▪ Abstract knox genes encode homeodomain-containing transcription factors that are required for meristem maintenance and proper patterning of organ initiation. In plants with simple leaves, knox genes are expressed exclusively in the meristem and stem, but in dissected leaves, they are also expressed in leaf primordia, suggesting that they may play a role in the diversity of leaf form. This hypothesis is supported by the intriguing phenotypes found in gain-of-function mutations where knox gene misexpression affects leaf and petal shape. Similar phenotypes are also found in recessive mutations of genes that function to negatively regulate knox genes. KNOX proteins function as heterodimers with other homeodomains in the TALE superclass. The gibberellin and lignin biosynthetic pathways are known to be negatively regulated by KNOX proteins, which results in indeterminate cell fates.

Read the full review for this F1000Prime recommended article: The interaction of two homeobox genes, BREVIPEDICELLUS and PENNYWISE, regulates internode patterning in the Arabidopsis inflorescence.

Conserved in a variety of evolutionarily divergent plant species, LOB DOMAIN ( LBD ) genes define a large, plant-specific family of largely unknown function. LBD genes have been implicated in a variety of developmental processes in plants, although to date, relatively few members have been assigned functions. LBD proteins have previously been predicted to be transcription factors, however supporting evidence has only been circumstantial. To address the biochemical function of LBD proteins, we identified a 6-bp consensus motif recognized by a wide cross-section of LBD proteins, and showed that LATERAL ORGAN BOUNDARIES (LOB), the founding member of the family, is a transcriptional activator in yeast. Thus, the LBD genes encode a novel class of DNA-binding transcription factors. Post-translational regulation of transcription factors is often crucial for control of gene expression. In our study, we demonstrate that members of the basic helix–loop–helix (bHLH) family of transcription factors are capable of interacting with LOB. The expression patterns of bHLH048 and LOB overlap at lateral organ boundaries. Interestingly, the interaction of bHLH048 with LOB results in reduced affinity of LOB for the consensus DNA motif. Thus, our studies suggest that bHLH048 post-translationally regulates the function of LOB at lateral organ boundaries.

Understanding molecular mechanisms that control cell fate in the shoot apical meristem is a fundamental question in plant development. Genetic and molecular studies demonstrate that maize KNOTTED1 (KN1) of the TALE (3-aa acid loop extension) class of homeodomain (HD) proteins is involved in shoot apical meristem function. We show that KN1 interacts with knotted interacting protein (KIP), a BEL1-like TALE HD protein. Interaction between KN1 and KIP is mediated by conserved domains in the N termini of both proteins. The KN1 DNA-binding sequence, TGACAG(G/C)T, was biochemically identified, and in vitro DNA-binding assays show that individually KN1 and the HD of KIP bind specifically to this motif with low affinity. The KN1–KIP complex, however, binds specifically to this DNA-binding motif with high affinity, indicating that the association of KN1 and KIP may function in transcriptional regulation.