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Effect of 2,4-Diacetylphloroglucinol on Pythium: Cellular Responses and Variation in Sensitivity Among Propagules and Species

First and sixth authors: Wageningen University and Research Centre (WUR), Department of Plant Sciences, Laboratory of Phytopathology, Binnenhaven 5, P.O. Box 8025, 6709 PD, Wageningen, The Netherlands; second, third, and fourth authors: UMR INRA/Université de Bourgogne BBCE-IPM, Centre de Microbiologie du Sol et de I'Environment, INRA, BV 1540, 21034 Dijon Cedex, France; and fifth author: INRA-CMSE, Laboratoire de Recherches sur la Flore Pathogène dans de Sol, 17 rue Sully, 21034 Dijon Cedex, France

The antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) plays an important role in the suppression of plant pathogens by several strains of Pseudomonas spp. Based on the results of this study, there is variation within and among Pythium spp. to 2,4-DAPG. Also, various propagules of Pythium ultimum var. sporangiiferum, that are part of the asexual stage of the life cycle, differ considerably in their sensitivity to 2,4-DAPG. Mycelium was the most resistant structure, followed by zoosporangia, zoospore cysts, and zoospores. Additionally, we report for the first time that pH has a significant effect on the activity of 2,4-DAPG, with a higher activity at low pH. Furthermore, the level of acetylation of phloroglucinols is also a major determinant of their activity. Transmission electron microscopy studies revealed that 2,4-DAPG causes different stages of disorganization in hyphal tips of Pythium ultimum var. sporangiiferum, including alteration (proliferation, retraction, and disruption) of the plasma membrane, vacuolization, and cell content disintegration. The implications of these results for the efficacy and consistency of biological control of plant-pathogenic Pythium spp. by 2,4-DAPG-producing Pseudomonas spp. are discussed.

Additional keywords: microbial ecology, soilborne pathogens, ultra-structure.