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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,
hormonal responses
moss protonema,
CDPKs, MBP kinase, abscisic acid, auxin, calcium, origin of
Introduction
The overall development in plants is regulated by
environmental and internal factors. How plants
perceive various signals, transduce
them and
ultimately alter the growth in terms of temporal .and
spatial patterns, is beginning to be comprehended.
The bryophytes represent the earliest group of land
plants where the phytohormone-mediated
morphogenetic responses comparable to that in higher plants
have been found (Bopp, 1990; Johri, 1990). The
protonema of mosses such as Funaria hygrometrica
Hedw. and Physcomitrella patens (Hedw.) B.S.G.
have been used to study the hormonal regulation of
development (Bopp & Atzorn, 1992; Cove & Knight,
* Author for correspondence:
Tel: 022-22152971 Ext-2255 ; Fax: 022-22152110
E-mail: [email protected]
Abbreviations:
A23187: calcium ionophore; ABA: abscisic acid; CaM: calmodulin;
CCaMK: calcium regulated calmodulin-dependent
PK; CDPK:
calcium-dependent protein kinase; CTC: chlorotetracycline; DHP:
I A-dihydropyridine; FhCDPK: Funaria hygrometrica CDPK gene;
LEA: late embryogenesis abundant; MAPKKK: mitogen activated
protein kinase kinase kinase; MBP kinase: myelin basic protein
kinase; PCIB: p-chlorophenoxyisobutyric acid; PK: Protein kinase;
SAPK: stress activated PK; SIMK: salt stress-induced MAPK.
1993; Johri, 1978). The hormonal responses are rapid
and discernible either in single cells or in a group of a
few cells and are thus more or less cell autonomous.
This review focusses on the hormonal regulation of
protonema development, the diversity of protein
kinases (PKs) from the moss, F. hygrometrica and the
importance of PKs in relation to abiotic stress
tolerance in plants. The possible origin of hormonal
responses is also discussed.
Hormonal Regulation of Cell differentiation in Moss
Suspension Cultures
A remarkable feature of the protonemal suspension
cultures of Funaria has been the long-term stability of
the cell line J-2 which has now been maintained for
over 32 years by repeated subculture in a low-calcium
medium. During this long time, it has neither lost the
potential to differentiate nor the responsiveness to
phytohormones (Johri, 1974). The caulonema differentiation is regulated by inoculum size, auxin concentration and nutrient level. It marks a major developmental switch, which is turned on by biologically
active auxins and ethyl ester of IAA but not by
indoleacrylic acid or 2,4-dichlorophenoxyacetic acid
(Johri & Desai, 1973; Johri & D'Souza, 1990). There is
10
INDIAN J BIOTECHNOL, JANUARY 2003
an increase In endogenous auxin during caulonema
formation (Atzorn et al, 1989a, 1989b). The
cytokinins induce the formation of bud initials, which
arise as side branches from caulonemal cells (Gorton
& Eakin, 1957). Cytokinin-over-producing mutants of
P. patens form the buds constitutively (Ashton et al,
1979).
ABA inhibits the growth and cytokinin-induced
bud formation (Valdon & Mummery, 1971). ABAtreatment of protonemal cells is reported to increase
tolerance against freezing and to confer adaptation to
drought (Nagao et al, 2001; Werner et al, 1993). The
endogenous level of ABA increases in moss plants
under arid conditions (Bopp & Werner, 1993) and an
application of ABA causes closure of stomata in the
Funaria sporophyte (Garner & Paolillo, 1973). In the
presence of ABA, the side branches on the protonema
remain short and develop into resting structures
referred to as the brood cells (Johri, 1988). Formation
of new polypeptides which share epitopes with the
higher plant LEA (late embryogenesis abundant)
proteins, have been demonstrated in the protonema
(Ainapure, 1998). The ABA- and osmotic stressinducible promoter elements from the wheat Em gene
are fully functional in the moss P. patens (Knight et
al, 1995). Thus, stress responses seem to be well
conserved between higher plants and mosses.
