Transcriptome-wide effects of expansin gene manipulation in etiolated Arabidopsis seedling.

Expansin is a non-enzymatic protein which plays a pivotal role in cell wall loosening by inducing stress relaxation and extension in the plant cell wall. Previous studies on Arabidopsis, Petunia × hybrida, and tomato demonstrated that the suppression of expansin gene expression reduced plant growth but expansin overexpression does not necessarily promotes growth. In this study, both expansin gene suppression and overexpression in dark-grown transgenic Arabidopsis seedlings resulted in reduced hypocotyl length at late growth stages with a more pronounced effect for the overexpression. This defect in hypocotyl elongation raises questions about the molecular effect of expansin gene manipulation. RNA-seq analysis of the transcriptomic changes between day 3 and day 5 seedlings for both transgenic lines found numerous differentially expressed genes (DEGs) including transcription factors and hormone-related genes involved in different aspects of cell wall development. These DEGs imply that the observed hypocotyl growth retardation is a consequence of the concerted effect of regulatory factors and multiple cell-wall related genes, which are important for cell wall remodelling during rapid hypocotyl elongation. This is further supported by co-expression analysis through network-centric approach of differential network cluster analysis. This first transcriptome-wide study of expansin manipulation explains why the effect of expansin overexpression is greater than suppression and provides insights into the dynamic nature of molecular regulation during etiolation.

Transcriptomic data of Arabidopsis hypocotyl overexpressing a heterologous CsEXPA1 gene.

Expansin increases cell wall extensibility to allow cell wall loosening and cell expansion even in the absence of hydrolytic activity. Previous studies showed that excessive overexpression of expansin gene resulted in defective growth (Goh et al., 2014; Rochange et al., 2001) [1,2] and altered cell wall chemical composition (Zenoni et al., 2011) [3]. However, the molecular mechanism on how the overexpression of non-enzymatic cell wall protein expansin can result in widespread effects on plant cell wall and organ growth remains unclear. We acquired transcriptomic data on previously reported transgenic Arabidopsis line (Goh et al., 2014) [1] to investigate the effects of overexpressing a heterologus cucumber expansin gene (CsEXPA1) on the global gene expression pattern during early and late phases of etiolated hypocotyl growth.

Transcriptomic data of Arabidopsis thaliana hypocotyl upon suppression of expansin genes.

Expansin is a cell wall loosening protein without hydrolytic activity, which allows cell expansion by influencing cell wall extensibility. Previous studies showed that the suppression of expansin genes (EXPA1, EXPA3, EXPA5 and EXPA10) resulted in defective organ growth and altered cell wall chemical composition [1,2]. However, the molecular mechanism on how the suppression of non-enzymatic expansin expression can result in widespread effects on plant cell wall and organ growth is still unclear. In this study, we performed transcriptomic analysis on the hypocotyls of previously reported transgenic Arabidopsis line [1] to investigate the effects of expansin gene suppression on the global gene expression pattern, particularly on the cell wall related genes.