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In plants the biomolecules are very much essential for the plant growth and development. Among biomolecules sugars serve as energy source and as structural component in plants. Koch (2004) reported that 80% of the CO2 assimilated during photosynthesis is used for synthesis of sucrose. Sucrose is the major transport form of organic carbon exported from the photosynthetic source to sink organs. Extensive losses in agricultural production have been observed due to environmental stresses (Mittler, 2006; Bohnert et al., 2006). Sucrose is performing dual functions one as transported carbohydrate in vascular part and second as signaling molecules (Baier et al., 2004). Sugars are responsible gene expression involved in plant metabolism viz, photosynthesis, glycolysis, N2 metabolism, cell cycle regulation etc. soluble sugars like hormones can act as primary messenger and regulate signals that control the expression of different genes involved in plant growth and metabolism (Rolland et al., 2006; Chen, 2007). Gupta and Kaur (2005) have reported that HXK is a sugar sensor in plants. Evolutionary conserved glucose sensor hexokinase 1 (HXK1) mixes different signals (nutrient and hormone signals) and use them for the expression of genes and plant growth in response environmental stress (Cho et al., 2006). It is not fully understood how the HXK1 controls the gene expression for encoding proteins involved in photosynthesis. Rolland et al. (2006) reported two glucose signal transduction pathways in plants: (1) hexokinase (HXK) dependent and hexokinase independent. HXK dependent system requires the phosphorylation of glucose while HXK independent system does not need (Smeekens, 2000). Recently expression of specific photosynthetic genes by HXK1 nucleur complex have been observed and the process is glucose metabolism independent and requires two partners VHAB1 and RPT5B (Chen, 2007). This leads to the conclusion that enzymes involved in metabolic activities can play part in signal transduction by expression of genes in the nucleus. Arabidopsis thaliana mutant hsr8 (high sugar response 8) exhibited increased sugar responsive growth and expression of genes (Li et al., 2007). Li et al. (2007) observed that hsr8 plants grown under light showed lower chlorophyll content and higher levels of starch and anthocyanin in response to glucose treatment. The hsr8 plant grown under dark showed glucose hypersensitivitive. Li et al. (2007) revealed a signal pathway altered expression of gene, metabolic and developmental changes. HXK transgenic Arabidopsis plants (AtHXK) have three distinct glucose signal transduction pathways. These are: First is AtHXK1 dependent pathway, here expression of gene was correlated with the AtHXK1 mediated signaling functions. Second pathway us glycolysis dependent and is regulated by catalytic activity of both AtHXK1 and the heterologus yeast HXK2. Third is AtHXK1-independent pathway, here expression of gene was independent of AtHXK1 (Xiao et al., 2000). Hence the role of HXK in sensing the sugar status is still under discussion (Reviewed by Rosa et al., 2009).
Kempa et al., (2007) reported that MsK4 (Medicago sativa glycogen synthase kinase 3-like kinases) have been involved in stress signaling with carbon metabolism. MsK4 is located in plastids and is associated with starch granules. Kempa et al. (2007) have also reported that kinase activity of MsK4 is induced rapidly under high salt concentration. Overexpression of MsK4 in transgenic plants showed tolerance to salinity stress accompanied with more starch accumulation and modified carbohydrate content (Kempa et al., 2007). The protein kinase KIN10 and KIN11 connects abiotic stress signals, sugar signals and developmental signals to regulate the plant metabolism (Baena-Gonzalez et al., 2007).
Sucrose non-fermenting-1 (SNF-1) related proteins, analogue of the protein kinase (SNF-) yeast signalling pathway have been reported in plants (Loreti et al., 2001). SNF-1 related proteins have a role in Sugar sensing (Purcell et al., 1998).
5 Abscisic acid in signaling
Ohkuma et al. in 1963 for the first time purified and crystallized Abscisic Acid (ABA) from cotton fruits and named it as Abscisin II. It was also isolated from sycamore leaves and was named as dormin. Chemical characterization of both revealed identical nature of the two compounds and it was later named as Abscisic Acid. Apart from being having an inhibitory role in the plant system, the hormone is also known to possess a stress protective function. ABA plays an important role in plant responses to drought and salt stresses.
