Resistance to Maize Dwarf Mosaic and the Corn Virus Disease Complex in Synthetic Populations of Dent Corn and Sweetcorn

Phenotypic recurrent selection for resistance in corn (Zea mays L.) to maize dwarf mosaic (MDM) and the com virus disease complex was conducted for five cycles in synthetic populations of dent com and sweet com. Selection was carried out concurrently at two locations under a natural epiphytotic of the com virus disease complex of MDMV and maize chlorotic dwarf virus (MCDV) near Waverly, Tennessee and under an artificially induced epiphytotic near Knoxville, Tennessee created by mechanical inoculation with MDMV and transplanting of infected host plants. Resistant plants were selected and interpollinated concurrently and only apparently symptomless plants were harvested. It was of interest to determine the effect of successive cycles of selection as well as the effect of selection under different disease conditions at two locations. Each cycle of selection at both locations in both synthetics was evaluated for virus reaction by determining the number of diseased plants and the severity of infection of the diseased plants. Host reaction to virus infection is largely quantitative, and genotypes with the same percentage of diseased plants may still vary in resistance because of differences in the severity of infection. Evaluation of the dent populations showed no improvement for virus reaction from C0 to C4 at either location. The Waverly selections had significantly fewer diseased plants than the Knoxville selections at an early rating representing MDMV infection. Evaluation of 100 S1 random selections from the C0 and C3 dent populations showed greater variability for virus reaction in C3. Because hybrid vigor seems to enhance virus tolerance in susceptible and resis-tant genotypes, it may be that the variability of C3 was due to increasing the number of resistant selections and to greater expression of virus reaction in the susceptible selections due to inbreeding depression. Selection in the sweet com synthetic resulted in reduction in the number and severity of diseased plants at both locations when evaluated at Waverly. The Waverly selections were more resistant than the Knoxville selections. Lack of response to selection in the dent synthetic may have been due to reduction in genetic variability for resistance, inbreeding depression, changes in disease pressure during cycles of selection, or to the inability to identify S0 plants with high gene frequencies for resistance because of the confounding effects of hybrid vigor and virus resistance in heterozygous plants. Selection among S0 individuals may have favored heterozygous genotypes and maintained undesirable alleles at higher frequencies than expected

Comparative in situ analyses of cell wall matrix polysaccharide dynamics in developing rice and wheat grain

Cell wall polysaccharides of wheat and rice endosperm are an important source of dietary fibre. Monoclonal antibodies specific to cell wall polysaccharides were used to determine polysaccharide dynamics during the development of both wheat and rice grain. Wheat and rice grain present near synchronous developmental processes and significantly different endosperm cell wall compositions, allowing the localisation of these polysaccharides to be related to developmental changes. Arabinoxylan (AX) and mixed-linkage glucan (MLG) have analogous cellular locations in both species, with deposition of AX and MLG coinciding with the start of grain filling. A glucuronoxylan (GUX) epitope was detected in rice, but not wheat endosperm cell walls. Callose has been reported to be associated with the formation of cell wall outgrowths during endosperm cellularisation and xyloglucan is here shown to be a component of these anticlinal extensions, occurring transiently in both species. Pectic homogalacturonan (HG) was abundant in cell walls of maternal tissues of wheat and rice grain, but only detected in endosperm cell walls of rice in an unesterified HG form. A rhamnogalacturonan-I (RG-I) backbone epitope was observed to be temporally regulated in both species, detected in endosperm cell walls from 12 DAA in rice and 20 DAA in wheat grain. Detection of the LM5 galactan epitope showed a clear distinction between wheat and rice, being detected at the earliest stages of development in rice endosperm cell walls, but not detected in wheat endosperm cell walls, only in maternal tissues. In contrast, the LM6 arabinan epitope was detected in both species around 8 DAA and was transient in wheat grain, but persisted in rice until maturity