Physiological and transcriptomic evidence for a close coupling between chloroplast ontogeny and cell cycle progression in the pennate diatom Seminavis robusta

Despite the growing interest in diatom genomics, detailed time series of gene expression in relation to key cellular processes, are still lacking. Here, weinvestigated the relationships between the cell cycle and chloroplast development in the pennate diatom Seminavis robusta. This diatom possesses two chloroplasts witha well-orchestrated developmental cycle, common to many pennate diatoms. By assessing the effects of induced cell cycle arrests with microscopy and flow cytometry, wefound that division and reorganization of the chloroplasts is initiated only after S-phase progression. Next, we quantified the expression of the S. robusta FtsZhomolog to address the division status of chloroplasts during synchronized growth and monitored microscopically their dynamics in relation to nuclear division andsilicon deposition. We show that chloroplasts divide and relocate during the S/G2 phase, after which a girdle band is deposited to accommodate cell growth.Synchronized cultures of two genotypes were subsequently used for a cDNA-AFLP-based genome-wide transcript profiling, in which 917 reproducibly modulated transcriptswere identified. We observed that genes involved in pigment biosynthesis and coding for light-harvesting proteins were up-regulated during G2/M phase and cellseparation. Light and cell cycle progression were both found to affect fucoxanthin-chlorophyll a/c-binding protein expression and accumulation of fucoxanthin cellcontent. Because chloroplasts elongate at the stage of cytokinesis, cell cycle-modulated photosynthetic gene expression and synthesis of pigments in concert with celldivision, might balance chloroplast growth and confirms that chloroplast biogenesis in S. robusta is tightly regulated.

Gillard, J., Devos, J, Huysman, M., De Veylder, L., D'Hondt, S., Martens, C., Vanormelingen, P., Vannerum, K., Sabbe, K., Chepurnov, V., Inzé, D., Vuylsteke, M., Vyverman, W. (2008) Physiological and transcriptomic evidence for a close coupling between chloroplast ontogeny and cell cycle progression in the pennate diatom Seminavis robusta. Plant Physiol. 148(3):1394-411.

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