Quality traits. Pre-harvest sprouting tolerance (PHS)

Contributed by Mark Sorrells (mes12@cornell.edu)

Genes and QTLs involved in PHS tolerance

There are many QTLs that have been reported to be involved in PHS tolerance. Anderson et al. (1) found eight regions related to PHS tolerance in white winter wheat populations that were obtained from the crosses: NY6432-18 (NY18) x "Clark's Cream"(CC) and NY6432-18 x NY6432-10. One of these QTLs is located on chromosome arm 1BS linked to locus Xbcd1434 and was initially the main source of PHS resistance for the IFAFS project; however, recent mapping results (Sorrells, unpublished) in a new DH population from a cross between Cayuga and Caledonia has revealed a QTL with larger effects in the cdo64 region. That region was initially less important because the marker was too far away from the QTL. Recently, a microsatellite marker, XBarc055 was found to be very close to the QTL and explains at least 32% of the variation over 3 years and 8 environments. Cayuga is a backcross-derived line with PHS resistance from Clark's Cream.

Lawson et al (3) working with Australian backgrounds showed that in two recombinant inbred populations, PHS was under a simple genetic control involving two genes, one of which was different from, but closely linked to, the locus responsible for red-color kernels.  In a third population the control of PHS was more complex. Mares and Mrva (9), also working with materials from Australia, identified 3 QTLs for grain dormancy on chromosome arms 2AL, 2DL, and 4AL. Kato et al. (7) identified three QTLs controlling seed dormancy on group 4 chromosomes of wheat in a population derived from the cross between AC Domain, a Canadian red-grain wheat with a high level of grain dormancy, and Haruyutaka, a Japanese red-grained wheat with low levels of dormancy. The major QTL was located on chromosome arm 4AL, between Xcdo795 and Xpsr115 and could be the same QTL that Mares and Mrva (9) found in Australian lines and that Anderson et al. (1) found in the NY18 x CC population. Comparative mapping between wheat and barley suggests that a homologous relationship exists between this QTL on 4AL and the barley gene, SD4 which is one of the loci involved in dormancy control in that cereal. The other two minor QTLs that Kato et al. found were located in the terminal regions of 4BL and 4DL.

Roy et al. (4) and Varshney et al. (8) found and mapped two loci related to PHS tolerance on chromosome arms 6BS and 7DL in a mapping population developed from the cross SPR8198 x HD2329. SPR8198 is a red-kernel wheat tolerant to PHS, while HD2329, a widely used cultivar in India, has white kernels and is susceptible to PHS.

Groos et al. (10) analyzed 194 recombinant inbred lines from the cross between cultivars "Renan" (red kernels and resistant to PHS) and "Récital" (white kernels and susceptible) and mapped QTLs for PHS tolerance and color independently. Three out of four QTLs for PHS tolerance overlapped with QTLs responsible for the red color of grains. They were located on the long arm of chromosome group 3. The fourth QTL was located on chromosome arm 5AS.

Bailey et al (5) mapped wheat and rice loci that were believed to be orthologous to the maize Vp1 genes. The product of Vp1 in maize is a transcription factor related to vivipary in maizel. The Vp1 loci of wheat are located 30 cM away from the R loci that control coat-imposed dormancy.

A common feature of many of the QTLs described is the variability they show across genetic backgrounds and environments. Thus, it is important to validate these QTLs before using them for breeding.