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In this study, 15 closely related barley genotypes were analyzed for the abundance of three b-1,3-glucanase transcripts immediately before and during infestation by the bird cherry-oat aphid (Rhopalosiphum padi L.). The barley lines are doubled haploid lines in backcross (BC) generations BC1 and BC2 from a cross between cultivar Lina and a wild barley accession. Previously, they have been characterized as susceptible (S) or resistant (R) to R. padi based on their ability to support nymphal growth. Here we also tested whether resistance was manifested as reduced aphid settling on the plants. Indeed, aphid numbers were lower on R than on S lines in all cases where there were significant differences between R and S lines. The choice of b-1,3-glucanase sequences is based on earlier results comparing two S and two R genotypes, suggesting that at least two of the three studied sequences are susceptibility factors. The comparisons of transcript abundance in plants with aphids showed for two of the b-1,3-glucanase sequences that there were several cases where an S genotype had significantly higher abundance than an R genotype, and in no case did an R line have significantly higher abundance than an S line. Thus, there was some support for the idea that b-1,3-glucanase sequences are susceptibility factors in the interaction between barley and R. padi.

Aphids are phloem feeders that cause large damage globally as pest insects. They induce a variety of responses in the host plant, but not much is known about which responses are promoting or inhibiting aphid performance. Here, we investigated whether one of the responses induced in barley by the cereal aphid, bird cherry-oat aphid (Rhopalosiphum padi L.) affects aphid performance in the model plant Arabidopsis thaliana L. A barley cDNA encoding the protease inhibitor CI2c was expressed in A. thaliana and aphid performance was studied using the generalist green peach aphid (Myzus persicae Sulzer). There were no consistent effects on aphid settling or preference or on parameters of life span and long-term fecundity. However, short-term tests with apterous adult aphids showed lower fecundity on three of the transgenic lines, as compared to on control plants. This effect was transient, observed on days 5 to 7, but not later. The results suggest that the protease inhibitor is taken up from the tissue during probing and weakly inhibits fecundity by an unknown mechanism. The study shows that a protease inhibitor induced in barley by an essentially monocot specialist aphid can inhibit a generalist aphid in transgenic Arabidopsis.

The activity of the promoter of a metallothionein gene expressed in actinorhizal nodules of Casuarina glauca Sieber ex Spreng., CgMT1, has previously been analysed in Casaurinaceae and in tobacco (Nicotiana tabacum L.), Arabidopsis and rice. In all these plants, the promoter showed high activity in the root cortex and epidermis, making it a useful tool for the expression of transgenes. Therefore, its activity was now analysed in transgenic root systems of Datisca glomerata (C. Presl) Baill, an actinorhizal plant from a different phylogenetic group than C. glauca, using the same CgMT1::GUS fusion as in previous studies. However, in contrast with all other plant species examined previously, the CgMT1::GUS construct showed no activity at all in D. glomerata hairy roots: the expression pattern in nodules resembled that found in C. glauca nodules. This is probably due to the changed hormone balance in hairy roots since experiments on the CgMT1::GUS construct in transgenic Arabidopsis showed that CgMT1 promoter activity was repressed by auxin or cytokinin, respectively. Yet, in hairy roots of the model legume Lotus japonicus L. induced by the same Agrobacterium rhizogenes strain, the CgMT1 promoter was active in roots and not in nodules. These results indicate that although the expression of pRi T-DNA genes leads to changes in root hormone balance, these changes do not abolish the differences in phytohormone levels or sensitivity between plant species. Therefore, gene expression data obtained using transgenic hairy root systems have to be viewed with care, not only due to the disturbed hormone balance, but also because the effects of the pRI-T-DNA genes can differ between species.

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