Genetic control of branching in Arabidopsis and ... - Directory UMM

Genetic control of branching in Arabidopsis and tomato
Gregor Schmitz and Klaus Theres*
The patterns of axillary bud formation and the growth
characteristics of side-shoots determine to a large extent the
form of plants. Characterization of mutants in the monopodial
plant Arabidopsis thaliana and in the sympodial tomato, as well
as cloning of some of the respective genes, contributes to a
better understanding of side-shoot development. Genes have
been identified that influence the initiation of axillary meristems
and the pattern of their subsequent development.
Addresses
Institut für Genetik, Universität zu Köln, Carl-von-Linné-Weg 10,
D-50829 Köln, Germany
*e-mail: theres@mpiz-koeln.mpg.de
Current Opinion in Plant Biology 1999, 2:51–55
http://biomednet.com/elecref/1369526600200051
© Elsevier Science Ltd ISSN 1369-5266
Abbreviations
GA gibberellin
IM inflorescence meristem
SAM shoot apical meristem
Introduction
During plant development, the process of branching is of
fundamental importance for the elaboration of the speciesspecific
patterns in root, shoot and inflorescence
morphology. In seed plants, shoot branching is initiated by
the formation of lateral meristems in the leaf axils [1],
which subsequently function as shoot apical meristems
(SAM). In plants like tomato and potato, groups of meristematic
cells, which seem to be derived directly from the
SAM of the main shoot, can be recognized early in development
in the axils of leaf primordia. On the basis of these
observations it was proposed that lateral meristems are
formed from detatched parts of the main shoot SAM [1]. In
Arabidopsis, axillary meristems can be detected only much
later after the transition of the SAM to reproductive development
[2]. Shoots may also arise from adventitious buds
on roots, hypocotyls, stems, and leaves [3], indicating that
meristems can also develop from partially or fully differentiated
cells. Further development of axillary buds into
side-shoots is controlled by the main shoot apex, which
very often exerts an inhibitory influence on lateral buds.
This phenomenon, known as apical dominance or correlative
inhibition, has been reviewed recently by Cline [4].
In some plant species the primary shoot apical meristem
remains active throughout the life span producing continuously
new lateral organs, which results in a
monopodial growth pattern (e.g. Arabidopsis,
Antirrhinum). In other species the SAM will soon undergo
the transition into a terminal floral meristem, or its
activity will cease. In these cases elaboration of the shoot
system is often continued by one or few axillary meris-
51
tems giving rise to sympodial growth patterns (e.g. tomato,
Petunia).
Very little is known about the molecular mechanisms controlling
branching in higher plants. Recent studies have
resulted in a detailed description of mutants, and molecular
analysis is just beginning to identify the first gene
products involved in this process. In this review we will
compare branching patterns in monopodial versus sympodial
seed plants as observed during shoot and inflorescence
development in Arabidopsis and tomato.
Shoot development in Arabidopsis and
tomato
The monopodial shoot of Arabidopsis is established in
three stages: first, vegetative type 1 metamers, consisting
of a very short internode, a leaf and a bud, form a rosette
(Figure 1). Then the inflorescence main stem is build up
from type 2 metamers with an elongated internode, a
cauline leaf and a bud, followed by type 3 metamers consisting
of an intermediate length internode and a floral bud
but without a leaf [5].
During the vegetative growth phase lateral meristems are
not detectable in the axils of rosette leaves. After the SAM
has been transformed into an inflorescence meristem (IM)
and the stem is starting to bolt, however, axillary meristems
become visible, first in the axils of cauline leaves and
then spreading in a basipetal wave to the axils of rosette
leaves [2,6]. Meristems in the leaf axils of type 1
and 2 metamers develop into additional flowering axes
(paraclades) consisting of type 2 and type 3 metamers [7,8],
whereas lateral buds of type 3 metamers develop into flowers.
Later, in the axils of most cauline leaves, and in some
of the rosette leaves, accessory side-shoots are formed
between the primary side-shoot and the surface of its subtending
leaf [7,8].
