Chemical structure of cichorinotoxin, a cyclic lipodepsipeptide that is produced by Pseudomonas cichorii and causes varnish spots on lettuce.

Hidekazu Komatsu,Tsutomu Hoshino,Takashi Shirakawa,Takeo Uchiyama

doi:10.3762/bjoc.15.27

Abstract

Pseudomonas cichorii, which causes varnish spots on lettuce and seriously damages lettuce production during the summer season in the highland areas of Japan (e.g., Nagano and Iwate prefectures) was isolated. The structure of a toxin produced by this organism was analyzed based on the detailed evaluation of its 2D NMR and FABMS spectra, and this compound has not been reported previously. We propose the name cichorinotoxin for this toxin. In conjunction with the D or L configurations of each amino acid, which were determined by Marfey’s method, we propose the structure of cichorinotoxin to be as follows: 3-hydroxydecanoyl-(Z)-dhThr1-D-Pro2-D-Ala3-D-Ala4-D-Ala5-D-Val6-D-Ala7-(Z)-dhThr8-Ala9-Val10-D-Ile11-Ser12-Ala13-Val14-Ala15-Val16-(Z)-dhThr17-D-alloThr18-Ala19-L-Dab20-Ser21-Val22, and an ester linkage is present between D-alloThr18 and Val22 (dhThr: 2-aminobut-2-enoic acid; Dab: 2,4-diaminobutanoic acid). Thus, the toxin is a lipodepsipeptide with 22 amino acids. The mono- and tetraacetate derivatives and two alkaline hydrolysates, compounds A and B, were prepared. We discuss here the structure–activity relationships between the derivatives and their necrotic activities toward lettuce.

Highlights

Pseudomonas cichorii causes varnish spots on lettuce
Two types of cyclic lipodepsipeptide phytotoxins are well known: relatively lower molecular weight compounds composed of 9 amino acids, such as those found in syringostatin [9], syringomycin [10,11] and syringotoxin [12], and higher molecular weight compounds consisting of 22 or 25 amino acids, which have been found in syringopeptins [13]
FABMS showed signals at m/z 2069 [M + H]+ and m/z 2070, and signals at m/z 2067 [M − H]+ and m/z 2068 were observed in the negative mode (Supporting Information File 1, Figure S2). This finding demonstrates that the molecular ion of cichorinotoxin (M+) is m/z 2068

Summary

Introduction

Pseudomonas cichorii causes varnish spots on lettuce. Varnish spots, called midrib rot or bacterial rot [1,2,3,4], are dark brown and can induce necrotic lesions [1,2,3,4,5,6,7].

Results

Conclusion