The phenotype of iojap plants: sorting-out of plastids vs. incoordination

Iojap is generally treated as a "striping" factor with sectored leaves, but the phenotype of ij ij plants as described and pictured by M. T. Jenkins (J. Hered. 15:467, 1924) and M. M. Rhoades (Unités Biol. Douées de Continuité Génétique, 1949, pp. 37-44) is variable and quite different from conventional striping (see MNL 53:30). We have converged ij into 6 different inbreds and can compare the expression of ij ij in seedlings according to several parameters, as follows:
 
      Pattern of White Tissue  
Inbred White tissue % Green areas First leaf Second leaf Morphology
Oh51a 80-100 light green symm. margins scattered normal
W23 R-g 5-90 medium green scattered scattered normal
Tr 5-50 light grainy* symm. margins scattered normal
Mo17 20-90 dark green symm. margins symm. midribs normal
K55 10-20 dark green symm. margins symm. midribs bladeless**
KY21 5-60 light grainy symm. margins symm. midribs normal

*Light grainy signifies pale green leaves (finely "spatter-painted"), usually with darker veins.

**Bladeless refers to the absence of lamina proliferation on later leaves, except for irregular, knotted or gnarled "remnants."

On a number of ij ij plants in Tr background this summer, tillers developed rapidly as a result of fortuitous death of the shoot apex at the 5-10 leaf stage. Such tillers were initially pure white in sharp contrast to the light grainy phenotype of neighboring plants. As they advanced further, however, a few of the leaves showed patches of grainy tissue, suggesting that the competence to become green was present in the plastids but perhaps was overwhelmed by unusually rapid proliferation at high temperatures. One such white tiller carried a dark green sector in two successive leaves, more than 1/4 of the width of the leaf (see accompanying note on reversion of ij).

Some observations suggest that seasonal variation may be elicited during maturation of the seeds. In particular, progeny matured in Florida in the winter (versus in Missouri in the summer) tend to have much more white tissue.

These diverse expressions were defined in derivatives with their cytoplasmic constitutions derived from the inbreds; preliminary observations indicate that cytoplasmic differences are not responsible for the variations.

The diverse expressions found in ij ij plants are difficult to square with expectations from sorting-out of plastids in morphogenetic lineages (see MNL 53:30). Among observations that tend to separate the phenotype from sorting-out are the following:

1. The light grainy expression in Tr and Ky21 backgrounds, which is extremely reduced in chlorophyll yet virtually unsectored.

2. Occurrence of green plastids cryptically in apparently pure white areas (manuscript in preparation).

3. Symmetrically patterned white areas contrary to morphogenetic lineages.

4. Stage-specific sectors, originating separately in leaf or stem initials (Walbot and Coe, PNAS 76:2760, 1979).

5. Variations in expression that appear to be related to seasonal conditions during cell-determination phases.

6. Expressions in tillers that appear to be related to sudden changes in growth.

7. "Running out" of green color, and aborted spikelets, in distal parts of tassels.

8. Localization of white seedlings in ear maps from ij ij x + + at the distal tip of the ear (see accompanying note), contrary to morphogenetic lineages.

While sorting-out may contribute to these expressions, incoordination between the cells and the proplastids (in replication, reproduction, determination, differentiation) must play a larger role than sorting-out.

E. H. Coe, V. Walbot and D. Thompson


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