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SYSTEMS BIOLOGY, MOLECULAR BIOLOGY, AND GENE REGULATION

Polyphenoloxidase Silencing Affects Latex Coagulation in Taraxacum Species^1,[W]

Westphalian Wilhelms-University of Münster, Institute of Biochemistry and Biotechnology of Plants, 48143 Muenster, Germany (D.W., C.R., B.M.M., D.P.); Fraunhofer Institute for Molecular Biology and Applied Ecology, 52074 Aachen, Germany (C.S.G., F.F., R.F., J.M., D.P.); and Department of Biology, University of York, Heslington, York YO10 5DD, United Kingdom (R.M.T.)

Latex is the milky sap that is found in many different plants. It is produced by specialized cells known as laticifers and can comprise a mixture of proteins, carbohydrates, oils, secondary metabolites, and rubber that may help to prevent herbivory and protect wound sites against infection. The wound-induced browning of latex suggests that it contains one or more phenol-oxidizing enzymes. Here, we present a comprehensive analysis of the major latex proteins from two dandelion species, Taraxacum officinale and Taraxacum kok-saghyz, and enzymatic studies showing that polyphenoloxidase (PPO) is responsible for latex browning. Electrophoretic analysis and amino-terminal sequencing of the most abundant proteins in the aqueous latex fraction revealed the presence of three PPO-related proteins generated by the proteolytic cleavage of a single precursor (pre-PPO). The laticifer-specific pre-PPO protein contains a transit peptide that can target reporter proteins into chloroplasts when constitutively expressed in dandelion protoplasts, perhaps indicating the presence of structures similar to plastids in laticifers, which lack genuine chloroplasts. Silencing the PPO gene by constitutive RNA interference in transgenic plants reduced PPO activity compared with wild-type controls, allowing T. kok-saghyz RNA interference lines to expel four to five times more latex than controls. Latex fluidity analysis in silenced plants showed a strong correlation between residual PPO activity and the coagulation rate, indicating that laticifer-specific PPO plays a major role in latex coagulation and wound sealing in dandelions. In contrast, very little PPO activity is found in the latex of the rubber tree Hevea brasiliensis, suggesting functional divergence of latex proteins during plant evolution.

² These authors contributed equally to the article.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Dirk Prüfer (dpruefer{at}uni-muenster.de).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.138743

^* Corresponding author; e-mail dpruefer{at}uni-muenster.de.

Received March 18, 2009; accepted July 10, 2009; published July 15, 2009.

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Peter V. Minorsky
Plant Physiol. 2009 151: 1-2. [Full Text]