Shoot development in tomato can be separated into two
different phases; during the initial vegetative phase the
SAM forms metamers consisting of an elongated internode,
a leaf, and a bud (Figure 1). After the formation of
between 7 and 11 metamers, the primary shoot SAM is
transformed into an IM. While the IM develops into an
inflorescence, a second phase of shoot development is
initiated by the outgrowth of the bud in the axil of the
youngest leaf primordium. This sympodial shoot grows
vigorously, it displaces the developing inflorescence to a
lateral position and transfers its subtending leaf to an
elevated position above the inflorescence [9,10]
(Figure 1). After the formation of three leaves, the SAM
of the sympodial shoot is also transformed into an IM
and develops into an inflorescence. The main axis is
again continued by the sympodial shoot in the axil of the

52 Growth and development
Figure 1
Arabidopsis
Vegetative side-shoot
Infloral side-shoot (paraclade)
Accessory side-shoot
Type3
metamers
Type2
metamers
Type1
metamers
Schematic representation comparing shoot development in
Arabidopsis and tomato. The primary shoot is illustrated in black,
lateral axes of successive order in grey. In Arabidopsis, the primary
meristem forms the main plant axis, consisting of a rosette, a bolting
stem and an indeterminate inflorescence. Side-shoots originating from
type1 and type2 metamers (paraclades) are infloral and repeat the
development of the bolting stem. Accessory side-shoots are often
formed between the primary side-shoot and its subtending leaf. In
tomato, the primary shoot meristem forms the first segment of the
sympodial stem terminating in the first inflorescence. In all leaf axils,
vegetative side-shoots are formed. The side-shoot in the axil of the
youngest leaf primordium. In wild-type tomato plants
this iterative phase of shoot development can progress
indefinitely [10].
Microscopic studies reveal, that, in contrast to
Arabidopsis, axillary buds in tomato are formed early in
development in all axils of leaf primordia [11]. Due to the
weak correlative inhibition by the shoot apex these buds
develop into side-shoots without a resting phase [12],
repeating at least part of the development of the primary
shoot [13]. As all nodes of a side-shoot can again form lateral
shoots, this pattern can be repeated indefinitely,
resulting in a very bushy growth habit.
Tomato
Infloral meristem
Flower
Leaf
Second
sympodial unit
First
sympodial unit
Primary
shoot
Current Opinion in Plant Biology
youngest leaf primordium grows vigorously, pushes the inflorescence
to a lateral position, and transfers its subtending leaf to an elevated
position above the inflorescence. After the formation of three nodes
the sympodial shoot also terminates in an inflorescence. In wild-type
tomato plants sympodial shoots of progressively higher order continue
the elaboration of the main axis indefinately. Inflorescences consist of a
series of terminal flowers originating from sympodial inflorescence axes
of progressively higher order. Accessory side-shoots can be found
regularly between the sympodial side-shoot and its subtending leaf,
but only rarely in other leaf-axils.
The formation of the tomato inflorescence has been
described as a cymose pattern, that is to say the primary
inflorescence meristem is transformed into a floral meristem
and a new lateral meristem originating from the
pedicel of the first flower continues inflorescence development
[10,14]. More recently, Allen and Sussex [15] found
that the primary inflorescence meristem is divided into two
halves: one of which develops into a flower, whereas the
other functions as an IM, continuing inflorescence development.
Sympodial shoot development of a second model
species, Petunia hybrida, is very similar to tomato. The main
difference is that the iterative unit in tomato comprises
three vegetative nodes, whereas only two vegetative nodes

are formed in Petunia [16]. Interestingly, microscopic analysis
has shown that in Petunia the SAM undergoes an equal
division resulting in the terminal flower meristem and the
sympodial shoot meristem [17]. This process shows strong
similarity to the branching pattern of the inflorescence apex
in tomato.
Influence of the subtending leaf on axillary
meristem formation
Several lines of evidence suggest that the subtending leaf
plays an important role in axillary bud development [18].
Characterization of the Arabidopsis revoluta mutant has
demonstrated that the extent of leaf growth and the development
of axillary shoots are negatively correlated. In the
mutant, leaves, stems, and floral organs grow extremely large,
whereas lateral shoots are often reduced or fail to develop [8].
The REVOLUTA gene seems to control the relative growth
rates of leaves and axillary shoots by regulating either the
incorporation of meristematic cells or the partitioning of
nutrients or growth factors between the two organs.
