Abstract
Both tracheary elements and fiber cells undergo programmed cell death (PCD) during xylem development. In this study we investigated the role of papain-like cysteine protease CEP1 in PCD in the xylem of Arabidopsis. CEP1 was located in the cell wall of xylem cells, and CEP1 expression levels in inflorescence stems increased during stem maturation. cep1 mutant plants exhibited delayed stem growth and reduced xylem cell number compared to wild-type plants. Transmission electron microscopy demonstrated that organelle degradation was delayed during PCD, and thicker secondary walls were present in fiber cells and tracheary elements of the cep1 mutant. Transcriptional analyses of the maturation stage of the inflorescence stem revealed that genes involved in the biosynthesis of secondary wall components, including cellulose, hemicellulose, and lignin, as well as wood-associated transcriptional factors, were up-regulated in the cep1 mutant. These results suggest that CEP1 is directly involved in the clearing of cellular content during PCD and regulates secondary wall thickening during xylem development.
Highlights
Secondary growth is an important biological process in woody plants
We found that mutations in CEP1 delayed plant growth and altered xylem cell number.To determine whether CEP1 is involved in programmed cell death (PCD) during xylem development in Arabidopsis, we compared the structure and global transcriptional expression patterns of the inflorescence stem of cep1 mutants with wild-type plants.Transmission electron microscopy revealed that the mutant plants displayed delayed organelle degradation during PCD and had thicker secondary walls in fiber cells and tracheary elements (TEs)
Expression increased during inflorescence stem development and reached a maximal level when the stem stopped elongating at 25–30 d after bolting, when TEs are formed via PCD (Chaffey et al, 2002).To investigate the location of CEP1 within the stem, the ProCEP1GFP reporter line was used and the GFP signal was detectable in the wall of xylem cells, but not in the other cells of the vascular tissue (Fig. 1D–I)
Summary
Secondary growth is an important biological process in woody plants. The wood that it produces provides raw material for construction, fuel, and paper manufacturing. Several transcription factors have been found to regulate secondary xylem development (Demura and Fukuda, 2007; Zhong et al, 2008), and plant hormones such as auxin, cytokinin, brassinosteroids, and ethylene are involved in the process (Milhinhos and Miguel, 2013). Both tracheary elements (TEs) and fiber cells undergo programmed cell death (PCD) during xylem development. The release of hydrolytic enzymes stored in the vacuole leads to the rapid degradation of organelles, nuclear DNA, and part 206 | Han et al
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