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Main conclusion Overexpression of chalcone-3-hydroxylase provokes increased accumulation of 3-hydroxyphloridzin in Malus . Decreased flavonoid concentrations but unchanged flavonoid class composition were observed. The increased 3-hydroxyphlorizin contents correlate well with reduced susceptibility to fire blight and scab. The involvement of dihydrochalcones in the apple defence mechanism against pathogens is discussed but unknown biosynthetic steps in their formation hamper studies on their physiological relevance. The formation of 3-hydroxyphloretin is one of the gaps in the pathway. Polyphenol oxidases and cytochrome P450 dependent enzymes could be involved. Hydroxylation of phloretin in position 3 has high similarity to the B-ring hydroxylation of flavonoids catalysed by the well-known flavonoid 3′-hydroxylase (F3′H). Using recombinant F3′H and chalcone 3-hydroxylase (CH3H) from Cosmos sulphureus we show that F3′H and CH3H accept phloretin to some extent but higher conversion rates are obtained with CH3H. To test whether CH3H catalyzes the hydroxylation of dihydrochalcones in planta and if this could be of physiological relevance, we created transgenic apple trees harbouring CH3H from C. sulphureus. The three transgenic lines obtained showed lower polyphenol concentrations but no shift between the main polyphenol classes dihydrochalcones, flavonols, hydroxycinnamic acids and flavan 3-ols. Increase of 3-hydroxyphloridzin within the dihydrochalcones and of epicatechin/catechin within soluble flavan 3-ols were observed. Decreased activity of dihydroflavonol 4-reductase and chalcone synthase/chalcone isomerase could partially explain the lower polyphenol concentrations. In comparison to the parent line, the transgenic CH3H-lines showed a lower disease susceptibility to fire blight and apple scab that correlated with the increased 3-hydroxyphlorizin contents. Electronic supplementary material The online version of this article (doi:10.1007/s00425-016-2475-9) contains supplementary material, which is available to authorized users.

To break the juvenile stage of apple (Malus x domestica Borkh.) we transferred the LFY gene of Arabidopsis into the genome of the apple cv. 'Pinova'. A total of five transgenic clones constitutively overexpressing the LFY gene were obtained. Approximately, 20 shoots of each clone were rooted and transferred to the glasshouse. No flowers were obtained on transgenic plants during the first 2 years of cultivation. Evaluation of the expression of possible LFY targets revealed that no transcripts could be detected for MdAP1-1 and MdAP1-2. MdTFL1 was unaffected. Based on the absence of the LFY core-binding sequence within promoter sequences of MdAP1-1 and MdAP1-2, it was concluded that LFY was not able to induce these genes. The LFY genes of apple were unaffected in transgenic plants and sequence alignments of the C-terminal amino acid sequence showed a high conservation of these proteins. A change in binding ability to DNA can therefore be excluded. Instead of early flowering, the transgenic plants showed an altered phenotype, which is similar to the columnar phenotype of the 'McIntosh Wijcik' mutant of apple. The transgenic plants showed shortened internodes and a significantly reduced length of the regrowing shoot. A negative correlation was observed between the length of the regrowing shoot and the LFY mRNA transcript level. Furthermore, the LFY transgenic apple plants showed an increased shoot diameter at node 20, which was positively correlated with the LFY mRNA transcript level. Based on our results, we assume an alternative role of LFY in apple.

Transgenic apple plants (Malus × domestica cv. ‘Holsteiner Cox’) overexpressing the Leaf Colour (Lc) gene from maize (Zea mays) exhibit strongly increased production of anthocyanins and flavan-3-ols (catechins, proanthocyanidins). Greenhouse plants investigated in this study exhibit altered phenotypes with regard to growth habit and resistance traits. Lc-transgenic plants show reduced size, transversal gravitropism of lateral shoots, reduced trichome development, and frequently reduced shoot diameter and abnormal leaf development with fused leaves. Such phenotypes seem to be in accordance with a direct or an indirect effect on polar-auxin-transport in the transgenic plants. Furthermore, leaves often develop necrotic lesions resembling hypersensitive response lesions. In tests, higher resistance against fire blight (caused by the bacterium Erwinia amylovora) and against scab (caused by the fungus Venturia inaequalis) is observed. These phenotypes are discussed with respect to the underlying altered physiology of the Lc-transgenic plants. The results are expected to be considered in apple breeding strategies.