The phabulosa-1d mutant of Arabidopsis forms leaves with
a purely adaxial identity and develops additional lateral
meristems on the abaxial side of the leaf [19 •• ]. These
observations indicate that adaxial leaf cells play an important
role in axillary bud development. Microscopic analysis
suggests that axillary meristems are actually initiated on
the adaxial leaf surface [8]. On the basis of these data, the
detached meristem concept was called into question and a
model was suggested according to which the SAM makes
leaves, that in turn initiate at their adaxial base the formation
of axillary meristems [19 •• ]. This model is in
agreement with the results of a clonal analysis demonstrating
that in Arabidopsis, axillary meristems are clonally
related to their subtending leaves [20]. Further support for
this model comes from overexpression of the tomato
homeobox gene LeT6, which results in the formation of
adventitious meristems on the adaxial leaf surface, indicating
a certain level of leaf cell indeterminacy and a
predisposition for meristem initiation [21].
Is the initiation of axillary meristems
controlled by plant hormones?
In tomato, several mutants are defective in axillary meristem
initiation. The lateral suppressor (ls) mutant is
characterized by the absence of side-shoots, except for the
sympodial shoot and the lateral shoot immediately below
(subfloral side-shoots). In this mutant the meristematic
cells in the axils of leaf primordia, which later form the
axillary bud, are missing [11,22]. In addition, ls plants have
a defect in petal development leading to the absence of
the second whorl of flower organs [23]. Using different
bioassays it has been demonstrated that the endogenous
activities of gibberellins, auxins, and abscisic acid in the
shoot tip are drastically increased, whereas cytokinin levels
are reduced [24]. The recent cloning of the Ls gene [25 •• ]
revealed that the Ls protein belongs to a family of proteins
of unknown biochemical function, that are named VHIID
Genetic control of branching in Arabidopsis and tomato Schmitz and Theres 53
domain proteins after a conserved sequence motif. This
protein family includes the Arabidopsis proteins GIB-
BERELLIC ACID INSENSITIVE (GAI) [26] and
REPRESSOR OF GA1-3 MUTANT (RGA) [27], both of
which act as inhibitors of GA signal transduction. This
leads to the working hypothesis that the Ls protein also
exerts a negative regulation of GA signal transduction
pathway controlling the formation of axillary meristems
and petal primordia.
A second group of tomato mutants, represented by blind
[28] and torosa [29,30], is characterized by the absence of
meristems in many leaf axils [31] and inflorescences comprising
only one or two flowers. When axillary meristems
are initiated, they develop into slow growing side-shoots or
into leaf-like structures [31,32]. Comparable to ls, the morphological
defects of the blind and torosa mutants are
correlated with imbalances in plant hormone levels [24,31].
Taken together, the hormonal imbalances of the ls, blind
and torosa mutants and the sequence similarities of the
Ls gene to GAI and RGA indicate that axillary meristem
formation may be under hormonal control.
Monopodial versus sympodial shoot
development
As the ls mutant containing a null allele is still able to form
the sympodial shoot [25 •• ] and the side-shoot below, the Ls
protein seems not to be required for initiation of the respective
axillary meristem. This finding demonstrates that the
formation of the subfloral side-shoots, including the sympodial
shoot, which is most crucial for sympodial development,
is subject to different control than other side-shoots.
Similarly, Arabidopsis meristems in the axils of cauline leaves
develop much faster than those in the axils of rosette leaves.
The revoluta mutant enhances these differences, because
paraclades are repressed weakly in cauline leaf axils, but
strongly in the axils of rosette leaves [8].
In tomato, the failure to form the sympodial shoot results
in an early termination of axis development. Such phenotypes
can frequently be observed among blind and torosa
plants, which indicates that the respective gene products
may be involved in the genetic control of sympodial
branching. In addition, both mutants show a dramatic
reduction in inflorescence branching suggesting that sympodial
branching, and inflorescence branching may be
similar processes controlled by a common mechanism.
