Clonal analyses of maize development by Coe and colleagues have demonstrated that the leaf blade is derived from a transverse ring of about 32 apical meristem cells. The number of cells in the longitudinal dimension of the leaf primordium is still unclear, however, nor is there a consensus about the number of apical cell layers that contribute to the leaf blade. Johri and Coe (MNL 55:31) have presented conclusive evidence that husk leaves are derived from at least two cell layers. Our recent results indicate that the foliage leaf is also derived from at least two apical cell layers (L-I and L-II) and from at least two rings of L-I cells.
Seedlings of the genotype Wd/wd; Pl/pl; B/b; R-r/R-g were irradiated with 500 R of x-rays (89 R/min, 2 mmAl, 138 kVp, 9 mA) 5 and 8 days after sowing, and were grown to maturity in pots in the greenhouse. At the time of irradiation 5-day-old plants had 8 leaf primordia, while 8-day-old plants had 9 primordia (Fig. 1).
Only sectors in leaves initiated after irradiation were considered in this study (i.e., leaves 9 and above in the case of 5-day-old plants, and leaves 10 and above in the case of 8-day-old plants).
Four kinds of sectors were observed. Some sectors were completely white throughout the length and thickness of the leaf blade (Fig. 2). Others were similar in that they were completely white along the length of the leaf, but differed in that they had a green epidermis (as indicated by the color of guard cells) (Fig. 3). A third class consisted of sectors that were white towards the distal end of the blade, and became green towards the proximal end (Fig. 4). The epidermis of such sectors was generally white, even in regions where the mesophyll was completely green. A fourth class of sectors exhibited the reverse condition (Fig. 5). In these cases the sectors were white towards the proximal end of the leaf, greener towards the distal end, and stopped before reaching the margin. These sectors always had a green epidermis.
Sectors in which the mesophyll and epidermis are of different genotypes (Fig. 3, 4, 5) provide evidence that the leaf blade is derived from at least 2 cell layers in the apical meristem. Sectors of the type illustrated in Figure 3 represent cases in which L-I cells always divided anticlinally, new cell walls perpendicular to the leaf surface. Instances in which the L-I divided periclinally during leaf development, thereby contributing cells to the internal part of the leaf blade, are represented by the sectors illustrated in Figures 4 and 5. Note that Figure 4 represents a white sector in the L-I layer, whereas Figure 5 represents a white sector in the L-II. In response to the criticism that such sectors might have been generated after leaf initiation, it should be pointed out that they are exactly the same width (1/32 of leaf width) as pure white sectors.
That the leaf blade is derived from at least two rings of L-I cells is apparent from the fact that epidermal sectors (marked by wd, b or pl) frequently end within the leaf blade. If the leaf blade were derived from a single ring of 32 L-I cells, then sectors would always end at the base of the leaf blade, that is, each of the 32 longitudinal L-I lineages would always be of a single genotype.
In conclusion, it should be pointed out these are only minimal estimates of the number of initial cells in a leaf primordium. They are valuable in that they confirm the picture provided by histological studies, and provide yet another indication that morphogenetic phenomena in plants involve signals that act at a multicellular level.
R. Scott Poethig
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