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 thalianaand 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: [email protected]
Current Opinion in Plant Biology1999, 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 species-specific 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 meris-tematic cells, which seem to be derived directly from the SAM of the main shoot, can be recognized early in devel-opment 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 devel-opment [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 differen-tiated 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 correla-tive inhibition, has been reviewed recently by Cline [4].
In some plant species the primary shoot apical meristem remains active throughout the life span producing con-tinuously new lateral organs, which results in a monopodial growth pattern (e.g. Arabidopsis, Antirrhinum). In other species the SAM will soon under-go 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-tems giving rise to sympodial growth patterns (e.g. toma-to, Petunia).
Very little is known about the molecular mechanisms con-trolling branching in higher plants. Recent studies have resulted in a detailed description of mutants, and molecu-lar 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 sympo-dial seed plants as observed during shoot and inflorescence development inArabidopsis 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 con-sisting 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 meris-tems 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 flow-ers. 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 sub-tending 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 intern-ode, 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
Genetic control of branching in
Arabidopsis
and tomato
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 lat-eral shoots, this pattern can be repeated indefinitely, resulting in a very bushy growth habit.
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 meris-tem and a new lateral merismeris-tem originating from the pedicel of the first flower continues inflorescence develop-ment [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 devel-opment. 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
Figure 1
Infloral side-shoot (paraclade ) Vegetative side-shoot
Accessory side-shoot
Infloral meristem
Flower
Leaf Type1
metamers Type2 metamers Type3 metamers
Arabidopsis
Second sympodial unit
First sympodial unit
Primary shoot
Tomato
Current Opinion in Plant Biology
Schematic representation comparing shoot development in Arabidopsisand 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
are formed in Petunia [16]. Interestingly, microscopic analy-sis 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 devel-opment 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 REVOLUTAgene 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-1dmutant ofArabidopsisforms 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 impor-tant 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 forma-tion of axillary meristems [19••]. This model is in agreement with the results of a clonal analysis demonstrat-ing 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, indicat-ing 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 meris-tem 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, lsplants 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 Lsgene [25••] revealed that the Ls protein belongs to a family of proteins of unknown biochemical function, that are named VHIID
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 com-prising 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 mor-phological 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 thelsmutant 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 respec-tive axillary meristem. This finding demonstrates that the formation of the subfloral side-shoots, including the sympo-dial shoot, which is most crucial for symposympo-dial development, is subject to different control than other side-shoots. Similarly, Arabidopsismeristems 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 pheno-types can frequently be observed among blindand 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 sym-podial branching, and inflorescence branching may be similar processes controlled by a common mechanism.
Phases of axillary shoot development
the base of each side-shoot. This phenotype is due to the activity of dominant alleles of the genes ARTand 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) con-vert 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 wild-type; however, the pattern of sympodial shoot development is abnormal. Whereas in the wild-type three nodes are formed before the transition to floral develop-ment, 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 theTFL1 gene of Arabidopsis [37•,38] and the CENTRORADIALIS (CEN) gene of Antirrhinum[39]. The similarity of TFL1, CEN, and SP to phosphatidylethanolamin-binding pro-teins 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 theSP gene of tomato regulates the progression through different phas-es of shoot development in a similar manner to theTFL1 gene ofArabidopsis. Unlike TFL1, however, the SP gene seems to have no effect on the floral transition of the pri-mary 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 axil-lary meristem initiation influencing the position of meristems and their growth rate. Molecular evidence indi-cates 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 repro-ductive 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 inflo-rescence 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 meris-tem and of the axillary merismeris-tem 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 develop-ment 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.
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