Phases of axillary shoot development
Axillary shoots repeat the pattern of development of the
primary shoot, starting with a vegetative phase and switching
later to reproductive growth. Mutations in several
genes either prolong or shorten these phases of axillary
shoot development, leading to a modified architecture of
the plant. In Arabidopsis, lateral shoots originating from the
axils of rosette or cauline leaves develop into paraclades
consisting of type 2 and type 3 metamers. In contrast, the
late flowering ecotype Sy-0 develops a rosette of leaves at

54 Growth and development
the base of each side-shoot. This phenotype is due to the
activity of dominant alleles of the genes ART and EAR, one
of which (EAR) can be substituted for by mutant alleles of
other late flowering genes. The aerial rosette phenotype
seems not to be due to a premature initiation of axillary
meristems, but instead they seem to have adopted the
identity of young primary SAMs leading to a prolonged
vegetative phase of the lateral shoots [2].
Mutations in the gene TERMINAL FLOWER 1 (TFL1) convert
the indeterminate Arabidopsis inflorescence into a
determinate one and condition a shorter vegetative phase of
the primary shoot [7,33]. In contrast, overexpression of
TFL1 results in a longer vegetative phase of both primary
and lateral shoots leading to a highly branched inflorescence
phenotype. These observations suggest that TFL1 is
involved in a mechanism regulating the progression through
the different phases of shoot development [34 •• ,35].
In the tomato mutant self pruning (sp), development of the
primary shoot and of the inflorescence are as in the wildtype;
however, the pattern of sympodial shoot
development is abnormal. Whereas in the wild-type three
nodes are formed before the transition to floral development,
in the mutant the numbers of vegetative nodes of
successive sympodial units are progressively reduced, until
the main axis terminates in a sympodial shoot without a
leaf [14,36 •• ]. Recently, the corresponding gene has been
isolated [34 •• ] and proved to be homologous to the TFL1
gene of Arabidopsis [37 • ,38] and the CENTRORADIALIS
(CEN) gene of Antirrhinum [39]. The similarity of TFL1,
CEN, and SP to phosphatidylethanolamin-binding proteins
of animals, known to bind to membrane protein
complexes, suggests that these proteins may play a role in
signalling processes in the apex. Overexpression of SP
results in plants with more than three vegetative nodes per
sympodial unit and a partially leafy inflorescence. These
results are consistent with the assumption that the SP gene
of tomato regulates the progression through different phases
of shoot development in a similar manner to the TFL1
gene of Arabidopsis. Unlike TFL1, however, the SP gene
seems to have no effect on the floral transition of the primary
shoot and on the architecture of the inflorescence.
Conclusions
Recently several factors influencing the process of shoot
branching in plants have been identified. The subtending
leaf seems to play an important role in the process of axillary
meristem initiation influencing the position of
meristems and their growth rate. Molecular evidence indicates
that the plant hormone GA may influence the
formation of axillary meristems.
A comparison of lateral shoot formation in the monopodial
plant Arabidopsis and the sympodial tomato reveal both
differences and similarities, and may uncover factors
responsible for the characteristic shoot architecture
observed in the two species. Whereas in tomato axillary
meristems are initiated early in development, in
Arabidopsis, this happens only after the transition to reproductive
growth. In both species the side-shoots preceding
the inflorescence develop faster than other side-shoots. In
tomato, the sympodial shoot overgrows the comparatively
slowly developing inflorescence resulting in a sympodial
shoot architecture, whereas in Arabidopsis the main inflorescence
develops with a similar growth rate as the
paraclades and remains in a terminal position. In tomato,
the genetic control of the initiation of the sympodial meristem
and of the axillary meristem below it is controlled
differently as compared to other lateral meristems. It
remains to be tested if this is also true for Arabidopsis.
Genes of the TFL1/CEN/Sp family encoding products
related to phosphatidylethanolamin-binding proteins of
animals control the length of all phases of shoot development
in Arabidopsis, but only the length of the vegetative
phase of sympodial shoots in tomato.
Acknowledgements
We thank F Salamini and Z Schwarz-Sommer for critical reading of the
manuscript and members of the laboratory for helpful comments and
discussion. Our work on genes that control branching is supported by the
Deutsche Forschungsgemeinschaft and the European Community.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1. Steeves TA, Sussex IM: Organogenesis in the shoot:
determination of leaves and branches. In Patterns in Plant
Development, 2nd edn. Cambridge: Cambridge University Press;
1989:124-146.
2. Grbic V, Bleecker AB: An altered body plan is conferred on
Arabidopsis plants carrying dominant alleles of two genes.
