Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 253

will provide resistance to the race UG99 the long-term strategy should focus on
rebuilding the “ Sr2 complex” to achieve long-term durability. The complex to be
built will involve an assemblage of slow rusting gene Sr2 with other unknown ad-
ditive genes of similar nature (Singh et al. 2006 ). Addressing the target swiftly has
been considered very crucial as migratory paths present a gloomy picture for wheat
production should adequate resistance not be incorporated in regional wheat variet-
ies (Hodson et al. 2005 , Reynolds and Borlaug 2006 ). To add to the swiftness would
be efficient tools (Mujeeb-Kazi et al. 2006b ; Randhawa et al. 2009 ) as an integral
means to drive the gene transfers (Mago et al. 2005 ) and give allelic output stability
(Mujeeb-Kazi 2003 , 2005 , 2006 ). The allelic diversity from unique genetic resourc-
es will also be a significant aid (Coghlan 2006 ; Rizwan et al. 2007 ; Simonite 2006 ).
Hence, the availability of broad-based genetic variability is a pre-requisite for
having a sound and successful wheat improvement program. Genomic diversity is
one unique option available and the maximum ease that permits exploitation of this
resource comes from the D and A genome diploids of the primary Triticeae gene
pool that have generated via pre-breeding the synthetic hexaploid germplasm (Mu-
jeeb-Kazi 2003 , 2006 ). Synthetic hexaploids created by crossing Triticum turgidum
with Aegilops tauschii tap the desirable genes present in the wild D genome dip-
loid species (Trethowan and Mujeeb-Kazi 2008 ; Trethowan and Van-Ginkel 2009 ).
These synthetic hexaploid wheats have been used as an intermediary for transfer-
ring resistance genes from the wild D genome ancestor to cultivated wheat. As both
synthetic hexaploid and bread wheat varieties have the same genomic constitution
with perfect homology they can be readily inter-crossed.
Several varieties with UG99 and its lineage lines have been released in various
countries based upon data gathered from Kenya and Ethiopia (Joshi et al. 2011 )
and as early as 2006 in CIMMYT a targeted program to increase yield and pos-
sess stem and yellow rust resistance got actively moving with its superior products
obtained as reported by Singh et al. ( 2011b ). It was encouraging to see that in the
various promising high yielding and resistant lines identified a significant number
had unique genetic resources in their pedigrees that included several of Ae. squar-
rosa (Syn. Ae. tauschii ) and Thinopyrum acutum . Almost all possessed APR genes
and others that contributed. The contribution to yield from alien resources has been
well demonstrated by the varietal release in Sichuan, China of Chaunmai 42 that
reported a yield enhancement of 22.7 % over the earlier cultivar Chaunmai 107
(Yang et al. 2009 ).
In initial stages has been the contribution with basic research aspects where sig-
nificant contributions related to UG99 pathotypes resistance has been reported. This
has come through screening of various accessions of the AA diploid progenitor
wheats (Rouse and Jin 2011 ), the D genome Ae. tauschii accessions (Rouse et al.
2011 ) and the tertiary gene pool diploid Th. bessarabicum promise (Xu et al. 2009 ).
All the above have opened up avenues for global researchers to embark on vola-
tile “pre-breeding” programs where from the closely related forms of the AA and
DD resource pay offs will be swifter while those from the E b E b genome transfers
more time consuming due to the genetic distance of that diploid resource but still
needed as translocations from this source may be additive to what has recently been