiological roles and mechanisms of action are decipherable. Foliar application or seed treatment of some amino acids, polyamines and biopolymers (chitosan, lipochitin oligosaccharides and thuricin 17) enable plants to overcome drought and salinity stress via activation of mechanisms for reactive oxygen species (ROS) scavenging, osmolyte regulation and chlorophyll accumulation. Interaction of nitric oxide (NO) with some vitamins and melatonin exhibits potential significance as BSs for mitigating stress by ROS scavenging and maintenance of intracellular ionic balance and membrane integrity. Near future is likely to see wide applications of nanoparticles (NPs) and nanomaterials (NMs) as BSs in view of their biphasic mode of action (bio-physical activation of membrane receptors followed by gradual release of BS into the plant cells).

Salicylic acid (SA) is a phenolic compound characterized with an aromatic ring and a hydroxyl group. SA and its derivatives [such as acetylsalicylic acid (aspirin)] are collectively called “salicylates.” Use of salicylates as pain killers was first discovered in fossils from El Sidrón caves in Spain. These fossil remains were found to contain poplar (Populus sp.) bark, indicating their use to relieve pain due to dental abscess. In 1828, a compound called “salicin” (salicylic acid beta-d-glucoside) was purified from the bark of white willow or weeping willow (Salix alba) trees by Johann Andreas Buchner (a German pharmacologist). Subsequently in 1838, salicin was separated into a sugar moiety and an aromatic compound that could be converted into 2-hydroxybenzoic acid, now referred as salicylic acid (SA). In 1859, Hermann Kolbe (a German Chemist) and his group chemically synthesized SA. Years later, Bayer and Company (a German pharmaceutical company) produced acetyl salicylic acid, with the trade name “Aspirin,” which became popular as an analgesic since then (Fig. 22.1). For many years after its discovery, SA continued to be considered as one of the several phenolic compounds produced by plants, with no well-established biological functions. In 1974, however, SA was for the first time described as a mobile signaling molecule localized in phloem, which can induce flowering in Lemna gibba G3 and Xanthium strumarium (Cleland and Ajami, Harvard University, MA, USA). In 1989, SA’s role as a plant hormone was finally established when voodoo lily (Sauromatum guttatum) was found to exhibit a 100-fold increase in its endogenous level during thermogenesis.

Biotic stress factors cause massive damage to crops and account for major economic losses globally. With the global temperature on the rise, a great threat looms on the global economy and food security as sustainable agriculture is expected to be devastated by larger outbreaks of diseases worldwide. Plants have developed versatile counter-strategies to overcome these stressors. These strategies rely on preformed defense or induced defense responses which, in turn, entail a complex crosstalk among different signaling pathways. Once the presence of a pathogen is detected, the plant host switches on its defense response. Very specific recognition of the invading pathogen is mediated via the detection of their Avr effector proteins by the cognate elements in the plant host, i.e., the R proteins, encoded by R genes. Successful recognition of Avr by the cognate host R proteins sets distinct signaling cascade(s) into motion to activate defense responses. The nucleotide-binding leucine-rich repeat proteins (NB-LRRs or NLRs) represent the major class of R proteins and insights gained regarding their structural complexity and underlying signal transduction has enabled researchers to exploit them for competent introgression of disease resistance in plants against a single or a broad range of pathogens. In addition to conventional breeding, more recent biotechnological interventions have also been employed to develop plants with improved disease resistance with the use of R genes. The domain of R gene-mediated disease resistance has gained much attention from plant breeders and through experimental interventions, many promising opportunities for conferring resistance for effective management of diseases have come forth. This review attempts to summarize the progress made hitherto and focuses on how the knowledge base so created has been used to find out solutions to the major biotic problems occurring throughout the world.

An essential feature of plant development is the ability of plant cells to grow and differentiate. Growth of an organism is defined as an irreversible increase in mass. Because mass is related to cell volume and cell number, growth refers to an irreversible increase in cell size (enlargement) or to an increase in cell size as well as cell number (cell division). Cell division, by itself, is not sufficient to result in growth. Differentiation, in contrast, refers to the cells acquiring qualitative differences among other cells of common origin, i.e., those derived from a cell or group of cells. It is by differentiation that cells in an organ or tissue become different from each other or specialized for different functions, e.g., the epidermis, or mesophyll, or xylem, or phloem cells in a leaf. Morphogenesis is the acquisition of form, how a plant or organ acquires its distinctive shape or form. The control of these two processes is central to a study of plant morphogenesis.