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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Shoot apices of Lolium temulentum excised after exposure of the plants to one long day (LD) undergo floral development in vitro, whereas those excised from plants in short days (SD) remain vegetative. Floral differentiation in vitro is reasonably normal and responds quantitatively to the preceding LD induction when three conditions are met. First, excision must not precede the arrival at the apex of the LD stimulus from the leaves. This appears to begin about 22 h after the start of the LD and is completed after a further 14 h, by which time all isolated apices have become capable of initiating inflorescence differentiation, i.e. florally determined. Second, for apices excised on the day after the LD (Day II) or early on Day III, the presence of gibberellic acid in the medium is required for floral differentiation to occur in most explants. By contrast, neither kinetin (N(6)-furfurylaminopurine) nor indole-3-acetic acid is required or beneficial, while abscisic acid in the medium is inhibitory to both survival and floral differentiation in excised apices. The third requirement is for an adequate supply of sugar, particularly after floral differentiation begins. Sucrose is taken up rapidly to reach high levels in the excised apices, but high sugar concentration in the medium, either alone or with gibberellic acid, does not suffice for floral differentiation to begin, and there is an absolute requirement for receipt of the LD stimulus.

The terminal, apical shoot meristem ofN. tabacum cv. Wisconsin 38 normally differentiates into a flower after producing 30 to 40 nodes. The influence of leaves and roots on the regulation of flowering was evaluated by counting the number of nodes produced after removal of leaves or the induction of adventitious roots. Leaf removal has no effect on the number of nodes produced before flower formation. Root induction significantly increases the number of nodes produced before flower formation. The plant behaves as if it were measuring the number of nodes between the meristem and the roots as a means of regulating meristem conversion from vegetative to floral differentiation.

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