Plant Molecular Biology 45: 387–398, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 387 Expression of a class 1 knotted1-like homeobox gene is down-regulated in pea compound leaf primordia Julie Hofer ∗ , Campbell Gourlay, Anthony Michael and T.H. Noel Ellis Department of Applied Genetics, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK ( ∗ author for correspondence; e-mail: hofer@bbsrc.ac.uk) Received 6 March 2000; accepted in revised form 14 September 2000 Key words: compound leaf, homeobox, pea, plant development Abstract Differences in knotted1-like (knox) gene expression may account for some of the diversity of leaf forms seen in nature. Class 1 knox genes are expressed in the compound leaf primordia of tomato but not in the simple leaf primordia of a range of species examined so far. In order to test the hypothesis that all compound leaves differ from simple leaves in this way, we isolated a class 1 knox cDNA from pea, Pskn1 (Pisum sativum knotted1) and examined its expression pattern. The encoded homeodomain of Pskn1 shares 88% identical residues with KNOTTED1 from maize and an adjacent ELK domain is present. The protein sequence of PSKN1 is 69% identical to TKN2, its nearest related sequence in tomato. Unlike TKn2, Pskn1 was not expressed in newly initiated compound leaves. The expression pattern of Pskn1 resembled those of other class 1 knox genes described in maize and Arabidopsis. Transcripts were detected in the shoot apical meristem and developing vasculature of the vegetative shoot, but expression was not detected in newly initiated and developing compound leaf primordia. The same pattern of expression was observed in the afila mutant, which is characterised by highly ramified compound leaves. Our results suggest that tomato and pea use different developmental processes in the generation of their compound leaves. Abbreviations: af, afila; BSA, bovine serum albumin; GCG, Genetics Computer Group; HD, homeodomain; kn1, knotted1; knox, knotted1-like; P, plastochron; Pskn1, Pisum sativum kn1; PCR, polymerase chain reaction; RFLP, restriction fragment length polymorphism; SAM, shoot apical meristem; stm, shoot meristemless; SSC, standard saline citrate; tl, tendril-less; T-FLO, tomato-floricaula; uni, unifoliata Introduction Homeobox-containing genes, first identified in Drosophila melanogaster (McGinnis, 1984; Scott and Weiner, 1984), comprise an increasingly well char- acterised gene superfamily involved in patterning in multicellular organisms (for reviews see Lawrence and Morata, 1994; Mann and Affolter, 1998). The dis- covery of homeobox genes in fungi (Schultz et al., 1990), members of ancient animal phyla (Shenk et al., 1993) and plants (Vollbrecht et al., 1991) is con- The nucleotide sequence data reported will appear in the Gen- Bank database under the accession numbers AF080104 (Pskn1) and AF080105 (Pskn2). sistent with the idea that the homeodomain (HD) has evolved from a common ancestral sequence before the divergence of these kingdoms. As in animals, plant homeobox genes are thought to be important in a number of developmental processes, for example in embryogenesis (Kawahara et al., 1995), ovule devel- opment (Reiser et al., 1995), vascular development (Baima et al., 1995) and shoot meristem maintenance (Sinha et al., 1993; Endrizzi et al., 1996; Long et al., 1996; Kerstetter et al., 1997). In plants, different classes of homeobox gene have been assigned according to the similarity of their shared protein sequence motifs (Ruberti et al., 1991; Bellmann and Werr, 1992; Kerstetter et al., 1994;