--Donald Miles

We have recently described an interesting developmental mutant from Mutator active lines which has a variable effect on the amount of leaf blade tissue that develops (Miles, MNL 63:66-67). We have named this locus leaf blade less (lbl) which describes the most extreme phenotype exhibiting complete loss of leaf blade without reduction of the midrib vascular tissue. The absence of blade tissue can vary from complete loss of blade tissue to only a 10% loss of the margins of the leaves. This variation has been classified into a series of five stages from the most extreme (stage 1) to the least affected phenotypes (stage 5). Figure 1A illustrates a mature lbl plant which shows the most extreme expression. Most leaves are stage 5 with no blade tissue while a few leaves express stage 3 with half leaves. Figure 1B shows a stage 3 half leaf.

Figure 1. A, Growth of an lbl plant at 30 C showing the extreme expression of the mutation. B, Shows a typical stage 3 leaf with one half of a blade.

We previously suggested that there was a temperature effect on the expression of lbl. To test this we germinated kernels and grew seedlings for 15 days under 28 C (14hr) days and 24 C nights in growth chambers. The light intensity throughout this experiment was maintained at 300µmol m2 s-1. Under this environment, lbl was expressed mildly as stage 3 to 5 of leaf development. At the end of 15 days, half of the mutant seedlings were shifted to 18 C and half to 30 C and grown for 75 more days to test the temperature effect. lbl continued to be expressed under both growth temperatures, though the expression was much more severe when grown at 30 C. At 30 C the development was shifted more to the extreme stages 1 to 4 while at 18 C the development was less severe, most often at stages 3 to 5. Certainly the expression of lbl is not on or off in response to these temperature differences but its effect is ameliorated by lower than normal growth temperature.

The surprising observation made on plants grown at 30 C was that stage 4 leaves often produced a small growth of `prop-root-like' tissue from the abaxial (lower) side of the midrib at the junction of the blades and the vascular tissue (Figure 2). Stage 4 is described as a leaf split at the tip into three structures, left and right leaf blades and central cylinder of vascular tissue which meet to form a complete leaf at about mid leaf. This `prop-root-like' growth was 0.5 to 1.0cm in length and appeared to consist primarily of vascular tissue. It always appeared just below the junction of the leaf blades with the vascular cylinder. It extended up or down but appeared to be growing with a positive gravitropic response. The meristem responsible, the precise cellular nature and the regulation of this growth are unknown.

Figure 2. Growth patterns of lbl stage 4 leaves from different plants at 30 C. The distal end of the leaf is to the top of the photograph. This shows the typical `prop-root-like' tissue projections on the abaxial side of the leaf at the junction of the midrib and the blades.

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