Evolution of Compound Leaf Development in Legumes: Evidence for Overlapping Roles of KNOX1 and FLO/LFY Genes

Plant leaves exist in a dizzying array of shapes and sizes, from the tiny duckweed (Lemna minor) to the enormous Raphia palm (Raphia farinifera), and the simpleleaved poplar (Populus sp) to the deeply lobed oak (Quercus spp) or the delicate compound-leaved Mimosa spp. Class I KNOTTED-like (KNOX1) genes have been found to play an important role in the development of compound leaves in many vascular plants (Bharathan et al., 2002). In simple-leaved plants, such as maize or Arabidopsis, class I KNOX1 genes are expressed in the shoot apical meristem, where they function to maintain indeterminacy. Their expression is repressed in leaf primordia and throughout leaf development, consistent with the determinate growth of these organs. In some cases, ectopic expression in simple leaves results in altered leaf morphology reminiscent of compound leaves (ectopic outgrowths and lobes, and in some cases, ectopic meristems). By contrast, many compound-leaved species show a reactivation of KNOX1 expression during leaf development. Furthermore, overexpression of a KNOX1 homolog in tomato leads to more highly branched compound leaf forms. These observations suggest that compound leaves generally evolve from simple leaves by the induction of KNOX1 genes in the developing leaf, causing leaves to become somewhat shoot like. In this issue of The Plant Cell, Champagne et al. (pages 3369–3378) show that expression of KNOX1 proteins is associated with compound leaf development in the major subfamilies of legumes (family Fabaceae; see figure), except for a large subclade known as the inverted repeat–lacking clade (IRLC), which includes fava bean (Vicia), pea (Pisum), and alfalfa (Medicago). Results of further experiments suggest that orthologs of FLORICAULA/LEAFY (FLO/LFY) function in place of KNOX1 genes in the development of compound leaves in the IRLC. The authors hypothesize that overlapping functionality of KNOX1 and FLO/LFY genes in ancestral Fabaceae species enabled the loss of KNOX1 function in compound leaf development in the IRLC. Champagne et al. first make use of a polyclonal KNOX1-specific antibody to show that KNOX1 expression is downregulated in incipient leaf primordia both in IRLC members, such as alfalfa and Chinese wisteria, and IRLC nonmembers such as soybean (Glycine) and bean (Phaseolus). However, a noticeable difference in KNOX1 expression was evident in the later development of compound leaves: developing compound leaves of IRLC members (pea, wisteria, alfalfa, and fava bean) all failed to show KNOX1 expression, whereas compound-leaved Fabaceae outside the IRLC (including soybean, bean, Mimosa, Lotus, and Acacia) all showed KNOX1 expression in developing leaves. This suggests that soon after the IRLC diverged from its closest living relatives there was a loss of KNOX1 expression during compound leaf development. From a phylogenetic analysis by Wojciechowski et al. (2000), based on plastid matK gene sequences, Cercis (redbud) and Bauhinia were found to be sisters to the rest of Fabaceae. Species in these genera lack complex compound leaves, though they may be bilobed, leading to a question of whether they are truly simple leaves or are simplified from compoundleaved ancestors. Champagne et al. found that KNOX1 protein expression in Cercis occurs both in the SAM and in developing mature leaves. Because this pattern is shared with other more distantly related compoundleaved Fabaceae (excluding members of the IRLC), the authors contend that this supports the hypothesis that the ancestral leaf type was compound and the unifoliolate leaf was derived by a fusion of leaflets. Given that KNOX1 function in compound leaf development has been lost in the IRLC, what controls formation of compound leaves in members of this clade? It was previously shown that in pea (Pisum sativum), a member of the IRLC, the gene UNIFOLATA (UNI) is required for compound leaf development (Hofer et al., 1997, 2001; Gourlay et al., 2000). UNI is an ortholog of Antirrhinum FLO and Arabidopsis LFY genes, which have a primary function in the regulation of floral meristem initiation in these species. The pea uni mutant exhibits floral