The thermal induction of HSP 18 mRNA in Zea mays callus
--Friedberg, JN, Yang, Z, Walden, DB

Greyson et al. (Devel. Genet. 18:244-253, 1996) have shown that the heat shock response in maize radicles and shoots is found primarily in cytoplasmically dense cells and meristematic regions. Neither the cell type or organized meristem are found in 'immature' callus. Thus to extend our observations, maize callus cultures were initiated; callus was subjected to antisense RNA in situ hybridization to determine if callus and/or cells from the callus are able to respond to thermal stress.

Callus was initiated from 16 to 21 day old embryos (inbred Ohio 43). Embryos were plated on MS basal medium containing 105 2, 4-dichlorophenoxyacetic acid (2,4-D). Calli approximately 30 days old were heat shocked at 43 C for 2 hours, fixed, embedded in wax, cut in 10 micron sections and mounted on slides. The in situ hybridization was performed as described by Greyson et al. and used the antisense scMHSP 18-9-2 RNA, DIG labelled probe (Table 1, Greyson et al., 1996).

The callus cells exhibited strong expression of the HSP 18 mRNA, indicating that the tissue was responding to the heat shock. The mRNA expression appeared to be localized either in the nucleus or in the periphery of the cells. Across the callus section, mRNA expression was variable. These results indicate that the ability to respond to heat shock may be limited to cycling cells in an active nuclear cycle and that 'mature' cells lose the ability to respond to a thermal induction.

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