Auxin-transport and -binding Sites are Involved in
Auxin Response
Similar to the other auxin-induced responses, the
bulk medium pH changes from an initial value of
about 5 to 6.5 during IAA response. In the absence of
auxin, the caulonema can differentiate, but after a lag
of 6 days, if the medium is buffered in the range of
pH 5 - 5.5. The responsiveness of cells to auxin is also
modulated by medium pH and nutrient level (Johri &
D'Souza, 1990). The lag is prolonged by the auxin
antagonist p-chlorophenoxyisobutyric
acid (PCIB)
which reduces polar, basipetal transport in Funaria
rhizoids (Rose & Bopp, 1983) and is also known to
compete with IAA for auxin-binding sites ( Jacobs &
Hertel, 1978). In the PCIB-treated cultures, there is no
inhibition of growth and in fact there is a profuse
stimulation of secondary chloronema formation. Thus,
both basipetal transport and IAA-binding sites seem
to be involved during caulonema differentiation and
chloronema inhibition. Most recently, the initial
events in embryo development in Fucus distichus, a
brown alga, have also been found to be linked to
auxin and auxin transport (Basu et al, 2002).
The Role of Calcium in Mosses
The role of calcium in cytokinin induced bud
formation in Funaria and Physcomitrella is welldocumented (Schumaker & Dietrich, 1997). Using the
lipophilic fluorescent calcium chelating probe-chlorotetracycline (CTC), a calcium rise following
cytokinin treatment was localised to the presumptive
initial cell site (Saunders
& Hepler,
1981).
Measurements of calcium currents using vibrating
microelectrodes along a caulonema filament have
shown that cytokinin application leads to an increase
in the magnitude of the inward current and causes a
change in the spatial properties of the current
(Saunders, 1986). In a caulonemal cell maximum
inward current is observed near the nucleus but after
cytokinin application, there is a decrease in the
current near the centre which is followed by a rise in
the apical end. This rise in the inward current predicts
the location of the initial cell and falls to resting levels
with the onset of the initial cell outgrowth. This
current can be blocked by gadolinium, which
competitively inhibits calcium transport.
The calcium channel blockers, verapamil and
D-600 prevent the cytokinin induced bud formation
and the reversal of this effect by the calcium
ionophore A23l87 shows that the calcium rise is
essential for bud induction. The application of
A23187 in fact leads to bud formation in the absence
of cytokinin (Saunders & Hepler, 1982). Inhibition of
bud formation by antagonists of l,4-dihydropyridine
(DHP) and the ability of DHP agonists to substitute
for the presence of cytokinin shows that DHP
sensitive voltage dependent calcium channels play an
important role in the cytokinin response (Conrad &
Hepler, 1988). G-proteins have been suggested to
regulate these DHP sensitive calcium channels in
Physcomitrella (Schumaker & Gizinski, 1996).
There is a rise in intracellular calcium concentration in Physcomitrella in response to physical
stimuli such as cold shock and touch (Russell et al,
1996). Thus, similar calcium-sensing mechanisms
seem to exist in mosses and higher plants. As CDPKs
are the most predominant among the downstream
targets of calcium in plants, the regulation of calcium
regulated kinases from Funaria has emerged as the
major thrust area.
Protein Kinases from Chloronema Cells
So far, at least five CDPKs and one calciumregulated CaM dependent PK (CCaMK) have been
characterised from chloronema cells. A calcium-
JOHRI & D'SOUZA: HORMONAL REGULATION OF PROTONEMA DEVELOPMENT
independent 38 kDa MBP kinase regulated by ABA is
also present. The calcium is required for the
autophosphorylation
as well as substrate phosphorylation by the CDPKs of Mr 44,48,63 and 70 kDa
(D'Souza & Johri, 1999). There is an enhancement in
the autophosphorylation of 44 kDa COPK in the
presence of auxin or under other physiological
conditions,
which
also
lead
to
caulonema
differentiation in auxin-free medium. Thus, this PK
could have a role in caulonema formation. This PK is
recognised by moss anti-calmodulin antibodies and is
also competed by the purified moss CaM during
immunoprecipitation showing that it has calmodulinlike domain.
The FhCDPK gene encodes a transcript of about
2.6 kb which is upregulated by nutritional deprivation.
The genomic clone shows the canonical autoinhibitory
region and the four EF hands (Mitra & Johri, 2000).
