The dominant mutant Vg1-R was first noted for greatly diminished glumes in both the tassel and ear (J. Hered. 30:143-145, 1939). Later it was found that Vg1-R also severely reduces the ligule (Laughnan, MNL30:67, 1956; Galinat, personal communication). Here we report on our studies of Vg1-R ligule development. We inspected developing ligules both macroscopically and microscopically in Vg1-R heterozygotes after three generations of introgression into Mo17. The first two leaves of the Vg1-R seedling produce long, wild type-appearing ligules. Before leaf three emerges from enveloping older leaves, the ligule of leaf 2 degenerates gradually until just the base remains. This process can be observed over a period of 24 hours. The ligule of leaf 1 remains unaffected. In longitudinal sections, degenerating ligules contain distal cells with collapsed walls, indicating that cell death plays a role in Vg1-R induced ligule loss.
Leaf 3 and all subsequent leaves also exhibit reduced ligules. Unlike leaf 2, however, the ligules of these later leaves are already diminished at the time of leaf emergence from enveloping lower leaves. Based on histological examination, ligules of later leaves appear to initiate normally from the adaxial leaf surface. At this point, we are in the process of investigating whether diminished growth and/or cell death is involved in ligule reduction in these later leaves. The fact that ligules of leaf 2 and subsequent leaves become simultaneously subject to reduction suggests that Vg1-R triggers a switch leading to diminished ligules throughout the plant.
Experiments are currently underway to determine the effect of wild type gene dosage on Vg1-R phenes, to determine whether Vg1-R is cell autonomous, and to transposon-tag Vg1-R.
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