Galactinol Synthase Gene Expression in Melon

Raffinose family oligosaccharides (RFOs) perform several physiological functions in plants. In addition to accumulating during seed formation, raffinose and stachyose are translocated in the phloem and may accumulate in response to low temperatures, drought, or salt stress. Although the synthesis of galactinol, as mediated by galactinol synthase (GAS), is the first committed step in RFO formation, its expression patterns are poorly understood in most species. We have cloned and characterized the expression of two galactinol synthase gene family members in melon (Cucumis melo L. Cantalupensis Group). Both CmGAS1 and CmGAS2 are highly expressed in mature leaves. Galactinol synthase transcription in leaves was not upregulated by either water or low temperature stresses. Transcripts of CmGAS1 were present in developing melon seeds at a time coincident with the formation of raffinose and stachyose. Based on the GAS expression and RFO accumulation patterns, we propose that RFOs in melon function in carbon translocation and seed desiccation tolerance. and ajugose (a hexasaccharide RFO) accumulates in the mesophyll cells of Ajuga reptans after exposure to low temperatures (Bachmann and Keller, 1995; Sprenger and Keller, 2000). These acclimation responses may be critical for the survival of low temperature tolerant species. Although the presence of RFOs has been documented in a variety of systems, there are few studies demonstrating the expression of the GAS genes. Sprenger and Keller (2000) published a study that described the expression patterns of two galactinol synthase genes, GolS-1 and GolS-2, in Ajuga reptans. Ajuga is a frost-hardy evergreen labiate that both accumulates and translocates RFOs. The gene, GolS-1 was primarily expressed in the mesophyll in response to low temperatures while GolS-2 was expressed in intermediary cells and most likely catalyzes the formation of RFOs destined for translocation. The Arabidopsis GAS genes AtGolS1 and AtGolS2 are expressed in response to drought and salt stress while AtGolS3 accumulates in response to low temperature stress (Taji et al., 2002). The expression patterns of other Arabidopsis GAS genes have not been characterized. We have cloned two GAS genes in melon (Cucumis melo L. Cantalupensis Group). One gene, CmGAS1, is highly homologous to the zucchini CpGAS1 gene identified by Kerr et al. (1993). A CmGAS1 promoter-gusA fusion was made and used to transform Arabidopsis and tobacco plants. In these transgenic plants, GUS expression localized to the smallest veins of mature Arabidopsis and tobacco leaves (Haritatos et al., 2000). Although galactinol is not normally synthesized in wild-type tobacco leaves, stain localized to the companion cells and sieve elements in the minor veins (Haritatos et al., 2000), indicating that the CmGAS1 promoter drives vein- specific gene expression. There are no previous reports document- ing the expression pattern of the second melon GAS gene, CmGAS2. In melon, raffinose and stachyose serve as phloem translocation sugars and also accumulate in maturing seeds. Other functions of RFOs in melon have not been documented. We have used RNA blots and RT-PCR techniques to characterize the mRNA expression patterns of the CmGAS1 and CmGAS2 genes in melon.