Expression of heat shock factor and heat shock protein 70 genes during maize pollen development.

We have analysed the expression of heat shock protein 70 (HSP70) and heat shock factor (HSF) gene during maize pollen development, HSFs being the transcriptional activators of hsp genes. In order to eliminate the sporophytic tissues of anthers, we have isolated homogeneous cell populations corresponding to five stages of maize pollen development from microspores to mature pollen. We show that in the absence of heat stress, hsp70 genes are highly expressed late-bicellular pollen as compared to other stages. HSP70 transcripts are significantly accumulated in response to a heat shock at the late microspore stage but to a much lower extent than in vegetative tissues. The latest stages of pollen development, i.e. mid-tricellular and mature pollen, do not exhibit heat-induced accumulation of HSP70 transcripts. Therefore, we analysed the expression of hsf genes throughout pollen development. We demonstrate that at least three hsf genes are expressed in maize and that transcripts corresponding to one hsf gene, whose expression is independent of temperature in somatic as well as in microgametophytic tissues, are present at similar levels throughout pollen development. In addition, we show that the expression of the two other hsf genes is heat-inducible in maize vegetative tissues and is not significantly increased after heat shock at any stage of pollen development. These results indicate that the loss of hsp gene expression at late stages of pollen development is not due to a modification of hsf gene expression at the mRNA level and that hsf gene expression is differentially regulated in vegetative and microgametophytic tissues.

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