The deduced amino acid sequence shows extensive
homology with other CDPKs namely, 73% identity
with the Fragaria CDPK and 71 % homology with
COPK isoform-7 of Arabidopsis. The homology to
the liverwort Marchantia or the moss Tortula COPKs
was lower (59-64%). The codon usage in another
moss Physcomitrella is also similar to higher plants
(Reski et al, 1998). Plants being monophyletic in
origin, the similarity of codon usage between Funaria
and Arabidopsis CDPK genes could reflect either
convergent evolution since the acquisition of CDPK
genes by these groups of plants or a lack of
divergence of the codons.
The 70 kOa moss CCaMK belongs to another very
important class of the calcium-regulated PKs that
have a kinase domain followed by a CaM-binding
domain homologous to neural visinin-like molecules
(Harmon et al, 2000; Patil et al, 1995). The purified
moss 70 kDa PK prefers lysine-rich histone as
substrate and is fully active in the presence of 50 ~M
free calcium (D'Souza & Johri, 2002b). The enzyme
is completely inactive at sub-optimal levels of free
calcium (23 ~M), but is activated by nanomolar levels
of the moss CaM (5-100 nM). At higher CaM levels
(100-1000 nM, optimum 400 nM), the autophosphorylation of the enzyme was also greatly stimulated
which in turn led to an enhanced substrate
phosphorylation. The activity of the moss CCaMK is
thus CaM-dependent at low levels of calcium, a
response which is likely to have a physiological
significance. At high calcium levels which are
nonphysiological,
the moss enzyme
becomes
II
independent of CaM and this response could be a part
of the calcium homeostasis
mechanism.
The
properties of the moss CCaMK are similar to lily and
tobacco CCaMKs. The specific role of moss enzyme
is yet to be understood.
Activation of a 38 kDa MBP Kinase by ABA
As mentioned earlier, a 38 kDa MBP kinase is
activated in chloronema cells within minutes of ABA
treatment (D'Souza & Johri, 2002a). The activation is
transient, independent of calcium, specific to ABA
among phytohormones and is also evoked by salt
stress but not by other abiotic stresses. Salts and other
compounds such as sugars, which change the
intracellular water potential in the moss protonema,
also activate it (Unpublished data of 0' Souza &
Johri). The effects of ABA and NaCI are additive and
both must be perceived independently and the signals
converge at the level of MBP kinase. The moss MBP
kinase seems to be similar to the stress activated PKs
(SAPK) or other osmotically-activated PKs. The latter
belong to the category of MAPKs such as the SIMK
which is a salt-stress and pathogen induced MAPK
recently described from Medicago sativa (Cardinale
et al, 2002). Since ABA is also involved in conferring
resistance against anhydrobiosis
in moss, the
overlapping activation of MBP kinase by salt and
ABA seems to be a part of signalling network. The
rapidity of the ABA response in Funaria indicates
that as a survival strategy different from that of
animals, the plant cells seem to be in a state of
readiness capable of mounting a rapid initial response.
After the initial response, the cells seem to synthesize
new signaling
proteins
and other protective
compounds. There is strong evidence for the ABAinducibility of the proteins sharing epitopes with the
alpha subunits of the heterotrimeric G-proteins in
moss F. hygrometrica (Panigrahi, 1998).
Manipulation of Calcium/CaM-dependent Protein
Kinases and Phosphatases to Confer Stress
Tolerance in Plants
The ABA and abiotic stress signaling pathways are
networked in such a way that several key elements
such as MAPKs, and CDPKs seem to be shared.
Therefore, by manipulating the signal transduction
components or pathways, it should be possible to
confer or improve the stress tolerance in plants
(Grover et al, 1999; Xing & Jordan, 2000). Monroy et
al (1993) demonstrated a role of calcium in the
12
INDIAN J BIOTECHNOL,
regulation of cold acclimation-specific
genes in
Medicago sativa. The COPKs have been found to be
one of the positive regulators of tolerance to salt and
cold stresses in several plants. Maize mesophyll
protoplasts expressing the GFP reporter gene driven
by an ABA-inducible HVAI promoter from barley
show an enhanced expression in response to ABA,
salt and cold stresses and darkness. On co-transfecting
these protoplasts with constitutively active forms of
the Arabidopsis COPKI and COPKla, the GFP
expression was found even in the absence of stress
signals or ABA. Thus, activated COPKs bypass the
requirement of the signals in evoking the stress
signaling (Sheen, 1996). The over-expression of rice
cold- and salt-inducible
OsCOPK7,
conferred
tolerance against both the stresses in transgenic rice
(Saijo et al, 2000). The COPKs have also been found
to be involved in the defense response against the
fungus Cladosporium fulvum in resistant tobacco
(Romeis et al, 2000); mechanical
strain and
dehydration stress in mung bean and Arabidopsis
respectively (Botella et al, 1996; Urao et al, 1994).
