Orchid (Orchidaceae) is one of the largest families in angiosperms and presents exceptional diversity in lifestyle. Their unique reproductive characteristics of orchid are attracted by scientist for centuries. One of the synapomorphies of orchid plants is that their seeds do not contain endosperm. Lipids are used as major energy storage in orchid seeds. However, regulation and mobilization of lipid usage during early seedling (protocorm) stage of orchid is not understood. In this study, we compared transcriptomes from developing Phalaenopsis aphrodite protocorms grown on 1/2-strength MS medium with sucrose. The expression of P. aphrodite MALATE SYNTHASE (PaMLS), involved in the glyoxylate cycle, was significantly decreased from 4 days after incubation (DAI) to 7 DAI. On real-time RT-PCR, both P. aphrodite ISOCITRATE LYASE (PaICL) and PaMLS were down-regulated during protocorm development and suppressed by sucrose treatment. In addition, several genes encoding transcription factors regulating PaMLS expression were identified. A gene encoding homeobox transcription factor (named PaHB5) was involved in positive regulation of PaMLS. This study showed that sucrose regulates the glyoxylate cycle during orchid protocorm development in asymbiotic germination and provides new insights into the transcription factors involved in the regulation of malate synthase expression.
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