An explanation of the sporadic behavior of morphological mutants

--Michael Freeling and Nicholas Harberd

This laboratory has been working with several dominant morphological mutants that specify the common phenotype of foci of extra cell division within the leaf. Kn1 (in blade), Kn2 (over auricle), Rs* (in sheath and ligular region) and Hsf* (sheath transforms blade at margins of blade) are examples. Each of the 16 mutants included within the four loci designations above shows a sporadic phenotype: the patterns of extra cell division are not always the same and genetic background modifies expression. We have logically sound explanations for most aspects of these mutants' phenotypes (see Freeling et al., 1988. In: Plant Gene Research; Temporal and Spatial Regulation of Plant Genes, D.P.S. Verma and R.B. Goldberg, eds., Springer-Verlag, Wien, pp. 41-62), but their sporadic behaviors were unexplained. Since the sporadic aspects of phenotype characterize genes rather than individual mutants--all the mutants are sporadic--transposons are not likely to be involved.

We now think it likely that there is a default cell division rule for the developing leaf (and, presumably, each other organ of the plant): the slower-dividing cells communicate with those adjacent cells that could divide faster and say "slow down so as not to tear the leaf apart." The signal might well be physical stress and strain, transmitted wall-to-wall and transduced into molecularly meaningful second messages. The existence of this default cell division rule is not proved, but there are excellent data. Harberd and Freeling (1989, Genetics, in press) have described the basic genetics of D8 and Mpl, two closely linked, GA-insensitive, dominant dwarf mutants that derepress anther primordia in the ear. Plants carrying one copy of D8 are late-maturing and have leaves that have about 1/3 the nonmutant number of cells in length and in width; leaf primordial cells divide more slowly than do cells in leaves of normal sibs. Using genetic mosaic analyses, genetic markers and x-irradiation, we were able to obtain plants that were almost entirely dwarf (D8/+), but had sectors of normal tissue. Loss of D8 in the epidermis didn't matter in any case, but loss from the internal tissues of the plant often mattered. The question: did the normal tissue outgrow the nearby dwarf tissue? The answer: yes, D8 behaved autonomously in sectors including several phytomers of ear or tassel,or when comprising large portions of glume (unpublished). However, the autonomy argument grew circular when smaller sectors within leaves were examined. Most behaved as if they were dwarf; that is, D8 behaved nonautonomously. However some (2/9) overgrew the dwarf leaf at the margin only. D8-loss sectors within the glume (unpublished) were often large. In these cases, the normal sector did attempt to grow faster (was larger), but there were always smooth morphological contours between the faster-growing and slower-growing regions rather than a sharp boundary reflecting genotype.

If there were a default cell division rule operating in the leaf, as we suggest, the sporadic mutant phenotypes are explained. For example, the mutant Kn1-0 conditions a phenotype that often includes many sporadic protuberances over lateral veins; these are called knots. Our explanations of knotted phenotype have centered on age-identity problems involving the cells surrounding lateral veins: these cells "think they are younger than their positions would indicate" (Freeling et al., 1988, ibid.). Thus, these retarded cells differentiate to younger fates (sheath in blade) or keep dividing when they ought to stop (knots). How come just some of the cells keep dividing?; i.e. why the sporadic knot formation? Suggested answer: all the too-young cells surrounding lateral veins do have the autonomous capacity to continue division, but the default leaf cell division rule usually keeps them in check. Sometimes the default rule is overridden, and a knot occurs which, itself, would exacerbate or relieve cell-cell strain and lead to quite complex and sporadic cell-cell strain relationships. We imagine similar explanations for the sporadic phenotypes specified by the other dominant leaf cell division/cell identity mutants.

Perhaps "developmental compartments" in plants might be defined as the domains within which this hypothetical default cell division rule operates. Groups of cells within different compartments would not communicate to each other their rates of growth and coordination would not happen, as with cells in different phytomers.

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