Among other protein kinases, an Arabidopsis
homologue of GSK3/shaggy-like kinase, AtGSK1, is
involved in salt stress responses (Piao et al, 2001).
The transgenic plants over-expressing
AtGSKI
showed enhanced tolerance to salinity and the NaCI
responsive genes were induced in the absence of salt.
There is also evidence for the involvement of
other proteins such as calciumlCaM-dependent
phosphatases--the calcineurin, in the stress tolerance.
The expression of AtCBLl (Arabidopsis thaliana
calcineurin B-like protein) is induced by stresses such
as cold, drought and wounding (Kudla et al, 1999).
Another gene with considerable
homology to
calcineurin B, is the salt overly sensitive 3 (SOS3)
gene of Arabidopsis. It has been found to mediate
calcium signaling associated with the acquisition of
cold tolerance (Liu and Zhu, 1997). SOS3 interacts
with a serine threonine kinase, the SOS2 and upon
increase in cytosolic calcium in response to high salt
stress, the activated SOS2/S0S3 complex seems to
modify the Na+ and K+ transporters thereby causing
salt tolerance (Halfter et al, 2000). Likewise, the
transgenic tobacco plants coexpressing the catalytic
and regulatory subunits of yeast calcineurin showed
an enhanced tolerance to salt stress (Pardo et al,
1998).
The above account
shows
understanding of the signaling
that a deeper
mechanisms and
JANUARY 2003
especially the role calciumlCaM dependent protein
kinases and phosphatases, enables one to devise novel
and viable strategies for engineering tolerance against
stresses. As the protein kinases such as COPKs
operate early in the signaling pathway, their
manipulation also makes it possible to regulate the
activity of several genes that function downstream.
Origin of Hormonal Response in Plants
Distribution of Phytohormones and their Main Role
The major groups of phytohormones are more or
less ubiquitously distributed in plants. Auxin,
cytokinin, ethylene, gibberellins and ABA have been
reported from algae (Jacobs, 1986; Johri, 1990), but
the evidence for their hormonal role is not
unequivocal. It is only in the bryophytes that besides
the presence, specific responses to above hormones
have been demonstrated. The evidence for the
presence and for specific effects of gibberellins in
bryophytes is however, not very strong. The
regulatory role of gibberellins as antheridiogens is
well documented in some of the ferns of the family
Schizaeaceae (Bopp, 1990; Johri, 1990).
The basic responses of auxin, cytokinin, ABA and
ethylene are remarkably conserved in plants. The role
of auxin in caulonema differentiation has already been
mentioned. Caulonema is similar to the rhizoids and
both represent the same cell type, which is the
forerunner of root system of tracheophytes (vascular
plants). Likewise, the cytokinin-induced bud initial
cell, which develops into a three-sided apical cell
forming the moss gametophore, is analogous to the
initial cell or a founder cell of a multicellular apical
meristem.
Thus, auxin and cytokinin
evoke
respectively the rhizogenic and shoot bud-forming
responses in plants. Similar to the higher plants, ABA
also arrests development and confers tolerance against
water stress in mosses and at the biochemical level at
least the action mechanism of ABA action seems' to
be highly conserved. Ethylene is produced by axenic
cultures of several species of algae, liverworts,
mosses and ferns and in general retards cell division
but promotes cell elongation (Johri, 1990). It
suppresses the ventral row of leaves in the leafy
liverwort, Plagiochila arctica (Basile & Basile,
1983). The elongation of seta in the sporophyte of the
thallose liverwort Pellia epiphylla involves a dual
regulation by auxin and ethylene. The elongating
setae contain adequate endogenous auxin and at the
same time can also respond to exogenous auxin.
