Plant Science 156 2000 185 – 192
Comparison of the phytotoxic activity of the phytotoxin destruxin B and four natural analogs
M.S.C. Pedras , C.J. Biesenthal, I.L. Zaharia
Department of Chemistry, Uni6ersity of Saskatchewan,
110
Science Place, Saskatoon, Sask., Canada S
7
N
5
C
9
Received 1 December 1999; received in revised form 6 March 2000; accepted 6 March 2000
Abstract
A quantitative bioassay utilizing staining of plant cell suspension cultures of Sinapis alba was employed to establish a structure-phytotoxic activity correlation among destruxin B, homodestruxin B, and desmethyldestruxin B, toxins produced by
Alternaria brassicae Berk. Sacc., the causative agent of Alternaria blackspot of brassicas. In addition, the phytotoxicity of destruxin B, homodestruxin B, and their respective metabolites hydroxydestruxin B and hydroxyhomodestruxin B were tested on
resistant and susceptible plant species utilizing in planta leaf assays and leaf uptake of toxin solutions. Overall, the results obtained from punctured leaf and cell staining assays indicated that homodestruxin B EC
50
3 × 10
− 4
M was the most toxic of the five compounds, followed by destruxin B EC
50
5 × 10
− 4
M, and desmethyldestruxin B EC
50
5 × 10
− 4
M. On the other hand, the hydroxylated destruxins hydroxydestruxin B EC
50
5 × 10
− 4
M were significantly less phytotoxic than the parent toxins. © 2000 Elsevier Science Ireland Ltd. All rights reserved.
Keywords
:
Alternaria blackspot; Alternaria brassicae; Desmethyldestruxin B; Destruxin B; Homodestruxin B; Host-selective; Hydroxydestruxin B; Hydroxyhomodestruxin B; Phytotoxicity; Phytotoxins
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1. Introduction
We have been evaluating the role of host-selec- tive toxins as part of a research program aimed at
understanding the chemical and biochemical basis of plant disease resistance [1]. Host-selective toxins
are, by definition, fungal or bacterial secondary metabolites toxic to plants that host the pathogen
but have lower phytotoxicity on non-host plants [2].
The phytopathogen
Alternaria brassicae
Berk. Sacc.,
causative agent
of Alternaria
blackspot disease, is one of the most destructive fungal pathogens [3] of the economically impor-
tant oilseeds rapeseed and canola Brassica napus and Brassica rapa and brown mustard Brassica
juncea. Because both destruxin B and homode- struxin B are toxins produced both in vitro [4,5]
and in planta [6] by A. brassicae Berk. Sacc., it was of great interest to establish their effect and
fate on resistant and susceptible plant tissues. To- wards this end, the metabolism of the host-selec-
tive toxins destruxin B 1 and homodestruxin B 2 by plants resistant and susceptible to Al-
ternaria blackspot was recently investigated using synthetic radiolabeled toxins [7]. It was established
that those toxins were transformed into the respec- tive hydroxydestruxin B 3 and hydroxyhomode-
struxin B 4. Importantly, the rate of the metabolic transformation correlated with the
plant’s disease resistance, i.e. significantly faster rates were observed for plants resistant to A.
brassicae. Furthermore, preliminary bioassay data indicated that both hydroxydestruxins 3 and 4
were less toxic caused smaller lesions on leaves to disease susceptible species such as canola than
Abbre6iations
:
BAP, 6-Benzylaminopurine; 2,4-D, 2,4-Dichlorophe- noxyacetic acid; EC, Effective concentration; HPLC, High pressure
liquid chromatography; HRMS, High resolution mass spectrometry; NAA, Naphthaleneacetic acid; NMR, Nuclear magnetic resonance;
TLC, Thin layer chromatography. Corresponding author. Tel.: + 1-306-9664772; fax: + 1-306-
9664730. E-mail address
:
pedrassask.usask.ca M.S.C. Pedras. 0168-945200 - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 1 6 8 - 9 4 5 2 0 0 0 0 2 5 3 - 3
destruxin B 1 and homodestruxin B 2 [7]. Sub- sequently, to determine the usefulness and poten-
tial application of these metabolic studies, it was essential to establish the phytotoxicity of destrux-
ins 1 – 4 Fig. 1. Although the phytotoxic activity of destruxin B and homodestruxin B was previ-
ously evaluated, there is no quantitative evaluation of their toxicity to the disease resistant species
Sinapis alba.
Consequently, we have developed a quantitative bioassay utilizing the staining of plant cell suspen-
sion cultures of S. alba to establish a structure- phytotoxic activity correlation among destruxin B
1, homodestruxin B 2, and desmethyldestruxin B 5 Fig. 1, a presumed toxin produced also by
