Mechanism of Russian wheat aphid resistance.
A.J. van der Westhuizen, X-M. Qian, and A-M. Botha.
The objective of this study is to gain information
on the mechanism of resistance that may contribute to the identification
of resistance markers and resistance genes.
Resistance is not found to be caused by a constitutive
factor, but involves a hypersensitive-type reaction elicited by
feeding aphids. The resistance mechanism is induced at a higher
level in resistant than susceptible wheat. Indications of the
hypersensitive reaction were increased respiration rate; accumulation
of phenolic compounds, which may include phytoalexins; and development
of necrotic spots. Specific intercellular proteins accumulated
in resistant wheat only in reaction to aphid infestation. Some
of them were related immunologically to pathogenesisrelated
proteins. Selective higher increases in chitinase, ß-1,
3-glucanase, and peroxidase activities, were found in resistant
wheat within 48 hours after aphid infestation. These increased
activities were due to increased de novo synthesis of these enzymes
and may serve as quantitative measures of resistance. The injurious
effect of Russian wheat aphid infestation on the chloroplast protein
Rubisco may be a major contributor to the poor survival abilities
of susceptible cultivars.
Although the accumulation of chitinases is selective,
ß-1,3-glucanase and peroxidases or their increased activities
may serve as biochemical markers of resistance. However, their
use would not be substantially timesaving, because plants
must be in an early 3-leaf stage and the physical symptoms usually
used for selection follow soon afterwards. Regarding the identification
of biochemical markers of resistance, our research now concentrates
on germlings (24ñ48 h old). Except in the case of unrelated
markers to defense, the resistance mechanism has to be elicited
first before accumulation of specific proteins or increased enzyme
activities could serve as markers. Several potential chemical
elicitors are being tested for selective induction of defense
related enzyme activities. The intercellular washing fluids of
infested resistant wheat were found to contain a selective elicitor.
Studies are continuing to determine its nature. The eliciting
effects of different chitin isomers have been tested on plantlets
and found to be nonspecific. Future studies will include key
enzymes of the phenolic synthesis pathway.
These research programs are financially supported
by the Wheat Board in agreement with the Small Grain Institute,
Bethlehem.
UNIVERSITY OF THE ORANGE FREE STATEóDEPARTMENT OF PLANT PATHOLOGY
Bloemfontein, 9300, South Africa.
Z.A. Pretorius, F.J. Kloppers, and C.M. Bender.
The destructive potential of leaf and stem rust on
certain bread wheat cultivars currently grown in high-risk areas
were confirmed in an artificially inoculated field trial at Greytown
during 1995. In comparison with fungicide-treated control plots,
mean plot yield was reduced by 45 % (first planting date) and
24 % (second planting). The corresponding reductions in 1,000-kernel
weight were 33 % and 18 %, respectively. Leaf and stem rust reached
100S and 80S severity levels on susceptible cultivars.
Improvement of leaf rust resistance.
In cooperation with the Small Grain Institute of
the ARC, 1,377 lines derived from the crosses `Karee/Lr29',
`Karee/Lr34', `Palmiet/Lr29',
and `Palmiet/Lr34' were evaluated for leaf
rust reaction in the field. Seed of these lines was harvested
for preliminary yield testing in 1996. Lines homozygous for leaf
rust resistance currently are being selected from `Karee*6/Lr35'
and `Karee*6/Lr37' populations and will be
tested accordingly by the Small Grain Institute. Palmiet or Karee
with Lr21, Lr32, Lr35, Lr36, Lr41,
or Lr42 have been backcrossed four times to the recurrent
parent.
Characterization of resistance to wheat leaf rust.
Cornel Bender completed a Master's thesis
on Lr12 and Lr13 resistance during 1995. Four Thatcher
(Tc) F3 lines (Lr13/Lr12-3, Lr13/Lr12-9,
Lr13/Lr12-19, and Lr13/Lr12-40), homozygous
for both Lr13 and Lr12, were selected, and their
resistance was compared with that of the parents (CT263 [= TC
Lr13] and RL6011 [= TC Lr12]), the single gene lines
`TC/13-22' and `TC/Lr12-16',
and Thatcher. Quantification of aborted penetration showed that
inhibition of fungal growth in wheat lines containing Lr12
and/or Lr13 was activated to a certain degree before haustoria
were formed. Colony size showed that fungal colonies were generally
smaller in lines containing both Lr12 and Lr13 than
in the parents, but not necessarily smaller than those in the
monogenic line `Tc/Lr13-22'. Host cell necrosis
was associated more frequently with infection sites, specifically
of pathotype UVPrt2, in the combination lines than in the parents.
Hypersensitivity index values indicated that host cell necrosis
was more severe following infection of the combination lines with
UVPrt2. Quantification of cell wall appositions showed that fewer
papillae occurred in Thatcher than in the other host genotypes.
