During development maize plants pass through a juvenile vegetative phase and an adult vegetative phase. Juvenile phytomers, or segments, unlike adult phytomers, possess prop roots and leaves with a visible waxy bloom. The primary shoot of a plant first produces juvenile leaves, then produces adult leaves, and finally terminates in a tassel. In some genetic backgrounds maize plants produce axillary branches, called tillers, which appear to have the same structure as the primary axis. We were interested in determining the duration of the juvenile phase of development in tillers originating at different positions. We wanted to determine whether tillers repeated the entire developmental program of the main stem or whether tillers sensed their position and underwent the transition from juvenile to adult development at a position parallel to that of the main stem.
Using the recessive mutation teosinte branched, which causes plants to produce a large number of tillers, we scored the duration of the juvenile phase of development on tillers originating at different positions along the primary axis of the plant. Phase change appears to occur in the same manner on tillers as it does on the primary axis of the plant. However, the developmental phase is not reset to a basal level in each tiller, and consequently tillers are not complete reproductions of the main stem. Tillers sense their position on the plant, and the duration of the vegetative phases reflects that position. As shown in Table 1, each tiller has a shorter juvenile phase than the main stem, or tillers from lower positions. The transition from the juvenile to the adult phase appears to occur after approximately the same number of phytomers from the base of the plant, either along the main axis or along a tiller. In the families examined, epicuticular wax is produced on the first five leaves of a plant, whether leaves are located on the main stem and the tiller or on the main stem only. Tillers also produce fewer leaves before tasseling than the main stem; moreover, tillers from higher nodes produce fewer leaves than tillers from lower nodes. However, tassel formation does not occur at the same position from the base of the plant on the tillers as it does on the main stem.
Table 1. Pattern of expression of phase-specific traits on the main
stem and tillers of teosinte branched plants.
|Shoot Position||Last Leaf with Wax||Last Node with Prop Roots||Last Leaf with an Axillary Shoot||Number of Leaves|
|Tiller in the axil of leaf 2||3.1±0.4c||5.5± 0.8||8.9±0.8b||12.4±0.8a|
|Tiller in the axil of leaf 3||2.1±0.6b||5.3±0.9b||8.0±1.1a||11.7±1.1b|
|Tiller in the axil of leaf 4||1.0±0.7a||4.3±0.6c||6.1±0.6||9.9±0.5a|
|Tiller in the axil of leaf 5||0.4±0.6||3.4±0.6||5.3±1.3||9.1±1.0|
These results indicate that vegetative phase change occurs globally throughout the plant and is not confined to the shoot apical meristem of the primary axis. The developmental phase of an axillary shoot reflects the developmental phase of the part of the main shoot from which it originated.
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