Twisted 1: a new morphological mutation

--R. Kelly Dawe and Michael Freeling

In its most extreme form, homozygous twisted 1 plants form ten to twenty extra leaves with whorled phyllotaxy (Fig. 1). This recessive mutant was recovered from a Robertson's Mutator background. In the upper two thirds of the plant, at least seven different characters are observed that are collectively responsible for the extreme phenotype shown in Figure 1. In no case have we observed the mutant phenotype in the lower six nodes of the plant. The expressivity of the mutant varies with the environment, being most extreme in our winter greenhouse. The common traits of the mutant are listed below with respect to the frequency of their occurrence:

1) multiple midribs. In the greenhouse every mutant plant shows at least one leaf with more than one midrib (Fig. 2C). There can be as many as four midribs on a single leaf. The wide, lax midribs that are sometimes observed (Fig. 2B) are thought to be the intermediate between one and two midribs. Leaves with no midrib are occasionally observed. The presence of an ectopic midrib clearly inhibits the development of the auricle (see diminished auricle in Fig. 3), and when there is no midrib, the auricle extends from margin to margin. Extra midribs often cause a displacement of the ligule towards the base of the leaf (Fig. 3).

2) Shortened internodes. About 70% of greenhouse plants have at least one shortened internode. Some are shown in Figure 4.

3) Opposite leaves. About 40% of greenhouse plants have opposite leaves. It is unclear whether these are the result of an extremely short internode or a legitimate switch in phyllotaxy. Opposite leaves are generally followed by series of additional leaves with an opposite decussate arrangement.

4) Barren nodes. What appear to be nodes without leaves are found on 30% of the plants in the greenhouse. As many a seven in a row have been observed (Fig. 4). Interestingly, the presence of barren nodes does not appear to change the total node number. If the plant counts vegetative nodes (see Irish and Nelson, 1988, Planta 175:9-12), it probably does not count leaves .

5) Twisted internodes. About 30% of greenhouse plants have at least one twist. We think twisting results from part of one node (or major veins) getting "stuck" on the node before (Fig 5, 6). In the most extreme case twisting continues indefinitely until plant senescence (Fig. 1). In these "infinite twists", leaf width decreases and leaf number increases.

6) Split plants. About 20% of greenhouse plants have between two to four tassels (Fig. 4). There can be as many as four nodes between where the plant splits and where tassels are formed.

7) Tube leaves. About 20% of greenhouse plants have at least one leaf with fused margins (Fig. 4).

Figures 1-5.
Figure 1. Top view of a twisted 1 plant in its most extreme form.
Figure 2. Multiple midribs. A, wild type leaf; B, leaf with wide, lax midrib; C, leaf with two midribs.
Figure 3. Auricle inhibition and ligule displacement. em, extra midrib. The upper and lower arrows delimit the distance of basipetal ligule displacement at the place where the extra midrib intersects the ligular region.
Figure 4. Shortened nodes, barren nodes, split plant, and tube leaf. ta, tassel; tl, tube leaf. Arrows indicate the positions of barren nodes (leaves were removed from upper two nodes). Note some nodes are shorter than others.
Figure 5. Twisted internode. Arrow shows direction of twist.

Figure 6. Proposed mechanism for twisted internodes. Dashed arch represents growing shoot. Solid horseshoes represent the bands of leaf initials. Leaf initials partially encircle the meristem and slightly overlap one another when viewed from the top (A). As growth proceeds, an internode develops and separates the leaf initials (B). In twisted plants, part of one node catches on the node below, binding the two bands of leaf initials (C, small filled circle). In the case illustrated, the leaf initials are bound on the rear side of the meristem. Internode growth is subsequently limited by the combined length of the leaf initials, and the internode twists to accommodate.

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