Genetic and molecular analysis of shoot branching in higher plants

Fig. 1: Scanning EM image of a tomato apex. Leaf primordia are visible as bulges at the flanks of the dome like structure. In the axils of leaf primordia (blue circle) lateral buds will be formed.

In seed plants, secondary axes of growth develop from lateral meristems formed in the axils of leaf primordia (Fig.1, blue circle). It is not clear, if these lateral meristems are derived directly as "detached meristems" from the main shoot apical meristem (SAM) or are initiated de novo. Further development of lateral meristems into buds and the outgrowth of buds are controlled by the SAM, probably through the action of plant hormones.
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 thaliana, Antirrhinum majus, Zea mays). In other species, the SAM will soon undergo the transition into a terminal floral meristem or its activity ceases. In these cases, elaboration of the shoot system is often continued by one or few axillary meristems giving rise to sympodial growth patterns (e.g. tomato, Petunia).

Fig. 2: Comparison of tomato wildtype (2a) and lateral suppressor (2b) leaf axils. In wildtype plants a lateral shoot is found in each leaf axil. These are missing in the axils of the ls mutant.

We have focused our attention on the mechanisms controlling the initiation of lateral meristems. Our studies started with the characterization of two tomato mutants, lateral suppressor (ls) and blind (bl), which show defects in lateral meristem formation. Histological analyses revealed that in the tomato ls mutant development of axillary meristems is inhibited in most of the leaf axils (Fig. 2). Similarly, lateral meristems cannot be detected in many leaf axils of blind-2 plants. Double mutants homozygous for ls and bl show additive effects, which indicates that at least two pathways control the initiation of axillary meristems in tomato (Schmitz et al., 2002). Both, the Ls (Schuhmacher et al., 1999) and the Bl (Schmitz et al., 2002) genes have been isolated from tomato. They encode a member of the GRAS family of putative transcriptional regulators and a MYB transcription factor, respectively.

Fig. 3: Pattern of LAS transcript accumulation in the vegetative Arabidopsis shoot apex. Longitudinal (3a) and transverse (3b) sections through shoot tips of Columbia were hybridized with a probe from the LAS gene.

To compare the genetic control of lateral meristem initiation in different plant species, we have extended our studies to Arabidopsis thaliana. The Arabidopsis LATERAL SUPPRESSOR (LAS) gene is required for the initiation of axillary meristems during the vegetative phase of development (Greb et al., 2003). LAS is expressed in axillary cells from which new meristems develop (Fig. 3).

To identify novel genes involved in specification and formation of axillary meristems, we have performed second-site EMS mutagenesis and screened for modifiers of the las-4 branching phenotype. Nine enhancer and five suppressor candidates have been identified and are currently being characterized. In addition, a group of three Bl-homologous genes from Arabidopsis has been characterized. These genes encode proteins with partially redundant functions that regulate axillary meristem formation in overlapping zones along the shoot axis.

A further goal of our studies is to use the isolated genes as tools to manipulate shoot architecture in crop plants. The known side-shoot mutants are of no value for crop production, because of their undesired side effects on fertility and yield. As the monostem character is a desired trait in several crop species (e.g. tomato), our experiments aim at an elimination of lateral shoots without the undesired side effects on inflorescence and fruit development.

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