What can the phylogeny of KNOX class I genes and their expression patterns in land plants tell us about the evolution of shoot development?

KNOX genes encode transcription factors (TFs), several of which act non-cell-autonomously. KNOX genes evolved in algae, and two classes, class I KNOX and class II KNOX genes, were already present in charophytes. In tracheophytes, class I KNOX genes are expressed in shoot apical meristems (SAMs) and thought to inhibit cell differentiation, whereas class II KNOX genes are expressed in mature organs regulating differentiation. In this review, we summarize the data available on gene families and expression patterns of class I and class II KNOX genes in embryophytes. The expression patterns of class I KNOX genes should be seen in the context of SAM structure and of leaf primordium development where the inhibition of cell differentiation needs to be lifted. Although the SAMs of angiosperms and gnetophytes almost always belong to the duplex type, several other types are distributed in gymnosperms, ferns, lycopods and bryophytes. KNOX gene families remained small (maximally five genes) in the representatives of bryophytes, lycopods and ferns examined thus far; however, they expanded to some extent in gymnosperms and, independently and much more strongly, in angiosperms. The growing sophistication of mechanisms to repress and re-induce class KNOX I expression played a major role in the evolution of leaf shape.