Gene expression and enzyme activity of cell wall degrading enzymes in the latex of opium poppy Papaver somniferum L.

Abstract

Maturing laticifer cells of the opium poppy, Papaver somniferum L., have their common wails perforated in order to develop a continuous system of latex vessels throughout the plant. In poppy, this latex is alkaloid-rich and is valued as a source of pharmaceuticals. The continuous network allows large volumes of the latex to be released from the plant upon wounding, and is the preferred method for collecting the latex for the illegal drug trade. By studying the process of laticifer cell wall degradation, it is hoped that a line of poppies will be developed that will not bleed and thus prevent such collection. Gene homology for cell wall degrading enzymes were found during EST screening of a latex cDNA library from the opium poppy. Northern analysis of cellulose, polygalacturonase [] subunit, 1,3-[]-glucanase, and xyloglucan endotransglycosylase homology showed expression levels not limited to laticifer cells. However, poppy gene homology to pectin methylesterase (PME), pectin acetylesterase (PAE) and sedate lease (PL) had specific expression in latex. Enzyme assays confirmed the presence of PME, PAE, and PL activity in poppy latex serum. None of the homology for PME, PAE, or PL were induced by ethylene in poppy seedlings. One EST clone had high amino acid homology to several putative ACC oxidase homologs, however biochemical assays of expressed gene product failed to show ACC oxidase activity. As more gene sequences are placed in databases without biochemical characterization, this serves as an important caveat when unknown genes are assigned an identity based only on sequence comparison to other genes. The plant cell wall is a very complex structure. The roles of specific enzymes in laticifer wall degradation, the changing conditions of the apoplast during laticifer maturation, and the developmental signals given by molecules stored within the cell wall, all need further study in order to understand, and perhaps modify, this developmental process.