Development 1996, 122:2395-2403.
3. Esau K: Anatomy of Seed Plants. New York: John Wiley and Sons; 1977.
4. Cline MG: Concepts and terminology of apical dominance.
Am J Bot 1997, 84:1064-1069.
5. Schultz EA, Haughn GW: LEAFY, a homeotic gene that regulates
inflorescence development in Arabidopsis. Plant Cell 1991,
3:771-781.
6. Hempel FD, Feldman LJ: Bi-directional inflorescence
development in Arabidopsis thaliana: acropetal initiation of
flowers and basipetal initiation of paraclades. Planta 1994,
192:276-286.
7. Alvarez J, Guli CL, Xiang-Hua Y, Smyth DR: terminal flower: a gene
affecting inflorescence development in Arabidopsis thaliana.
Plant J 1992, 2:103-116.
8. Talbert PB, Adler HT, Paris DW, Comai L: The REVOLUTA gene is
necessary for apical meristem development and for limiting cell
divisions in the leaves and stems of Arabidopsis thaliana.
Development 1995, 121:2723-2735.
9. Picken AJF, Stewart K, Klapwijk D: Germination and vegetative
development. In The Tomato Crop. London: Chapman and Hall
1986:111-166.
10. Sawhney VK, Greyson RI: On the initiation of the inflorescence and
floral organs in tomato (Lycopersicon esculentum). Can J Bot
1984, 50:1493-1495.
11. Malayer JC, Guard AT: A comparative developmental study of the
mutant side-shootless and normal tomato plants. Am J Bot 1964,
51:140-143.
12. Tucker DJ: Hormonal regulation of lateral bud outgrowth in the
tomato. Plant Sci Lett 1977, 8:105-111.

13. Pierik RLM, Dieleman JA, Heuvelink E: Flowering of tomato in vivo
and in vitro in relation to the original position of the axillary bud
on the main axis. Sci Hort 1994, 59:55-60.
14. Atherton JG, Harris GP: Flowering. In The Tomato Crop. London:
Chapman and Hall 1986:167-200.
15. Allen KD, Sussex IM: Falsiflora and anantha control early stages of
floral meristem development in tomato (Lycopersicon
esculentum Mill.). Planta 1996, 200:254-264.
16. Napoli CA, Ruehle J: New mutations affecting meristem growth
and potential in Petunia hybrida Vilm. J Hered 1996, 87:371-377.
17. Souer E, van der Krol A, Kloos D, Spelt C, Bliek M, Mol J, Koes R:
Genetic control of branching pattern and floral identity during
Petunia inflorescence development. Development 1998,
125:733-742.
18. Snow M, Snow R: The determination of axillary buds. New Phytol
1942, 41:13-22.
19. McConnell JR, Barton MK: Leaf polarity and meristem formation in
•• Arabidopsis. Development 1998, 125:2935-2942.
In this paper, a mutant is described that shows a strong adaxialization of the
leaf blade. Mutant plants form additional axillary meristems on the underside
of the leaf. The authors question the detached meristem concept and propose
a new model for axillary meristem formation.
20. Irish VF, Sussex IM: A fate map of the Arabidopsis embryonic shoot
apical meristem. Development 1992, 115:745-753.
21. Janssen BJ, Lund L, Sinha N: Overexpression of a homeobox gene,
LeT6, reveals indeterminate features in the tomato compound
leaf. Plant Physiol 1998, 117:771-786.
22. Sossountzov L, Maldiney R, Sotta B, Sabbagh I, Habricot Y, Bonne M,
Miginiac E: Immunocytochemical localization of cytokinins in
Craigella tomato and a sideshootless mutant. Planta 1988,
175:291-304.
23. Williams W: The effect of selection on the manifold expression of
the ‘suppressed lateral’ gene in the tomato. Heredity 1960,
14:285-296.
24. Tucker DJ: Endogenous growth regulators in relation to side shoot
development in the tomato. New Phytol 1976, 77:561-568.
25. Schumacher K, Schmitt T, Rossberg M, Schmitz G, Theres K:
•• The Lateral suppressor gene of tomato encodes a new member
of the VHIID protein family. Proc Natl Acad Sci USA 1999, in press.
