The bird cherry-oat aphid (Rhopalosiphum padi L.) is an important pest on cereals causing plant growth reduction without specific leaf symptoms. Breeding of barley (Hordeum vulgare L.) for R. padi resistance shows that there are several resistance genes, reducing aphid growth. To identify candidate sequences for resistance-related genes, we performed microarray analysis of gene expression after aphid infestation in two susceptible and two partially resistant barley genotypes. One of the four lines is a descendant of two of the other genotypes. There were large differences in gene induction between the four lines, indicating substantial variation in response even between closely related genotypes. Genes induced in aphid-infested tissue were mainly related to defence, primary metabolism and signalling. Only 24 genes were induced in all lines, none of them related to oxidative stress or secondary metabolism. Few genes were down-regulated, with none being common to all four lines. There were differences in aphid-induced gene regulation between resistant and susceptible lines. Results from control plants without aphids also revealed differences in constitutive gene expression between the two types of lines. Candidate sequences for induced and constitutive resistance factors have been identified, among them a proteinase inhibitor, a serine/threonine kinase and several thionins.

Summary• Interactions between a mycorrhiza-defective tomato ( Lycopersicon esculentum ) mutant, rmc , and different species of arbuscular mycorrhizal (AM) fungi were investigated and compared with those with the wild-type cv. 76R.• Both cv. 76R and rmc were challenged with Glomus intraradices , G. mosseae , G. coronatum, G. versiforme , G. etunicatum , G. fasciculatum , Gigaspora margarita and Scutellospora calospora using a nurse pot inoculation system.• Cv. 76R demonstrated normal colonization patterns for all fungal species. By contrast, the development of different fungal species with rmc was impaired at different steps. Development of G. intraradices , G. etunicatum and G. fasciculatum was arrested on the root surface. However, G. mosseae , G. coronatum , G. margarita and S. calospora frequently penetrated the root epidermis, but colonization of the cortex was rare. G. versiforme achieved relatively normal colonization in rmc compared with the other species.• This is the first report on the variation of colonization patterns in a mycorrhizadefective mutant by different species of AM fungi, and highlights the need for previously described mutants in legumes to be challenged by more than one fungus.

Abbreviation: VAM, vesicular-arbuscular endomycorrhiza. Note that since not all VAM fungi produce vesicles, the term arbuscular mycorrhiza (AM) has been suggested as a more inclusive nomenclature. Here we have retained the older term, which is familiar to a broader audience.

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The effects of infestation by the bird cherry-oat aphid (BCA), (Rhopalosiphum padi L) and the Russian wheat aphid (RWA) (Diuraphis noxia Mordvilko) on callose deposition and transcription of genes related to callose accumulation were investigated in barley (Hordeum vulgare L. cv. Clipper). The BCA, which gives no visible symptoms, induced very limited callose deposition, even after 14 days of infestation. In contrast, RWA, which causes chlorosis, white and yellow streaking and leaf rolling, induced callose accumulation already after 24 h in longitudinal leaf veins. The deposition was pronounced after 72 h, progressing during 7 and 14 days of infestation. In RWA-infested source leaves, callose was also induced in longitudinal veins basipetal to the aphid-infested tissue, whereas in sink leaves, more callose deposition was found above the feeding sites. Eight putative callose synthase genes were identified in a database search, of which seven were expressed in the leaves, but with similar transcript accumulation in control and aphid-infested tissue. Five out of 12 examined beta-1,3-glucanases were expressed in the leaves. All five were upregulated in RWA-infested tissue, but only two in BCA-infested tissue, and to a lesser extent than by RWA. The results suggest that callose accumulation may be partly responsible for the symptoms resulting from RWA infestation and that a callose-inducing signal may be transported in the phloem. Furthermore, it is concluded that the absence of callose deposition in BCA-infested leaves is not because of a stronger upregulation of callose-degrading beta-1,3-glucanases in this tissue, as compared to RWA-infested leaves.