The number of haustoria observed per colony did not indicate
any clear, repeatable differences between lines. Flag leaf infection
types showed that Lr12 is effective to most pathotypes
of P. recondita f. sp. tritici occurring in South
Africa. Conversely, Lr13 is ineffective to the dominant
pathotypes. Ratings on primary and flag leaves, and the resistance
components of latent period, uredium density, and uredium size,
did not distinguish between the digenic lines and the most resistant
parent. In the absence of a pathotype virulent to both genes,
the combination lines were highly resistant in the field.
Personnel.
Following a research position, Dr. F.J. Kloppers
was appointed as senior lecturer in the department as of 1 January,
1996.
Publications.
Crous PW, Petrini O, Marais GF, Pretorius ZA, and
Rehder F. 1995. Occurrence of fungal endophytes in cultivars
of Triticum aestivum L. in South Africa. Mycoscience
36:105-111.
Kloppers FJ and Pretorius ZA. 1995. Histology of
infection and development of Puccinia recondita f. sp.
tritici in a wheat line with Lr37. J Phytopath
143:261-267.
Kloppers FJ, Pretorius ZA, and Van Lill D. 1995
. The influence of Lr29, Lr35 and Lr37 on leaf
rust severity, yield loss and quality characteristics in wheat.
SA J Plant and Soil 12:55-58.
Pretorius ZA Van Niekerk BD, Kloppers FJ, and Vorster
AL. 1995. Managing certain recently named Lr genes in
breeding wheat for resistance to Puccinia recondita f.
sp. tritici in South Africa. SA J Plant and Soil 12:32-37.
UNIVERSITY OF STELLENBOSCH
Department of Genetics, Stellenbosch 7600, South Africa.
G.F. Marais, R. Prins, A. Antonov, F.L. Middleton, H.S. Roux, and A.S. Marais.
Selection programs aimed at developing improved durum
cultivars for the lower Orange River irrigation areas and triticale
cultivars for the southwestern Cape were continued. No new releases
were made, but promising advanced lines with good quality were
selected.
An attempt to transfer leaf rust resistance genes
from Aegilops and Triticum species to common wheat
was continued. Resistance genes from 70 accessions were expressed
fully in hybrids, and these are now in various stages of backcrossing.
Twenty-nine lines, with deletions in the Lr19
(Indis) translocated chromosome segment, were used to physically
map a number of Thinopyrum loci. The relative positions
of the marker loci on the translocated segment were determined
as: Sd1, Xpsr165, Xpsr105, Xpsr129,
Lr19, Wsp-D1, Sr25, Y1, and Y2.
When compared to the map of the wheat group 7 chromosomes, the
data confirmed the reported homoeology between the Lr19 segment
and chromosome arm 7DL of wheat. Also, it is evident that the
Lr19 translocation in Indis is very similar to the Lr19
segment in the T4 source and that the former may not derive from
Thinopyrum distichum as was previously thought. The mutation
data suggest that Sd1 and Lr19 may not be single
loci. Polymorphisms produced by eight recombined forms (ph-induced)
of the Lr19 (Indis) translocations then were related to
the physical map of the region. At least five of the recombinants
proved to be double crossover products. Each has lost Sd1,
yet they are still associated with mild segregation distortion
and sometimes strong self-elimination. However, it is not clear
whether the segregation distortion results from additional Sd
loci or the altered chromosome structure following relocation
of the segments. Evidence was found that Lr19 resistance
may be determined by two loci. The successful transfer of a gene
for RWA resistance from `Imperial' rye to the 1BL·1RS
translocation prompted an attempt to replace the secalin locus
on this translocation with wheat chromatin. We attempted the
transfer in two ways: (i) by deleting the homoeologous pairing
inhibitor, Ph1b, locus (through the use of the ph1b
deletion mutant), and (ii) by creating plants that are nullisomic
for chromosome 3D (Ph2b absent), but trisomic or tetrasomic
3B (have extra doses of the 3B pairing promotors).
The RWA-resistance gene, Dn5, was found to
be located on chromosome arm 7DL (telosomic analysis). The gene
segregates independently from the centromere and appears to be
located at about 32 map units from the endopeptidase locus, Ep-D1.
A dominant gene for stem rust resistance (germplasm line 87M66-2-1),
which is derived from T. tauschii accession RL5289, was
located on chromosome arm 1DS. The locus appears to be separate
from, but linked to, Sr33.
A germplasm line, KS87VP9, which has a dominant
gene for male sterility, was obtained from the USDA, and the gene
was transferred through backcrossing to a well-adapted spring
wheat background. This then was used as one of the parents in
a multicross involving 16 wheat parents. The sterility gene was
found to work very well, giving regular and complete male sterility
to 50 % of the progeny. A procedure was worked out whereby large
numbers of selections can be intercrossed with comparative ease,
making use of the sterility gene. The multicross F1 will be planted
in the field in the coming season and will serve as the starting
material for a recurrent selection procedure aimed at improvements
in yield, disease resistance, and quality. The aim is to eventually
incorporate and evaluate the procedure as part of a pedigree-
selection system.
Publication.
Du Toit F, Wessels WG, and Marais GF. 1995. The
chromosome arm location of the Russian wheat aphid resistance
gene, Dn5. Cereal Res Commun 23(1-2):15-17.