Tobacco (Nicotiana tabacum) and soybean (Glycine max) tissue culture cells were exposed t o a heat shock and protein synthesis studied by SDS-polyacrylamide gel electrophoresis after labeling with radioactive amino acids. A new pattern of protein synthesis is observed in heat-shocked cells compared t o that in control cells. About 12 protein bands, some newly appearing, others synthesized in greatly increased quantities in heat-shock cells, are seen. Several of the heat-shock proteins (HSPs) in both tobacco and soybean are similar in size. One of the HSPs in soybean (76K) shares peptide homology with its presumptive 25°C counterpart, indicating that the synthesis of at least some HSPs may not be due to activation of new genes. The optimum temperature for maximal induction of most HSPs is 39-40°C. Total protein synthesis decreases as heatshock temperature is increased and is barely detectable at 45°C. The heatshock response is maintained for a relatively short time in tobacco cells. After 3 hr at 39"C, a decrease is seen in the synthesis of the HSPs, and after 4 hr practically n o HSPs are synthesized. After exposure t o 39°C for 1 hr, followed by a return of tobacco cells t o 26"C, recovery t o the control pattern of synthesis requires greater than 6 hours. These results indicate that cells of flowering plants exhibit a heat-shock response similar t o that observed in animal cells.Key words: heat-shock, proteins, tobacco, soybeanThe exposure of cells of several different animal species t o heat-shock, ie, a sudden increase in the incubation temperature, results in the inhibition of synthesis of most cell proteins and in the new synthesis of a relatively few proteins. This phenomenon has been extensively described for Drosophila [ 1 , 2 ] , as well as for other insects [3] and in avian and mammalian cells [4].

A fate map for the shoot apical meristem of Zea mays L. at the time of germination was constructed by examining somatic sectors (clones) induced by γ-rays. The shoot apical meristem produced stem, leaves, and reproductive structures above leaf 6 after germination and the analysis here concerns their formation. On 160 adult plants which had produced 17 or 18 leaves, 277 anthocyanin-deficient sectors were scored for size and position. Sectors found on the ear shoot or in the tassel most often extended into the vegetative part of the plant. Sectors ranged from one to six internodes in length and some sectors of more than one internode were observed at all positions on the plant. Single-internode sectors predominated in the basal internodes (7,8,9) while longer sectors were common in the middle and upper internodes. The apparent number of cells which gave rise to a particular internode was variable and sectors were not restricted to the lineage unit: a leaf, the internode below it, and the axillary bud and prophyll at the base of the internode. These observations established two major features of meristem activity: 1) at the time of germination the developmental fate of any cell or group of cells was not fixed, and 2) at the time of germination cells at the same location in a meristem could produce greatly different amounts of tissue in the adult plant. Consequently, the developmental fate of specific cells in the germinating meristem could only be assigned in a general way.

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