THE UNIVERSITY OF ADELAIDE
Grain Biochemistry Group, Waite Campus, Plant Science, Glen Osmond, SA 506, Australia.
Daryl Mares, Kolumbina Mrva, Robert Asenstorfer, Imelda Soriano,
Judith Rathjen, and Michael Quinn.
A highly significant QTL on chromosome 4A was associated with dormancy in three wheat genotypes, AUS1408, SW95-50213, and a dormant single gene red genotype, AUS1490, of diverse origin. Additional SSR markers, gwm269, and barc170, located near the center of the QTL have recently been identified and should provide near-diagnostic tools for MAS. The phenotype of lines containing the 4A alleles from the dormant parent varied from dormant to intermediate dormant with both the range and absolute values dependent on temperature during grain ripening. As temperature during ripening increased, dormancy decreased, and the range for lines containing the 4A dormancy alleles increased. A doubled-haploid population, dormant x intermediate dormant, that is fixed for the 4A dormancy allele but varies with respect to putative additional dormancy genes, has now been phenotyped and currently is being genotyped using markers specific for a number of chromosome regions previously reported to be associated with grain dormancy.
LMA is a genetic defect that can give rise to high grain a-amylase activity, low falling number (typically 200-300 sec but in some instances < 200 sec.), in the absence of sprouting and depending on the environmental conditions during the middle stages of grain filling. The defect is present at low levels in Australian wheat breeding programs and has been identified in genotypes from the U.K., Japan, China, South Africa, Mexico (CIMMYT) and the U.S. (California) and in primary and derived synthetic wheats. Once introduced, the defect is very difficult to eliminate from breeding programs.
Our current work focuses on populations involving different sources of LMA, the underlying biochemical mechanisms involved, the duration and temperature differential required for maximal expression, and a comparison of LMA expression in genotypes with semidwarfing genes such as Rht8 that do not depend on insensitivity to GA. Earlier work indicated that the expression of LMA was reduced in the presence of Rht1 or Rht2 and almost completely inhibited in the presence of Rht1 + Rht2 or Rht3, suggesting that GA was involved in this genetic defect. A recent study comparing patterns of cell death in the aleurone of LMA affected and germinating grains with GA-treated aleurone provided further evidence. Cell death was absent in control genotypes but pockets of dead or dying cells were observed distributed at random through the aleurone of LMA-affected grains but were concentrated near the scutellum in germinating grains.
Polyphenol oxidase (PPO). Australian cultivars vary from high (unacceptable for alkaline noodles due to excessive darkening) to low in benchmark cultivars such as Sunco. Recently, a bread wheat genotype with PPO levels significantly less than Sunco and number of primary synthetic wheats with zero PPO have been identified. These appear to provide incremental improvements in color stability (i.e., reduced rate of darkening). In the absence of PPO, there is still significant darkening contributed by non-PPO enzymic activity. Synthetic derived zero PPO has been backcrossed into locally adapted, semidwarf backgrounds and separated from undesirable agronomic traits such as adhering glumes.
Flavonoids. Water and 0.1 M hydroxylamine extract compounds
from whole meal or flour that are colorless at neutral pH but
that turn yellow at higher pH (e.g., as in yellow alkaline noodles,
YAN). The germ tissues contain both the free form and phenolic
esters of apigenin-C-diglycosides that contribute to the total
yellow color of YAN, whereas the seed coat or bran contains other
phenolic compounds that have a minor role. Unfortunately, the
endogenous hydroquinone precursor of these latter compounds reacts
with hydroxylamine to give a product that has a high color yield
at alkaline pH. As a result, the total color of hydroxylamine
extracts cannot be used to give an accurate measure of genetic
variation or to predict YAN color.
Lipoxygenase (LOX). This enzyme is implicated in the degradation
of yellow xanthophyll pigments in wheat-based end-products. Variation
in both 'Sunco/Tasman' and 'Opata/Synthetic' was associated with
a QTL on chromosome 4B located close to the centromere and in
the case of 'Sunco/Tasman', Rht1. A number of other populations
involving low and high LOX genotypes have now been phenotyped
and will be used to validate the 4B QTL. Many durum wheats have
very low LOX and although low LOX is not common in bread wheats,
very low or near-zero LOX synthetic wheats have now been identified.