We reported (Greyson et al, Devel. Genet. 18, 1996) that maize 18 kDa heat shock protein (HSP) mRNA increases following heat shock in the root-tip and shoot-tip. The HSP mRNA is found in specific cells. In the heat-shocked plumule, the mRNA is found mostly in the meristem, young leaves, and the vasculature of the coleoptile rather than in the more mature parenchyma cells. In the radicles, more mRNA is found in the meristematc region (1-3mm from the tip), and in the vasculature, than in the more mature distal region (4-10mm from the tip). We proposed that meristematic cells and cytoplasmically dense cells respond to heat shock.

To test our hypothesis, maize seeds were grown in the dark for seven days to permit lateral roots to develop. The lateral roots were heat-shocked, fixed, and embedded in wax. In situ hybridization was carried out according to Greyson et al. (MNL 71, 1996), employing the probe described therein.

Strong hybridization signals were observed within the tissues of lateral roots, but no signal was found in the primary root at the site where the lateral roots initiate (Fig.1).

Figure 1. Seven day old heat-shocked maize radicle (with both primary and lateral roots) in situ hybridized with SHSP 18-9-2 antisense RNA probe.

These results confirm that in the primary root, the cells of the more mature tissues no longer respond to heat shock, but as a lateral root is initiated, the cells in the lateral root (meristematic cells) regain the ability to respond to heat shock. (This work is supported by NSERC to D.B.W.).

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