Physiological and Transcriptomic Evidence for a Close Coupling between Chloroplast Ontogeny and Cell Cycle Progression in the Pennate DiatomSeminavis robusta       

Abstract

Despite the growing interest in diatom genomics, detailed time series of gene expression in relation to key cellular processes are still lacking. Here, we investigated the relationships between the cell cycle and chloroplast development in the pennate diatom Seminavis robusta. This diatom possesses two chloroplasts with a well-orchestrated developmental cycle, common to many pennate diatoms. By assessing the effects of induced cell cycle arrest with microscopy and flow cytometry, we found that division and reorganization of the chloroplasts are initiated only after S-phase progression. Next, we quantified the expression of the S. robusta FtsZ homolog to address the division status of chloroplasts during synchronized growth and monitored microscopically their dynamics in relation to nuclear division and silicon 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 cDNAamplified fragment length polymorphism-based genome-wide transcript profiling, in which 917 reproducibly modulated transcripts were identified. We observed that genes involved in pigment biosynthesis and coding for light-harvesting proteins were up-regulated during G2/M phase and cell separation. Light and cell cycle progression were both found to affect fucoxanthinchlorophyll a/c-binding protein expression and accumulation of fucoxanthin cell content. Because chloroplasts elongate at the stage of cytokinesis, cell cycle-modulated photosynthetic gene expression and synthesis of pigments in concert with cell division might balance chloroplast growth, which confirms that chloroplast biogenesis in S. robusta is tightly regulated.

Transcript level coordination of carbon pathways during silicon starvation‐induced lipid accumulation in the diatom Thalassiosira pseudonana

“…Cellular growth and chloroplast replication are tightly regulated cellular processes (Wittenberg & Reed, 2005; Hudik et al ., 2014) and are shown to be coordinated in diatoms (Gillard et al ., 2008). A strong transcript level response at 4 h of silicon starvation was documented in T. pseudonana and occurred while nearly half of the population continued to progress through the cell cycle (Fig.…”

“…Alternatively, recent functional genomics studies in diatoms and other microalgae have shown that transcription of large suites of genes is coordinated with growth‐related processes such as chloroplast division, release from nitrogen and silicon starvation, cell wall synthesis, onset of cell division, and circadian shifts, suggesting that many genes may be under the control of master regulators (i.e. redox state, transcription factors) that choreograph genome‐wide transcription (Gillard et al ., 2008; Monnier et al ., 2010; Allen et al ., 2011; Shrestha et al ., 2012; Ashworth et al ., 2013). Consequently, changes in transcript abundance can be interpreted as an adaptive response to specific environmental conditions, as a part of a coordinated regulatory program associated with growth, or a combination of both factors.…”

AUREOCHROME1a-Mediated Induction of the Diatom-Specific Cyclin dsCYC2 Controls the Onset of Cell Division in Diatoms (Phaeodactylum tricornutum)

“…Some pennate species have been reported to show a similar G1 and G2/M arrest, as reported for Cylindrotheca fusiformis (Brzezinski et al, 1990), while others display only a G1 arrest, as in Phaeodactylum tricornutum (Huysman et al, 2010) and Seminavis robusta (Gillard et al, 2008). For those species with only a light-dependent segment at the G1 phase, the immediate release of dark-arrested cells has proven to be a useful characteristic to synchronize and study the cell division process (Gillard et al, 2008;Huysman et al, 2010).…”

The evolution of the regulatory mechanism of chloroplast division

“…20 In addition, in the diatom (a stramenopile) Seminavis robusta, a single cell of which contains two chloroplasts of a red algal secondary endosymbiotic origin, the nucleus-encoded ftsZ transcript accumulates during the S/G 2 phase, when the chloroplasts divide. 21 Overall, these reports suggest that the cell cycle-based expression of the chloroplast division genes allows the chloroplast to divide in a specific phase of the host cell division cycle.…”