Is the tassel a developmental compartment in the young meristem?

In a fate-map experiment designed to explore the autonomy and expression of the Kn1 gene (Genetics 111:617, 1985; and to be published elsewhere), we noticed a number of leaf sectors that continued into the tassel (Figure 1). This result was unexpected because Johri and Coe (Dev. Biol. 97:154, 1983) found that tassel sectors excluded the rest of the plant body and vice versa.

Seeds of the genotype Lw Kn1-N2 Adh1-S/lw kn Adh1-FkFgamma25 were germinated 48 hours on 30 C benches, and then irradiated with 1000 rads at 50-80 rads/min. Approximately 750 plants were examined. Cell lineages that lost the dominant Lw gene were white. Of 15 sectors found in the flag leaf (or, in one case, the second highest leaf), 10 continued into the tassel (Table 1); 5 plants had sectors in the flag leaf or second highest leaf that did not continue into the tassel (Table 2). Tassel-only sectors were not seen, but it is possible that our "searching image" was for leaf sectors only. The fraction of plants with lw sectors that included the tassel, 10/750, is close to that seen by Johri and Coe, 44/3000.

The size of the tassel sectors varied. In two cases the sectors were large, including one-half to onethird of the main spike and some of the lateral spikes (Table 1,a-b). In most cases, the sectors included only 1 row of spikelets of the main spike, plus or minus a sectored spikelet (Table 1,c-j). The number of cells extant at the time of irradiation that contribute to making the tassel can be estimated from the percentage of tassel that is occupied by the sector. From our limited sample, the apparent cell number conforms to previous findings: 20% of the tassels developed from 2-3 cells, 30% developed from 5 cells, 50% developed from 8-9 cells.

There are several possible explanations that might explain how our data differ from previous results, but none detract from the conclusion: there is no obligatory compartment boundary delineating leaf segments from tassel segments in the 2-day post-germination meristem. The plant shown in Table 1 (g) is particularly interesting: here a sector comprising 1/6 - 1/30 of the leaf area is represented as all of the ear and 1/9 of the tassel as well. A developmental compartment boundary, such as the anterior-posterior line bisecting each segment in Drosophila melanogaster must present an obligatory barrier to cell division (see D. Brower, Cell 41:361, 1985). Lineage restrictions resulting from programmed barriers to cell division may exist, but perhaps only in older "determined" apical meristems.

Figure 1. A leaf sector formed from the loss of Lw that extends into the tassel. The plant in the photograph is M of Table 1.

Table 1. Position of sectors found in leaves and tassel for 10 plants. The leaves are numbered from the tassel to the base: leaf 1 is the flag leaf. The leaves and tassels that are included in the sector are drawn. For example, the sector of (d) included leaves 3, 2 and 1, one lateral spike and the main spike of the tassel. All the tassel sectors included the main spike of the tassel with the exception of (g) which had only two sectored lateral spikes. The sector of (g) included the ear as diagrammed. The tassel sector of (j) was small, just entering the main spike and ending.

Table 2. Position of sectors in the flag leaf or second highest leaf that did not enter the tassel for 5 plants. Other plants with sectors on lower nodes are not included.

Sarah Hake and Michael Freeling
 
 


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