THE EFFECT OF A PHYTOTOXIN FROM ALTERNARIA BRASSICICOLA ON BRASSICA POLLEN

Imbalanced lignin biosynthesis promotes the sexual reproduction of homothallic oomycete pathogens

Lignin is incorporated into plant cell walls to maintain plant architecture and to ensure long-distance water transport. Lignin composition affects the industrial value of plant material for forage, wood and paper production, and biofuel technologies. Industrial demands have resulted in an increase in the use of genetic engineering to modify lignified plant cell wall composition. However, the interaction of the resulting plants with the environment must be analyzed carefully to ensure that there are no undesirable side effects of lignin modification. We show here that Arabidopsis thaliana mutants with impaired 5-hydroxyguaiacyl O-methyltransferase (known as caffeate O-methyltransferase; COMT) function were more susceptible to various bacterial and fungal pathogens. Unexpectedly, asexual sporulation of the downy mildew pathogen, Hyaloperonospora arabidopsidis, was impaired on these mutants. Enhanced resistance to downy mildew was not correlated with increased plant defense responses in comt1 mutants but coincided with a higher frequency of oomycete sexual reproduction within mutant tissues. Comt1 mutants but not wild-type Arabidopsis accumulated soluble 2-O-5-hydroxyferuloyl-L-malate. The compound weakened mycelium vigor and promoted sexual oomycete reproduction when applied to a homothallic oomycete in vitro. These findings suggested that the accumulation of 2-O-5-hydroxyferuloyl-L-malate accounted for the observed comt1 mutant phenotypes during the interaction with H. arabidopsidis. Taken together, our study shows that an artificial downregulation of COMT can drastically alter the interaction of a plant with the biotic environment.

Sporophytic response to pollen selection for Alachlor tolerance in maize

In order to assess the efficiency of male gametophytic selection (MGS) for crop improvement, pollen selection for tolerance to herbicide was applied in maize. The experiment was designed to test the parallel reactivity to Alachlor of pollen and plants grown in controlled conditions or in the field, the response to pollen selection in the sporophytic progeny, the response to a second cycle of MGS, and the transmission of the selected trait to the following generations. The results demonstrated that pollen assay can be used to predict Alachlor tolerance under field conditions and to monitor the response to selection. A positive response to selection applied to pollen in the sporophytic progeny was obtained in diverse genetic backgrounds, indicating that the technique can be generally included in standard breeding programs; the analysis of the data produced in a second selection cycle indicated that the selected trait is maintained in the next generation.

Pollen selection

In recent years a large number of reports have been published hinting about the possible role of selection at the male gametophyte level in natural conditions and its possible use as a tool in plant breeding programs. In this work, the evidence that has so far emerged on gametophytic selection is reviewed, and its relationship to pollen competition under natural conditions is analyzed. Information supporting an overlap in the genetic expression between the gametophytic and sporophytic phases is also reported, mainly in terms of isozymes and RNA hybridization. The correspondence between both phases is reviewed through their common response to different external agents such as temperature, salinity, metals, herbicides, fungal toxins, and other factors. Finally, the implications that pollen selection may have in plant breeding are evaluated.

Effects of Alternaria alternata f.sp. lycopersici toxins on pollen

Effects of the phytotoxic compounds (AAL-toxins) isolated from cell-free culture filtrates of Alternaria alternata f.sp. lycopersici on in vitro pollen development were studied. AAL-toxins inhibited both germination and tube growth of pollen from several Lycopersicon genotypes. Pollen from susceptible genotypes, however, was more sensitive for AAL-toxins than pollen from resistant plants, while pollen of species not belonging to the host range of the fungus was not significantly affected by the tested toxin concentrations. AAL-toxins elicit symptoms in detached leaf bioassays indistinguishable from those observed on leaves of fungal infected tomato plants, and toxins play a major role in the pathogenesis. Apparently, pathogenesis-related processes and mechanisms involved in disease resistance are expressed in both vegetative and generative tissues. This overlap in gene expression between the sporophytic and gametophytic level of a plant may be advantageously utilized in plant breeding programmes. Pollen may be used to distinguish susceptible and resistant plants and to select for resistances and tolerances against phytotoxins and other selective agents.

Kanamycin resistance during in vitro development of pollen from transgenic tomato plants

Effects of kanamycin on pollen germination and tube growth of pollen from non-transformed plants and from transgenic tomato plants containing a chimaeric kanamycin resistance gene were determined. Germination of pollen was not affected by the addition of kanamycin to the medium in both genotypes. Kanamycin, however, severely affected tube growth of pollen from non-transformed plants, while pollen from plants containing the chimaeric gene were less sensitive and produced significantly longer tubes at kanamycin concentrations between 200-400 mgl-1. Apparently, this resistance for kanamycin correlates with the expression of the chimaeric gene during male gametophytic development.

TOWARDS THE SELECTION IN VITRO FOR RESISTANCE TO ALTERNARIA BRASSICICOLA (SCHW.) WILTS., IN BRASSICA NAPUS SSP. OLEIFERA (METZG.) SINSK., WINTER OILSEED RAPE

Culture filtrates of Alternaria brassicicola (Schw.) Wilts, were found to be toxic to secondary embryoids of Brassica napus ssp. oleifera (Metzg.) Sinsk. The culture filtrates were partially purified and added to tissue culture media to produce a selection medium. Different secondary embryoid lines of B. napus were found to differ in their sensitivity to the selection medium, and it was possible to select lines which showed increased resistance to the selection medium. Some plants that were regenerated from secondary embryoids were found to be more resistant to the pathogen than were seed-grown plants of the same cultivar, although there was no correlation between sensitivity to the selection medium and susceptibility to the pathogen.