Fig 6. Proposed KNOX functions during land plant evolution.

Along the phylogeny of plants, the primary functions for BELL (depicted as B) and KNOX (K) proteins, as well as the gene copy number, are presented for Chlamydomonas (a unicellular Chlorophyte alga), Physcomitrella (a moss), and Arabidopsis (a flowering plant). The ancestral conditions at branches were deduced from our phylogenetic analyses (S1 and S17 Figs.). In each life cycle, a red arrow indicates meiosis, and haploid (grey) and diploid (green) stages are color-coded. In Chlamydomonas the plus gamete expresses a BELL (depicted as B) protein while the minus gamete expresses a KNOX (K) protein; upon gamete fusion the KNOX and BELL proteins heterodimerize and regulate zygotic gene expression. Prior to the origin of land plants, a gene duplication in an ancestral KNOX gene generated two subclasses, KNOX1 (K1) and KNOX2 (K2) genes. In Physcomitrella, KNOX1 activity maintains tissue proliferation during sporophyte (diploid) development while KNOX2 represses the haploid genetic program during the diploid generation. In Arabidopsis, KNOX1 activity promotes meristem maintenance, and our study demonstrates that KNOX2 activity promotes tissue differentiation, perhaps via repression of meristematic functions, in the diploid generation. We propose that (1) the gene duplication producing KNOX1 and KNOX2 paralogs and ensuing neofunctionalization was instrumental in the evolution of a complex multicellular diploid generations in land plants and (2) the diversification of KNOX/BELL modules during land plant evolution facilitated the evolution of ever more complex diploid sporophyte body plans.