Heat-shock inducibility of RNA from a small heat shock protein encoding gene is independent of its developmental induction
--Robert A. Bouchard and David B. Walden

We have previously shown that RNA accumulation from specific members of the maize gene family comprising small heat-shock protein (shsp) genes is developmentally modulated in a gene-specific manner within tissues containing developing microsporocytes (Bouchard, Frappier, Liu, Raizada, Atkinson, and Walden, Maydica 38:135-144, 1993). Transcripts detected by the probes specific for two such shsp genes, uwo9 (Mhsp18-1) and uwo11(Mhsp18-9), show developmental induction in anthers or spikelets containing late-prophase through meiotic division microsporocytes relative to somatic tissue controls. However, transcripts detected by the probe specific for shsp gene uwo10 (Mhsp18-3), also derived from a cDNA clone recovered from the heat-shocked somatic tissue cDNA library, show no developmental induction relative to control somatic tissues in anthers or spikelets containing late-prophase through meiotic division microsporocytes, or in tissues harboring microsporocytes at any other stage of development. This observation suggested two possibilities to us. Either the product of shsp gene uwo10 remains uninduced in microsporogenous tissues because a gene-specific site or inducer for developmental induction is lacking, or this particular shsp gene is repressed or modified in these tissues so that it can no longer respond to induction when the other shsp gene does.

To differentiate between these possibilities, we compared the levels of uwo10 RNA seen in control and heat-shocked microsporogenous tissues, as well as in somatic tissues. The tissues used were: isolated staged anthers containing late prophase microsporocytes (late prophase anthers), spikelets containing early and late prophase anthers (prophase spikelets), the immature inner leaves surrounding the source tassels (tassel sheaths), and five day old plumules (plumules). RNA was isolated from control and heat-shocked samples and RNA-dots were probed using a gene-specific subclone of 18-3. The observed elevations in heat-shocked versus control samples were as follows: in late prophase anthers, >570%; in prophase spikelets, 550%; in tassel sheaths, >700%; in five day plumules, >500%. Thus, the uwo10 gene retains the same capacity for heat shock induction in late prophase anthers and spikelets as in somatic tissues, despite the fact that the microsporogenous tissues show no developmental accumulation of this shsp RNA at stages when the products of other shsp genes show developmental accumulation. These results favor the hypothesis that a gene-specific site or inducer for developmental induction is lacking for gene uwo10 in microsporogenous tissues. 

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