In vitro interactions between barley TALE homeodomain proteins suggest a role for protein-protein associations in the regulation of Knox gene function

This paper describes two-hybrid interactions amongst barley homeodomain pro teins encoded by the Three Amino acid Loop Extension (TALE) superfamily. Th e class I KNOX protein BKN3 is shown to homodimerise and to associate with proteins encoded by the class I and II Knox genes BKn-1 and BKn-7. Furtherm ore, JUBEL1 and JUBEL2, two BELL1 homologous proteins, are identified and c haracterised as interacting partners of BKN3. Differences in the requiremen ts of BKN3 derivatives for interactions with KNOX and JUBEL proteins imply the involvement of overlapping but slightly different domains. This set of results is an example for interactions amongst different classes of plant T ALE homeodomain proteins, as previously described for related animal protei ns. Apparently identical spatial and temporal expression patterns of BKn-1, BKn-3, BKn-7, JuBel1 and JuBel2, as determined by in situ hybridisation, a re compatible with possible interactions of their protein products in plant a. Contradictory to the common model, that the transcriptional down-regulat ion of certain class 1 Knox-genes is the prerequisite for organ differentia tion, transcripts of all five genes were, similar to Tkn1 and Tkn2/LeT6 of tomato, detected in incipient and immature leaves as well as in meristemati c tissues. A characteristic phenotype is induced by the overexpression of J uBel2 in transgenic tobacco plants.