The cloning of a novel member of the VHIID domain genes that specifically
regulates the formation of lateral meristems and petals in tomato is
described. The hormonal imbalances in the ls mutant and the sequence similarities
of the Ls gene to known inhibitors of GA signalling indicate that the
formation of axillary meristems may be controlled by plant hormones.
26. Peng J, Carol P, Richards DE, King KE, Cowling RJ, Murphy GP,
Harberd NP: The Arabidopsis GAI gene defines a signaling
pathway that negatively regulates gibberellin responses.
Genes Dev 1997, 11:3194-3205.
27. Silverstone AL, Ciampaglio CN, Sun T-P: The Arabidopsis RGA gene
encodes a transcriptional regulator repressing the gibberellin
signal transduction pathway. Plant Cell 1998, 10:155-169.
Genetic control of branching in Arabidopsis and tomato Schmitz and Theres 55
28. Rick CM, Butler L: Cytogenetics of the tomato. Adv Genet 1956,
8:267-382.
29. Stubbe H: Mutanten der Kulturtomate Lycopersicon esculentum
Miller III. [Title translation: Mutants of the cultured tomato
Lycopersicon esculentum Miller III.] Kulturpflanze 1959, 7:82-112.
30. Stubbe H: Mutanten der Kulturtomate Lycopersicon esculentum
Miller V. [Title translation: Mutants of the cultured tomato
Lycopersicon esculentum Miller V.] Kulturpflanze 1964,
12:121-152.
31. Mapelli S, Kinet JM: Plant growth regulator and graft control of
axillary bud formation and development in the TO-2 mutant
tomato. J Plant Growth Regul 1992, 11:385-390.
32. Tucker DJ: Apical dominance in the tomato: some further
observations on isogenic lines showing varying degrees of sideshoot
development. Ann Bot 1979, 43:571-577.
33. Shannon S, Meeks-Wagner DR: A mutation in the Arabidopsis TFL1
gene affects inflorescence meristem development. Plant Cell
1991, 3:877-892.
34. Ratcliffe OJ, Amaya I, Vincent CA, Rothstein S, Carpenter R,
•• Coen ES, Bradley DJ: A common mechanism controls the life cycle
and architecture of plants. Development 1998, 125:1609-1615.
Overexpression of the Arabidopsis TFL1 gene demonstrates that this gene
regulates not only the length of the infloral phase, but also the length of other
phases of shoot development. The authors show that the length of the different
developmental phases determines, to a large extent, the growth habit
of a plant.
35. Schultz EA, Haughn GW: Genetic analysis of the floral initiation
process (FLIP) in Arabidopsis. Development 1993, 119:745-765.
36. Pnueli L, Carmel-Goren L, Hareven D, Gutfinger T, Alvarez J, Ganal M,
•• Zamir D, Lifschitz E: The SELF-PRUNING gene of tomato regulates
vegetative to reproductive switching of sympodial meristems and
is the ortholog of CEN and TFL1. Development 1998,
125:1979-1989.
The tomato Self-pruning gene has been cloned and proved to be homologous
to the Antirrhinum CEN and the Arabidopsis TFL1 gene. The fact that
the sp mutant shows a progressive reduction of the sympodial units but no
inflorescence phenotype is explained by the nature of the sympodial inflorescence,
with the main axes terminating after the formation of only one
flower. Overexpression of the Sp and CEN cDNAs in tomato results in partial
transformation of flowers to leaves and, in an anantha background, to a
lengthening of the sympodial units.
37. Bradley D, Ratcliffe O, Vincent C, Carpenter R, Coen E:
• Inflorescence commitment and architecture in Arabidopsis.
Science 1997, 275:80-83.
The Terminal Flower 1 gene was isolated using a probe of the Antirrhinum
Centroradialis gene and shown to be the Arabidopsis homologue. A correlation
between induction of flowering and the expression of TFL1 was shown.
38. Ohshima S, Murata M, Sakamoto W, Ogura Y, Motoyoshi F: Cloning
and molecular analysis of the Arabidopsis gene Terminal Flower 1.
Mol Gen Genet 1997, 254:186-194.
39. Bradley D, Carpenter R, Copsey L, Coral V, Rothstein S, Coen E:
Control of inflorescence architecture in Antirrhinum. Nature 1996,
379:791-797.