JOHRl & D'SOUZA: HORMONAL REGULATION OF PROTONEMA DEVELOPMENT
Elongating setae release more ethylene than those not
undergoing elongation (Thomas et al, 1983), a feature
characteristic of auxin-rich tissues undergoing cell
elongation. Auxin application also enhances ethylene
production in P. arctica (Law et al, 1985). In the
moss F. hygrometrica, the formation of tnema cells in
old cultures seems to be related to ethylene
production (Rohwer & Bopp, 1985). Ethylene has
also been found to promote megasporangium
formation in the lycopod Selaginella
wallacei
presumably by blocking the final cell division of the
sporogenous tissue cells (Brooks, 1973). Similar to its
effects in some of the aquatic higher plants, ethylene
application stimulates the elongation of frond rachis
in the semi-aquatic fern Regnellidium diphyllum
(Walters
& Osborne,
1979). There is thus
overwhelming evidence for the remarkably conserved
ethylene effects between cryptogams and angiosperms
and an interaction with auxin seems to be involved in
many cases.
Possible Acquisition of Hormonal Function by
Secondary Metabolites
Growth substances acting as phytohormones are
also present in several bacteria and fungi as products
of secondary metabolism and the notion of a
metabolite acquiring a signaling or a hormonal
function has received some attention. Some of the sex
hormones (sexual pheromones) of algae and fungi
have been characterised chemically (Kochert, 1978;
AI-Hasani & Jaenicke, 1992) and chemical signaling
through pheromones had already evolved in the
sexual reproduction and somatic cell repair in algae
(Waaland, 1986). According to Kochert (1978), the
pheromones of unicellular eucarionts could be the
ancestors of hormones for all multicellular eucarionts.
The action of secondary metabolites (present in
ancestral forms) as hormones could have acquired a
regulatory role with the evolution of multicellular
orgamsms.
Origin of Specific Receptors
Following Kochert's
general idea about the
possible origin of hormones from pheromones,
Schraudolf (1985, 1986) has compared the similarities
between the pheromone system of Schizaeaceae and
the gibberellin responses in seed plants. According to
him, "the reaction of antheridiogens in Schizaeaceous
ferns represents the 'moment of becoming a hormone'
for gibberellin like molecules in phanerogams"
13
(Schraudolf, 1985). He further argues that "In contrast
to animal evolution, the phylogeny of plants seems to
be characterized by a post-evolution of hormonereceptor molecule. This event was a prerequisite for a
common and ubiquitously distributed metabolic
product to take over the function of a regulatory
molecule. The phylogeny of phytohormones therefore
has to be written as a phylogeny of their receptors"
(Schraudolf, 1985). Following the same argument, we
wish to propose that it is not difficult to visualise the
origin of specific receptors or the high-affinity
perception mechanisms at the level of the ancestor of
land plants and the consequent acquisition of
hormonal role by auxin, cytokinin, ABA, ethylene
and gibberellins. In Arabidopsis the receptors for
ethylene and cytokinin have been identified to be the
sensory hybrid-type histidine kinase and the twocomponent systems are involved in the signal
transduction (Hwang & Sheen, 2001; Urao et al,
2000). The ethylene receptor acts upstream of a Raf
protein kinase (a mitogen activated protein kinase
kinase kinase or MAPKKK), and the overall signaling
pathway is similar to the osmoregulation pathway in
yeast. The two-component histidine receptors seems
to have been derived from the cyanobacterial genome
as a result of horizontal transfer of genes during the
origin of the ancestor of plants (Meyerowitz, 2002;
Urao et al, 2000). The receptors for the other
hormones have yet to be identified. As bryophytes
represent the earliest group of land plants with
hormonal responses, we expect the histidine protein
kinase receptors to be present in them and clones
homologous with ethylene receptor have been
reported from P. patens (Fujiwara & Tohe, 2001).
The auxin transport and biosynthesis mechanisms in
the protonema of Funaria are comparable to higher
plants and are thus highly conserved (Rose & Bopp,
1983; Jayaswal & Johri, 1985).