ABA has been shown to regulate many agronomical aspects of plant development like synthesis of proteins and lipids, seed desiccation tolerance and dormancy, germinative, vegetative and reproductive growth (Leung and Giraudat, 1998; Rock, 2000; Rohde et al., 2000b). Also it mediates the responses of plants to many abiotic stresses like drought, salt, cold stress and botic stress like pathogen (Leung and Giraudat, 1998; Rock, 2000; Rohde et al., 2000b; Shinozaki and Yamaguchi-Shinozaki, 2000). This implies that ABA is involved in both long term development processes as well as short term physiological effects. Long term processes involve changes in pattern of gene expression whereas short term responses involve changes in the activity of various signaling molecules and fluxes of ion channels across the membranes. Both set of responses require the action of signaling elements which amplify the primary signal generated when the hormone binds to its receptors.
Signaling through ABA causes the production and accumulation of second messengers like Ca2+, phosphatidic acid (PA) or reactive oxygen species (ROS) in the cell which play an important role in ABA signal transduction. Reversible protein phosphorylation is an early and central mechanism that occurs in ABA signal transduction (Schmidt et al., 1995; Leung et al., 1997; Himmelbach et al., 2003; Sokolovski et al., 2005). This mechanism involves several protein kinases and phosphatases (Leung and Giraudat, 1998; Finkelstein et al., 2002). For example, ABA-activated serine-threonine protein kinase (AAPK) which is a guard cell specific protein kinase in Vicia faba or orthologous OPEN STOMATA 1/SNF1- RELATED PROTEIN KINASE 2.6 (OST1/SnRK2.6) which regulates ABA-induced stomatal closure (Li et al., 2000; Mustilli et al., 2002). Fujii et al. (2007) reported other SnRK2, SnRK2.2 and SnRK2.3 that regulate ABA response in germination, growth and gene expression. PKABA1 is another protein kinase involved in suppressing the gibberellins (GA) inducible gene expression in barlet aleurone layers (Gomez-Cadenas et al., 1999). Apart from the above mentioned calcium independent protein kinases, the role of several calcium independent protein kinases have also been revealed in ABA signaling. These either belong to the CDPK or to the SnRK3 family. CDPK1 and CDPK1a have been reported to activate ABA dependent promoters (Sheen, 1996). Also, CDPK3 and CDPK6 possess a role in regulating guard-cell aperture during responses to environmental stimuli (Mori et al., 2006). Zhu et al. (2007) reported the involvement of CPK4 and CPK11 which positively regulate ABA signal transduction in seed germination, seedling growth and stomatal movement. The mechanism involved is probably the phosphorylation of Abscisic Acid Responsive Element-Binding Factor 1 (ABF1) and Abscisic Acid Responsive Element-Binding Factor 4 (ABF4). Finally, several reports confirm the involvement of mitogen-activated protein kinases (MAPKs) in ABA dependent response to different stresses (Boudsocq and Laurière, 2005; Zhang et al., 2006) and germination (Lu et al., 2002). ABA, in case of stress, regulates gene expression in both positive and negative manner (Chandler and Robertson, 1994). Under stress conditions, the gene expression results in the production of transcripts that are responsible for hardening or stress tolerance.
6 Brassinosteroids in signaling
Brassinosteroids (BRs) are plant steroidal hormones having growth promoting activities. Grove et al. (1979) discovered the brassinolide (BL), (the most active form of BR) from the pollens of Brassica napus. BRs play significant role in seed germination, seedling photomormhogenesis, root and stem elongation, vascular differentiation, senescence, flowering and resistance to biotic and abiotic stresses (Clouse and Sasse, 1998). Biosynthetic pathway of BRs was elucidated through chemical analysis and isolation of additional BR-biosynthetic mutants, defective in genes encoding proteins which catalyse the plant steroid conversion to BR precursors (Asami et al., 2005). First BR biosynthesis inhibitor, brassinazole is another powerful tool for elucidation of BR signaling pathway (Asami et al., 2000). Genetic, genomic and proteomic approaches lead to the establishment of BR signaling pathway by providing important role in the mechanism of receptor activation and regulating components by process of phosphorylation (Tang et al., 2010)
In Arabidopsis, extensive genetic screens for loss - of - function BR signaling mutants is detected in one locus, BRI1 encoding LRR RLK (Clouse et al., 1996; Kauschmann et al. 1996; Li and Chory 1997; Noguchi et al., 1999). Phenotypes of BRI1 mutants are similar as that of BR-deficient mutants, but these are not rescued by the BRs addition. Components of BR signaling pathway have been characterised in additional suppressor and gain of function screens, which involve second LRR RLK, the BRI1 Associated Receptor Kinase-1 (BAK1) (Li et al., 2002); the Glycogen Synthase Kinase-3 (GSK3)-like kinase, BR Insensitive-2 (BIN2) (Li et al. 2001b; Li and Nam 2002), the serine/-carboxypeptidase BRI1 suppressor-1 (BRS1) (Li et al., 2001a), the phosphatase BRI1 suppressor-1 (BSU1) (Mora-Garcia et al., 2004) and the transcription factors brassinazole-resistant 1 (BZR1) (Wang et al., 2002) and BZR2 (BRI1-EMS suppressor 1 (BES1) (Yin et al., 2002).
