Induction of Malate Synthase Gene Expression in Senescent and Detached Organs of Cucumber.

Expression of the malate synthase (MS) gene is activated in cotyledons of cucumber seedlings during postgerminative growth and then repressed as the cotyledons become photosynthetic. MS gene expression is subsequently reactivated in the cotyledons as they senesce a few weeks later. In situ hybridization revealed that MS RNA is distributed throughout the organ during postgerminative growth and senescence, showing that the same cells express the gene at different stages of development. MS RNA also appears in senescing leaves and petals of cucumber plants. In addition, we found that MS RNA appears in mature expanded leaves and roots when they are removed from the plant and incubated in darkness for several days, thus providing a potential experimental system for the manipulation of MS gene expression. Leaves from transgenic Nicotiana plumbaginifolia containing the cucumber MS promoter fused to the [beta]-glucuronidase (GUS) reporter gene accumulated GUS activity when detached, demonstrating an activation of transcription from the MS promoter following leaf excision. These results are discussed in terms of the metabolic regulation of MS gene expression.