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Hormonal Regulation of Moss Protonema Development and the Possible Origin of Plant Hormonal Responses in Bryophytes

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Hormonal Regulation of Moss Protonema Development and the Possible Origin of Plant Hormonal Responses in Bryophytes Indian Journal of Biotechnology Vol 2, 1anuary 2003, pp 9-16 Hormonal Regulation of Moss Protonema Development and the Possible Origin of Plant Hormonal Responses in Bryophytes M M Johri* and Jacinta S D'Souza Department of Biological Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India The protonema of mosses is a far simpler paradigm to understand the mechanism of hormonal action and tolerance to abiotic stresses in plants. Its developmental biology, responses to hormones and the similarity of signaling mechanisms with higher plants are reviewed. There is strong evidence for second messenger role of calcium ions in the action of cytokinin. Multiple calcium-dependent protein kinases (CDPKs) are present in the protonema. The Funaria hygrometrica CDPK gene (FhCDPK) shows the characteristic catalytic and autoinhibitory domains, the four EF hands and the highest homology to CDPKs from higher plants but far lower to liverwort or other moss CDPK genes. A 38 kDa myelin basic protein kinase (MBP kinase) is activated within minutes by abscisic acid (ABA) and salinity. As ABA also confers tolerance against desiccation and freezing and the wheat ABA-inducible promoter is fully functional in mosses, the ABA signaling mechanism seems to be highly conserved. In plants, the CDPKs are involved not only in hormonal signaling but also in the acclimation response against abiotic stresses. The manipulation of signal transduction components such as transcription factors, CDPKs and calcineurin have emerged as viable strategies to genetically engineer the stress tolerant plants. There is increasing evidence to support the origin of plant hormonal responses at the level of bryophytes. Keywords : Funaria hygrometrica, moss protonema, CDPKs, MBP kinase, abscisic acid, auxin, calcium, origin of hormonal responses Introduction 1993; Johri, 1978). The hormonal responses are rapid The overall development in plants is regulated by and discernible either in single cells or in a group of a environmental and internal factors. How plants few cells and are thus more or less cell autonomous. perceive various signals, transduce them and This review focusses on the hormonal regulation of ultimately alter the growth in terms of temporal .and protonema development, the diversity of protein spatial patterns, is beginning to be comprehended. kinases (PKs) from the moss, F. hygrometrica and the The bryophytes represent the earliest group of land importance of PKs in relation to abiotic stress plants where the phytohormone-mediated morpho- tolerance in plants. The possible origin of hormonal genetic responses comparable to that in higher plants responses is also discussed. have been found (Bopp, 1990; Johri, 1990). The protonema of mosses such as Funaria hygrometrica Hormonal Regulation of Cell differentiation in Moss Hedw. and Physcomitrella patens (Hedw.) B.S.G. Suspension Cultures have been used to study the hormonal regulation of A remarkable feature of the protonemal suspension development (Bopp & Atzorn, 1992; Cove & Knight, cultures of Funaria has been the long-term stability of the cell line J-2 which has now been maintained for *Author for correspondence: over 32 years by repeated subculture in a low-calcium Tel: 022-22152971 Ext-2255 ; Fax: 022-22152110 medium. During this long time, it has neither lost the E-mail: [email protected] potential to differentiate nor the responsiveness to Abbreviations: phytohormones (Johri, 1974). The caulonema differen- A23187: calcium ionophore; ABA: abscisic acid; CaM: calmodulin; CCaMK: calcium regulated calmodulin-dependent PK; CDPK: tiation is regulated by inoculum size, auxin concen- calcium-dependent protein kinase; CTC: chlorotetracycline; DHP: tration and nutrient level. It marks a major develop- IA-dihydropyridine; FhCDPK: Funaria hygrometrica CDPK gene; mental switch, which is turned on by biologically LEA: late embryogenesis abundant; MAPKKK: mitogen activated active auxins and ethyl ester of IAA but not by protein kinase kinase kinase; MBP kinase: myelin basic protein indoleacrylic acid or 2,4-dichlorophenoxyacetic acid kinase; PCIB: p-chlorophenoxyisobutyric acid; PK: Protein kinase; SAPK: stress activated PK; SIMK: salt stress-induced MAPK. (Johri & Desai, 1973; Johri & D'Souza, 1990). There is 10 INDIAN J BIOTECHNOL, JANUARY 2003 an increase In endogenous auxin during caulonema The Role of Calcium in Mosses formation (Atzorn et al, 1989a, 1989b). The The role of calcium in cytokinin induced bud cytokinins induce the formation of bud initials, which formation in Funaria and Physcomitrella is well- arise as side branches from caulonemal