Recently, BR signalling model has been refined by proteomics studies by identifying the components like BR signalling kinases (BSKs), which are not found in previous screens, generating a complete signalling pathway from a RLK to transcription factors in plants (Tang et al., 2008).
BRs regulate the signalling pathway identical to that of classic receptor tyrosine kinases (RTKs) and transform growth factor-β (TGF- β)- mediated signalling in plants (Feng and Derynck, 1997; Schlessinger, 2000, 2002). In Arabidopsis genome sequence have more than 600 RLK members (Shiu et al., 2004) lead to identical signalling mechanisms. Plant RLKs and signalling pathways provide activation to signalling networks, which are controlled by plant hormones (De Smet et al., 2009).
7 ETHYLENE IN SIGNALING
Ethylene is a gaseous plant hormone, plays significant role in developmental processes like seed germination, senescence, fruit ripening, root nodulation, leaf abscission, programmed cell death, stress and pathogen attack (Bleecker and Kende, 2000; Johnson and Ecker, 1998). Ethylene has 'triple response' effect on plant growth of etiolated dicotyledonous seedlings. This response leads to radial swelling of hypocotyl, inhibition of hypocotyls and root cell elongation and exaggerated curvature of the apical hook. Genetic screens of Arabidopsis based on the triple-response phenotype. More than dozen of mutants are divided into three distinct categories. Constitutive triple-response mutants i.e. ethylene -insensitive overproduction (eto1), eto2, eto3, constitutive triple-response1 (ctr1) and responsive to antagonist1 (ran1) / ctr2; ethylene- insensitive2 (ein2), ein3, ein4, ein6 and tissue-specific ethylene-insensitive mutants i.e. hookless1 (hls1), ethylene insensitive root1 (eir1) and various auxin-resistant mutants (Bleecker and Kende, 2000; Johnson and Ecker, 1998; Stepanova, 2000). Ethylene belongs to the family of membrane-associated receptors, which include ETR1 / ETR2, Ethylene Response Sensor1 (ERS1)/ERS2 and EIN4 in Arabidopsis (Sakai et al., 1998; Hua et al., 1995; Hua et al., 1998; Chang et al., 1993). Ethylene attaches to its receptor by copper transporter RAN1 delievered copper co-factor. Functions of receptor is inactivated by the hormone binding (Hua and Meyerowitz, 1998). EIN2, EIN3, EIN5 and EIN6 act downstream of CTR1 and positively regulate the ethylene response. EIN2 acts as an integral membrane protein, EIN3 acts as transcription factor and expression of intermediate target gene like Ethylene Response Factor1 is regulated.
Ethylene belongs to the family of five receptors (ETR1, ETR2, ETS1, ERS2 and EIN4). Receptor family is divided into two subfamilies on the basis of structural similarities. Type-I subfamily contains ETR1 and ERS1 having amino-terminal ethylene-binding domain, (which is also known as Sensor domain) and carboxy-terminal histidine (His) kinase domain, whereas type-II subfamily receptors contain ETR2, ERS2 and EIN4, which involve an amino-terminal ethylene binding domain and a degenerate His kinase domain. Receptors of ethylene negatively regulates the ethylene responses (Bleecker and Kende, 2000; Chang and Stadler, 2001). Dominant ethylene in sensitivity mutations in receptor ETR1 lead to signaling (Schaller and Bleecker, 1995). Loss-of-function (LOF) mutants have no ethylene response phenotypes. Recently, LOF mutations were isolated in ERS1 gene (Zhao et al., 2002; Wang et al., 2003) with etr1, etr2, ers2 and ein4 mutants, Double LOF etr1 ers1 mutants possess strong constitutive-ethylenr response phenotypes (Wang et al., 2003). These phenotypes are present in plants containing strong allele of ran1, which cause loss-of-function of all receptors of ethylene (Woeste and Kieber, 2000). ETR possess His kinase activity in vitro, which is important for receptor function (Gamble et al., 1998). For other aspects of receptor functionality like localization, protein stability or interaction with other factors, His kinase activity is essential.
