Phytohormones as Signaling Molecules

7.1 Brassinosteroids

Brassinosteroids (BRs) are plant steroidal hormones having growth-promoting activities (Hacham et al. 2011). 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, photo-morphogenesis, root and stem elongation, vascular differentiation, senescence, flowering, and resistance to biotic and abiotic stresses (Clouse and Sasse 1998; Divi and Krishna 2010; Divi et al. 2010) ; The biosynthetic pathway of BRs was elucidated through chemical analysis and isolation of additional BR-biosynthetic mutants, defective in genes encoding proteins which catalyze 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 an 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 LOF BR signaling mutants are 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 addition of BRs. Components of BR signaling pathway have been characterized 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 insen-sitive-2 (BIN2) (Li et al. 2001b; Li and Nam 2002), the serine/carboxypeptidase BRI1 sup-pressor-1 (BRS1) (Li et al. 2001a), the phosphatase BRI1 suppressor-1 (BSU1) (Mora-Garcia et al. 2004), and the transcription factors brassi-nazole-resistant 1 (BZR1) (Wang et al. 2002) and BZR2 (BRI1-EMS suppressor 1 (BES1)) (Yin et al. 2002) .

Recently, BR signaling model has been refined by proteomic studies by identifying the components like BR signaling kinases (BSKs), which are not found in previous screens, generating a complete signaling pathway from an RLK to transcription factors in plants (Tang et al. 2008).

BRs regulate the signaling pathway identical to that of classic receptor tyrosine kinases (RTKs) and transform growth factor-b (TGF-b)-mediated signaling in plants (Feng and Derynck 1997; Schlessinger 2000, 2002) . In Arabidopsis, genome sequence has more than 600 RLK members (Shiu et al. 2004) leading to identical signaling mechanisms. Plant RLKs and signaling pathways provide activation to signaling networks, which are controlled by plant hormones (Smet et al. 2009).

7.2 Ethylene

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 (Johnson and Ecker 1998. Bleecker and Kende 2000; Binder et al. 2010). 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 are 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 (Johnson and Ecker 1998; Bleecker and Kende 2000; Stepanova and Ecker 2000) . Ethylene belongs to the family of membrane-associated receptors, which include ETR1/ETR2, ethylene response Sensor1 (ERS1)/ ERS2 and EIN4 in Arabidopsis (Chang et al. 1993 . Hua et al. 1995, 1998. Sakai et al. 1998). Ethylene attaches to its receptor by copper transporter RAN1-delivered copper cofactor. Functions of receptor are 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) and is divided into two subfamilies on the basis of structural similarities. Type-I subfamily contains ETR1 and ERS1 having amino-terminal ethyl-ene-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 leads to signaling (Schaller and Bleecker 1995). 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 etr1ers1 mutants possess strong constitutive-eth-ylene 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 possesses 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 occurs 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-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 causes LOFctr1 mutant phenotype. Type-I receptors, i.e., ETR1 and ERS1 play significant role in ethylene signaling. This role is not due to His kinase activity of type-I receptors.

7.3 Jasmonates

Jasmonates (JA) regulate plant growth and development. In the reproductive development of plants, jasmonate signaling plays an important role (Stintzi and Browse 2000; Avanci et al. 2010) by giving protection to plants from abiotic stresses (Traw and Bergelson 2003; Huang et al. 2004; Avanci et al. 2010 ; Lackman et al. 2011) and from pathogens and insects (Farmer and Ryan 1990; Engelberth et al. 2004; Smith et al. 2009; Ma et al. 2010). In Arabidopsis; three mutants namely jarl, coil; and jinl; 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 bio-synthetic pathway of JA causes susceptibility of plants to various insects and pathogens (Engelberth et al. 2004; Lewsey et al. 2010); for example, susceptibility of coil to Alternaria bras-sicicola and Pythium mastophorum (Feys et al. 1994; . Oxo-phytodienoic acid, JA-amino acids, and JA-glucosyl are the intermediates of JA biosynthesis which act as the signaling molecule in JA pathway (Staswick et al. 2002).

7.4 Salicylic Acid

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 (Hayat et al. 2007, 2010). In mung bean, SA helps in the 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 and Maheswari 1978; Larque 1979) and alleviation of abiotic stress (Ahmad et al. 2011).

SA signaling has been evaluated in case of plants exposed to abiotic stress. Plant tissues when exposed to abiotic stress release more superoxide anions which further increase the level of hydrogen peroxide (Doke et al. 1994; Ahmad et al. 2010). The increased level of hydrogen peroxide has the ability to stimulate the accumulation of SA (Ahmad et al. 2011). Hence, there is a connection between increase in H2 O2 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, banana, pea, and mung bean (Janda et al. 1999; yDing et al. 2002; Kang and Saltveit 2002; Kang et al. 2003; Tasgin et al. 2003; Krantev et al. 2009; Popova et al. 2009; Khan et al. 2010). Joseph et al. (2010) have reviewed the exogenous application of SA ant its protective role in different plants under salt stress. Scott et al. (2004) showed the inhibitory effect of SA on the growth of Arabidopsis exposed to chilling conditions. Under low temperatures, the salicylate is reported to accumulate as free and glucosyl SA. Based on studies 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 pretreatment 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.

7.5 Auxins

Auxin, the dynamic plant hormone, controls the growth and developmental processes by modulating the levels of auxin/indole acetic acid proteins (Mockaitis and Estelle 2008; Iglesias et al. 2011). 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 (Berleth and Sachs 2001; Perrot-Rechenmann 2010). Auxin signaling initiates with the interaction of auxin receptors. Auxin is considered as a multifunctional hormone and the signal is transduced through several signaling pathways. Iglesias et al. (2010) reported that auxin signaling participates in the adaptive response against oxidative stress and salinity in Arabidopsis. For wild-type auxin response, a large screen for mutants with changed auxin sensitivity was used to define genes for normal functioning. AXR1, AXR2, AXR3, AXR4, and AXR6 are five different loci and TIR1 is the sixth one (Leyser 2002).

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