cells (Gorton documented (Schumaker & Dietrich, 1997). Using the & Eakin, 1957). Cytokinin-over-producing mutants of lipophilic fluorescent calcium chelating probe-- P. patens form the buds constitutively (Ashton et al, chlorotetracycline (CTC), a calcium rise following 1979). cytokinin treatment was localised to the presumptive ABA inhibits the growth and cytokinin-induced initial cell site (Saunders & Hepler, 1981). bud formation (Valdon & Mummery, 1971). ABA- Measurements of calcium currents using vibrating treatment of protonemal cells is reported to increase microelectrodes along a caulonema filament have tolerance against freezing and to confer adaptation to shown that cytokinin application leads to an increase drought (Nagao et al, 2001; Werner et al, 1993). The in the magnitude of the inward current and causes a endogenous level of ABA increases in moss plants change in the spatial properties of the current under arid conditions (Bopp & Werner, 1993) and an (Saunders, 1986). In a caulonemal cell maximum application of ABA causes closure of stomata in the inward current is observed near the nucleus but after Funaria sporophyte (Garner & Paolillo, 1973). In the cytokinin application, there is a decrease in the presence of ABA, the side branches on the protonema current near the centre which is followed by a rise in remain short and develop into resting structures the apical end. This rise in the inward current predicts referred to as the brood cells (Johri, 1988). Formation the location of the initial cell and falls to resting levels of new polypeptides which share epitopes with the with the onset of the initial cell outgrowth. This higher plant LEA (late embryogenesis abundant) current can be blocked by gadolinium, which proteins, have been demonstrated in the protonema competitively inhibits calcium transport. (Ainapure, 1998). The ABA- and osmotic stress- The calcium channel blockers, verapamil and inducible promoter elements from the wheat Em gene D-600 prevent the cytokinin induced bud formation are fully functional in the moss P. patens (Knight et and the reversal of this effect by the calcium al, 1995). Thus, stress responses seem to be well ionophore A23l87 shows that the calcium rise is conserved between higher plants and mosses. essential for bud induction. The application of A23187 in fact leads to bud formation in the absence Auxin-transport and -binding Sites are Involved in of cytokinin (Saunders & Hepler, 1982). Inhibition of Auxin Response bud formation by antagonists of l,4-dihydropyridine Similar to the other auxin-induced responses, the (DHP) and the ability of DHP agonists to substitute bulk medium pH changes from an initial value of for the presence of cytokinin shows that DHP about 5 to 6.5 during IAA response. In the absence of sensitive voltage dependent calcium channels play an auxin, the caulonema can differentiate, but after a lag important role in the cytokinin response (Conrad & of 6 days, if the medium is buffered in the range of Hepler, 1988). G-proteins have been suggested to pH 5 - 5.5. The responsiveness of cells to auxin is also regulate these DHP sensitive calcium channels in modulated by medium pH and nutrient level (Johri & Physcomitrella (Schumaker & Gizinski, 1996). D'Souza, 1990). The lag is prolonged by the auxin There is a rise in intracellular calcium concen- antagonist p-chlorophenoxyisobutyric acid (PCIB) tration in Physcomitrella in response to physical which reduces polar, basipetal transport in Funaria stimuli such as cold shock and touch (Russell et al, rhizoids (Rose & Bopp, 1983) and is also known to 1996). Thus, similar calcium-sensing mechanisms compete with IAA for auxin-binding sites ( Jacobs & seem to exist in mosses and higher plants. As CDPKs Hertel, 1978). In the PCIB-treated cultures, there is no are the most predominant among the downstream inhibition of growth and in fact there is a profuse targets of calcium in plants, the regulation of calcium stimulation of secondary chloronema formation. Thus, regulated kinases from Funaria has emerged as the both basipetal transport and IAA-binding sites seem major thrust area. to be involved during caulonema differentiation and chloronema inhibition. Most recently, the initial Protein Kinasesfrom Chloronema Cells events in embryo development in Fucus distichus, a So far, at least five CDPKs and one calcium- brown alga, have also been found to be linked to regulated CaM dependent PK (CCaMK) have been auxin and auxin transport (Basu et al, 2002). characterised from chloronema cells. A calcium- JOHRI & D'SOUZA: HORMONAL REGULATION OF PROTONEMA DEVELOPMENT II independent 38 kDa MBP kinase regulated by ABA is independent of CaM and this response could be a part also present. The calcium is required for the of the calcium homeostasis mechanism. The autophosphorylation as well as substrate phosph- properties of the moss CCaMK are
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