Transcriptional analysis of cell growth and morphogenesis in the unicellular green alga Micrasterias(Streptophyta), with emphasis on the role of expansin

Katrijn Vannerum,Lieven De Veylder,Alain Goossens,Frederik Leliaert,Wim Vyverman,Dirk Inzé,Riet De Rycke,Ursula Lütz-Meindl,Jeroen Gillard,Jacob Pollier,Marie Jj Huysman,Marnik Vuylsteke

doi:10.1186/1471-2229-11-128

Abstract

BackgroundStreptophyte green algae share several characteristics of cell growth and cell wall formation with their relatives, the embryophytic land plants. The multilobed cell wall of Micrasterias denticulata that rebuilds symmetrically after cell division and consists of pectin and cellulose, makes this unicellular streptophyte alga an interesting model system to study the molecular controls on cell shape and cell wall formation in green plants.ResultsGenome-wide transcript expression profiling of synchronously growing cells identified 107 genes of which the expression correlated with the growth phase. Four transcripts showed high similarity to expansins that had not been examined previously in green algae. Phylogenetic analysis suggests that these genes are most closely related to the plant EXPANSIN A family, although their domain organization is very divergent. A GFP-tagged version of the expansin-resembling protein MdEXP2 localized to the cell wall and in Golgi-derived vesicles. Overexpression phenotypes ranged from lobe elongation to loss of growth polarity and planarity. These results indicate that MdEXP2 can alter the cell wall structure and, thus, might have a function related to that of land plant expansins during cell morphogenesis.ConclusionsOur study demonstrates the potential of M. denticulata as a unicellular model system, in which cell growth mechanisms have been discovered similar to those in land plants. Additionally, evidence is provided that the evolutionary origins of many cell wall components and regulatory genes in embryophytes precede the colonization of land.

Highlights

Streptophyte green algae share several characteristics of cell growth and cell wall formation with their relatives, the embryophytic land plants
By sampling this period at five consecutive time points we obtained samples with different proportions of cells at the major morphogenetic stages (Figure 1A,C,D). cDNA-AFLP expression profiling of these samples allowed the assignment of differentially expressed genes to either the onset of cell division (Figure 1A2; Figure 2 (C1a and C1b)), the bulge (Figure 1A3; Figure 2 (C2)), the lobe (Figure. 1A4-A9; Figure 2 (C3)), or the doublet stage, during which the secondary cell wall is formed (Figure 2 (C4 and C5))
The relative abundance was monitored of 4574 transcriptderived fragments (TDFs) during the cell growth of M. denticulata (Figure 3, Additional file 2), for which the expression patterns were altered visibly across time in 1420 and significantly (P

Summary

Introduction

Streptophyte green algae share several characteristics of cell growth and cell wall formation with their relatives, the embryophytic land plants. The multilobed cell wall of Micrasterias denticulata that rebuilds symmetrically after cell division and consists of pectin and cellulose, makes this unicellular streptophyte alga an interesting model system to study the molecular controls on cell shape and cell wall formation in green plants. After completion of the primary wall (during the doublet stage), a rigid cellulosic secondary cell wall pierced by pores is deposited, followed by shedding of it.

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Conclusion