In the mechanism of ethylene signaling, ethylene perception and signaling occur at endoplasmic reticulum (Chen et al. 2002; Gao et al. 2003). For ER association, the amino-terminal membrane-spanning sensor domain of ETR1 is essential. ER localization of ETR1 is not affected by the introduction of etr1-1 mutations or BR application. CTR is found at ER (Gao et al., 2003). CTR1 contains an amino-terminal domain and carboxy-terminal kinase domain, that is linked with Raf-like mitogen activated protein kinase (MAPK). CTR1 interacts with His kinase domains of ETR1 and ERS1 (Clark et al., 1998). ER localization of CTR1 with ER-associated receptors. ER-associated CTR1 level inhibits due to removal of ethylene receptors and distribution of CTR1 and receptor interactions.CTR1-ETR1 interaction depends on two lines of evidences in vivo. Co-purification of ETR1 leads to affinity purification of CTR1 from the Arabidopsis ER-membrane fraction, which describes the ETR1 and CTR1 presence in the protein complex (Gao et al., 2003). Overexpression of amino-terminal domain of CTR1 cause LOFctr1 mutant phenotype. Type-1 receptors i.e. ETR1 and ERS1 play significant role in ethylene signaling. This role is not due to His kinase activity of type-1 receptors.
8 JASMONATES IN SIGNALING
Jasmonates (JA) regulate plant growth and development. In the reproductive development of plants, jasmonate signaling plays important role (Stintzi and Browse, 2000) by giving protection to plant from abiotic stresses (Traw and Bergelson, 2003; Huang et al., 2004) and from pathogens and insects (Farmer and Ryan, 1990; Engelberth et al., 2004). In Arabidopsis, three mutants namely jar1, coi1 and jin1, which are defective in JA response and one triple mutant defective in JA biosynthesis (fad3-2fad-72fad8) help in understanding the functioning of JA in plants (Staswick et al. 1992; feys et al. 1994; Berger et al., 1996; McConn and Browse, 1996). Disruption of biosynthetic pathway of JA cause susceptibility of plants to various insects and pathogens (Engelberth et al., 2004). For example, susceptibility of coil to Alternaria brassicicola and Pythium mastophorum (Fay et al., 1994). Oxo-phytodienoic acid, JA-amino acids and JA-glucosyl are the intermediates of JA biosynthesis which act as signaling molecule in JA pathway (Staswick et al., 2002).
9 Salicylic acid in signaling
Salicylic acid (SA) is a naturally occurring phenolic compound having carboxylic acid group attached to the benzene ring. SA has important role in various aspects of plant development. In mungbean, SA helps in increase in yield and pod number (Singh and Kaur, 1980). It also possesses tuber inducing capacity in potato (Koda et al., 1992). It has positive influence on productivity and nitrogen content in maize (Singh and Srivastava, 1978; Asthana and Srivastava, 1978), flowering and helps in reducing transpiration by regulation of stomata (Khurana ans Maheswari, 1978; Larque, 1979).
SA signaling has been evaluated in case of plants exposed to abiotic stress. Plant tissues when exposed to heat stress, release more superoxide anions which further increase the level of hydrogen peroxide (Doke et al., 1994). The increased level of hydrogen peroxide has the ability to stimulate the accumulation of SA. Hence, there is a connection between increase in H2O2 level and SA accumulation (Rao et al., 1997). Role of SA has also been described by many workers during cold tolerance in plants like maize, rice, wheat, cucumber, tomato and banana (Janda et al., 1999; Dang et al., 2002; Kang and Saltveit, 2002; Kang et al., 2003; Tasgin et al., 2003). Scott et al. (2004) showed the inhibitory effect of SA on growth of Arabidopsis exposed to chilling conditions. Under low temperatures, the salicylate is reported to accumulate as free and glucosyl SA. Studies based on various wild species and mutants in Arabidopsis, it was proposed that SA induces low temperature growth inhibition. Wang and Li (2006) showed increase in cytoplasmic Ca2+ levels after the pre-treatment of grape plants with SA. This increased Ca2+ helps in maintaining the integrity of plasma membrane during the stress conditions. Also, it was shown that SA treated plants had higher levels of antioxidants like glutathione and ascorbic acid.
10 Auxin in signaling
Exogenous application of auxin to plants causes alteration in the transcription of gene families, changes in the rate of cell division and cell elongation, range of electrophysiological responses and changes of tissue pattern and differentiation. Auxin signaling initiates with the interaction of auxin receptors. Auxin is considered as a multi-functional hormone, its signal transduced through several signaling pathways. For wild-type auxin response, large screen for mutants with changed auxin sensitivity were used to define genes for normal functioning. AXR1, AXR2, AXR3, AXR4 and AXR6 are five different loci and TIR1 is the sixth one.