Many Signaling Components could have Evolved at
the Level of Common Ancestor
All land plants, the embryophytes, are believed to
have a monophyletic
origin. Based on the
comparative morphology and molecular phylogeny,
the liverworts are believed to be basal and distinct.
and either mosses or hornworts represent a living
sister group to vascular plants (Kendric & Crane
1997). Among the most recent studies, the analysis of
the mitochondrial nadl gene by Hashimoto & Sato
(2001) suggests the monophyly of mosses and
14
INDIAN J BIOTECHNOL,
tracheophytes and the paraphyly of liverworts to these
two taxa. It has been proposed that during the
transition from an aqueous to the gaseous medium,
the important metabolic pathways including the plant
hormones in tracheophytes (vascular plants) arose
from the pre-existing elements of primary metabolism
in charophycean algae and bryophytes. Due to lack of
information, it is difficult to comment about the
presence of pre-existing elements of hormonal
mechanisms in charophycean algae, but as already
discussed, there is now unequivocal evidence for the
origin of ethylene and cytokinin receptors from
histidine kinase of blue green algae. As already
mentioned, the auxin transport and biosynthetic
pathways seem to be conserved in moss and higher
plants; it is plausible that the similarities extend
beyond this to the level of signaling components and
transduction
mechanisms
involving
calcium!
calmodulin, CDPKs and MAP kinases. Interestingly,
the presence of similar hormonal mechanisms in the
mosses and seed plants, the two plant groups that had
evolved and diverged half a billion years ago,
suggests that the modules were possibly present in the
presumed unicellular common ancestor of plants. As
similar signaling mechanisms are utilized in the
animals also, the signaling elements or components of
signal transduction must have evolved and been
present before the separation of plant and animal
lineages. For instance the calcium signaling including
CaM operates both in plants and animals. The CDPKs
on the other hand, are specific to plants and must have
evolved later in the ancestor of plants only. Besides
plants, sequences coding for LEA proteins are also
present in the genomes of micro-organisms and
nematode and could have originated in the ancient
cell types as a common strategy against anhydrobiosis
(Browne et al, 2002). The available evidence thus
suggests that the receptors and signaling elements in
the extant plants had their origin from the elements in
the microbial and other ancient organisms. These
elements are now regulated by the eukaryotic
promoters and have been shuffled around to generate
different cascades. As pointed out by Schraudolf
(1985), the crux of the issue in phytohormonal
responses is the problem of the origin of specific
receptors.
With rapid advances
in molecular
phylogeny hormonal signaling pathways, it may
become possible to trace the events that ultimately
culminated in the origin of hormonal perception and
signal transduction mechanisms in plants.
JANUARY 2003
Conclusions
The moss protonema has emerged as an excellent
developmental
system to study the signaling
mechanisms during hormonal action and responses to
abiotic stresses. The overall hormonal regulation of
cell differentiation
involves several interacting
factors--both inhibitory and the promo tory ones. The
side branch initial on a caulonema filament is
pluripotent and depending on the phytohormone
applied it can differentiate either into a chloronemal, a
caulonemal or a bud initial cell. The role of a
particular hormone can thus be visualized in
channeling or fixing the terminal destiny of a progeny
cell to a specific developmental fate by inhibiting the
others. The available information suggests that the
specific responses to hormones such as auxin,
cytokinin and ABA could have evolved early at the
level of bryophytes and even in these cryptogams the
level of overall mechanism of their action seems
comparable to that of the higher plants. Based on
similar basipetal polar transport and auxin effects
Hertel (1983), had visualised the importance of
mosses to understand the origin, differentiation and
diversification of transport and action of plant growth
substances. The stage seems to be set towards a fuller
realisation of this idea. The partial sequence of
Funaria calcium-dependent PK gene shows extensive
homology with CDPK genes isolated from higher
plants. Reski et al (1998) had also observed a high
degree of conservation between moss and seed plant
sequences and the codon usage in moss P. patens is
very similar to that of the dicotyledenous plants. We
need to learn more about the identity of other
signaling molecules in plants. The analysis of protein
kinase genes in the genome of mosses is likely to
provide additional and new information about the
origin and diversification of histidine kinase type of
receptors. With the demonstration of highly efficient
homologous recombination in P. patens (Schaefer &
Zryd, 1997), a far more rapid progress can be
expected in future.
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