Host selection

The Involvement of Allelochemicals in the Host Selection of Parasitic Angiosperms... 

LYNN Parasitic Angiosperms Allelochemicals and Host Selection... 

While the activity and the structural specificity of the xenognosins were very encouraging, Astragalus gummifer is native to the Middle East and would not be expected to have co-evolved as a host for the southeastern U.S. native, Agalinis purpurea. The questions of whether molecules of this type were actually exuded from host roots in sufficient quantities to constitute host selection still remained. For that reason, John Steffens switched his attention to Lespedeza... 

Konstantopoulou M A, Krokos F D and Mazomenos B E (2002), Chemical stimuli from corn plants affect host selection and oviposition behavior of Sesamia nonagrioides (Lepidoptera Noctuidae) , J Econ Entomol, 95, 1289-1293. 

Host-guest inclusion complexes, 262—263 antibiotic hosts, 231—233 cahxarene hosts, 228—231 chiral crown ether hosts, 213—218 cyclic oligosaccharide hosts, 218—222 cyclodextrin host selectivities, 223/ host molecular size, 221 hnear ohgosaccharide hosts, 222—228 ir- TT stacking interactions, 217 proteic hosts, 231 Human 15-hpoxygenase, 52/... 

The DNA or cDNA library is then introduced into a preparation of bacterial host cells. Usually, the first host selected is a laboratory strain of E. coli which has been grown and pretreated with inorganic salts to make uptake of DNA easier. The ability to take up foreign DNA is called competence, cells which have been specially prepared for the purpose are called competent cells. Other methods to transfer DNA into cells include electroporation (application of an external electric field to permeabUize the cell wall), transfection (where a recombinant bacterial virus is used to transfer the DNA to the target cell) or ballistic methods (by using DNA-coated particle projectiles). The last method has been used to introduce foreign DNA into plant cells and mammalian cells. 

A recent review of the metabolites of L. maculans and L biglobosa produced in diverse culture conditions [19] emphasized that both species biosynthesize host-selective and non-selective phytotoxins. Importantly, it was shown that the composition of metabolite profiles of L. maculans depended on the composition of the culture medium. In a chemically defined liquid medium, isolates virulent on canola produced mainly sirodesmin PL (1), a non-host-selective phytotoxin, minor sirodesmins with one, three, or four sulfurs bridging the dioxopiperazine ring (sirodesmin H (3) [20], sirodesmin J (4) and K (5) [21]) and phomalirazine (6) (Fig. 9.1). The various sirodesmins 1-5 and phomalirazine (6) caused necrotic lesions of different intensities on leaves of both resistant and susceptible plants. Phomalide (7), the first host-selective phytotoxin isolated from virulent isolates of L. maculans, caused disease symptoms (necrotic, chlorotic, and reddish lesions) on canola (susceptible to L maculans) but not on brown mustard or white mustard... 

Fig. 9.1 Chemical structures of non-selective phytotoxins 1-6 and host-selective phytotoxin 7 produced by canola virulent isolates of Leptosphaeria maculans. Phytotoxins 1-7 are produced in a chemically defined medium...

L. maculans isolates Laird 2 and Mayfair 2 (virulent on brown mustard but not on canola) produced in a chemically defined medium the host-selective phytotoxin depsilairdin (8) (Fig. 9.2), containing a novel amino acid residue ((25,35,45)-3,4-dihydroxy-3-methylprolyl) and a sesquiterpene moiety (lairdinol A, synthesized recently [25,26]). Depsilairdin (8) caused disease symptoms similar to those caused by the pathogen on brown mustard, that is, strong necrotic and chlorotic lesions, but no lesions on canola. 

The Alternaria black spot fungi A. brassicae and A. brassicicola produce host-selective toxins as well. While A. brassicicola produces brassicicolin A (11) as the major host-selective phytotoxin [28], A. brassicae produces destruxin B (12) (Fig. 9.4) [29]. Consistent with the virulence of these phytopathogens, both brassicicolin A (11) and destruxin B (12) appeared to be more phytotoxic to the susceptible cruciferous species B.juncea than to the tolerant B. napus. 

Pedras MSC Biesenthal CJ (1998) Production of the host-selective phytotoxin phomalide by isolates of Leptosphaeria maculans and its correlation with sirodesmin PL production. Can J Microbiol 44 547-553... 

Pedras MSC, Chumala PB, Quad JW (2004) Chemical mediators the remarkable structure and host-selectivity of depsUairdin, a sesquiterpenic depsipeptide containing anew amino acid. Org Lett 6 4615 617... 

Brosch G, Ransom R, Lechner T, Walton JD, Loidl P. (1995) Inhibition of maize histone deacetylases by HC Toxin, the host-selective toxin of cochliobolus carbonum. Plant Cell 7 1941-1950. 

Recently, we have found that pathogens with a restricted host range and unrelated to common crop pathogens do produce novel phytotoxins. These novel phytotoxins have both unusual chemical structures and surprising, in some cases unprecedented, bioiogical activities. We will summarize these studies below, beginning with the ieast selective phytotoxins and progressing to phytotoxins with some host selectivity. 

The parent quinone may well be the ultimate phytotoxin. Alteichin does not show host selectivity in either the whole leaf or protoplast assays. In all test plants it causes necrotic flecks in leaf puncture wounds (2.7 mM 2% ethanol)) within 12 h of application. 

Biologically, the most interesting aspect of (-)-dihydropyrenophorin is that it causes reddish lesions on Johnson grass at 10 6 10 7, and 10"8 M wherein no other plant species tested shows any sensitivity whatever at these concentrations. Thus, it would appear that VI is host selective. To our knowledge Johnson grass is not a host of D. avenae. 

Nevertheless, we are encouraged by the findings on bipolaroxin (18) to believe that host-selective compounds do exist. 

---