Aphids are key pests in many crops, damaging plants both directly by feeding and indirectly by transmitting plant viruses. Efforts to breed for resistance have resulted in cultivars with resistance to Russian wheat aphid (RWA; Diuraphis noxia
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 AHA AG H. v. vulgare H. v. spontaneum Breeding lines Significantly lower acceptance to aphids after Alva - induction
Lina H.sp.5 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
Fig. 3 Aphid growth rate relative to the control Lina (AG) and aphid host acceptance (AHA) of Alva- exposed plants relative to air-exposed plants are correlated (rs¼ 0.48) and two out of three breeding
lines (triangles) are similar to the BC parent Lina in AHA. Modified from Ninkovic and A˚ hman (2009)
(Mordvilko)), greenbug (Schizaphis graminum (Rondani)) (Berzonsky et al.2003), lettuce aphid (Nasonovia ribisnigri (Mosley)) (Liu and McCreight 2006) and soybean aphid (Aphis glycines Matsura) (Li et al.2007). WithR. padi it has been more difficult to reach such a goal.R. padi is a pest of cereals in cold temperate regions, causing both direct plant damage and secondary damage as a vector of barley yellow dwarf virus (Blackman and Eastop1984). It has a wide range of hosts among grasses. Since this aphid does not cause visible plant symptoms in cereals, as RWA and greenbug do, selection for resistance is more difficult and time- consuming. Some attempts to breed for resistance to R. padi have been made (Weibull1994; A˚ hman et al.2000), and in an ongoing effort to breed for resistance to R. padi in barley, the selection method is based on measurements of aphid growth, using a wild barley (H. vulgare spp. spontaneum) accession as resistance source. Nymphal growth on this wild barley is approximately half of that on cultivated barley (Delp et al. 2009). Successive generations of back-crossing (BC) and selection for reduced aphid growth have resulted in barley lines with lower resistance levels, but still with significantly lower aphid growth rate than on the susceptible BC-parent (Ninkovic and A˚ hman 2009). Quantitative trait loci (QTL) analysis of the F1-population of the initial cultivar x wild barley cross revealed a QTL explaining ca. 20% of the variation in aphid growth rates (Louise O’Donoughue, 1994, personal communication). A marker for this QTL is now used as a primary selection criterion; only lines carrying the marker are further tested for aphid growth rate.
In most of the BC, the recurrent parent was the cultivar Lina. Coincidently, this parent is amenable to induction by certain barley volatiles such as those from cvs. Alva and Barke (Table1), and two out of three tested resistant BC-lines carrying Lina-genome responded with significantly reduced acceptance toR. padi after Alva- induction (Ninkovic and A˚ hman2009), indicating that the trait is heritable (Fig.3). Even though the allelobiosis-induced type of resistance from Lina and the type of resistance from wild barley was incidentally combined in some of the breeding lines, it is highly desirable to be able to combine different types of resistance traits with the allelobiosis type in a more controlled manner. There are several reasons to strive for multigenic resistance to aphids. One is that strong monogenic resistance to aphids such as RWA, greenbug (Berzonsky et al.2003) and lettuce aphid has been rapidly overcome. Another is that only a moderate level of resistance toR. padi has been identified in barley that is possible to use in crosses (Weibull 1987). Thus it is probably necessary to combine more than one resistance factor in order to obtain efficient, durable resistance to this aphid. This encourages further efforts to exploit allelobiosis in the ongoing breeding programme for resistance toR. padi.
However, since it would be even more time consuming to screen specifically for the allelobiosis type of resistance than screening with the aphid growth test, it is of utmost importance that molecular tools are developed as understanding of the mechanisms of allelobios is increases. In a QTL-test using a breeding population with variation in the aphid growth trait but lacking the previously mentioned QTL marker, a further QTL was detected on a chromosome segment originating from the parent Lina (Cheung et al.2010). One possibility is that this chromosomal region is important for the
allelobiosis responder function in that cultivar. Currently, a microarray analysis of cv. Lina exposed to cv. Alva is followed up in various ways, to narrow down the number of candidate genes (Karpinska, 2009, personal communication).
Monoculture is the norm for growing crops, whereas screens for resistance to pests, as well as to diseases, in breeding programmes are normally carried out with mixtures of many different plant genotypes. Thus there is a risk for induced effects via allelobiosis in the screenings, which are then not realised in monoculture in the field (Ninkovic and A˚ hman2009). On the other hand, knowledge about allelobiosis suggests that it may be favourable to grow combinations of cultivars known to induce resistance to a key pest, and enhance attraction of its natural enemies (Chapter ‘Volatile Chemical Interaction Between Undamaged Plants: Effects at Higher Trophic Levels’). Although allelobiotic responding ability is rarer among more modern than among older cultivars (Martin Kellner, 2009, personal commu- nication), it does exist in modern ones (Table1), which would enable immediate adoption. Part of the seed industry in Sweden is now using the opportunity to have their marketed barley varieties screened for allelobiotic abilities in our laboratory and subsequently tested as cultivar mixtures in field tests for pest abundance, yield and other agronomic characteristics. Further, if allelobiosis-eliciting volatile blends become available as formulations, it will also be possible to grow a responding cultivar in monoculture and apply these formulations, with the timing based on forecasts of pest attack.