KNOX Proteins Research Articles

In vitro interactions between barley TALE homeodomain proteins suggest a role for protein-protein associations in the regulation of Knox gene function.

This paper describes two-hybrid interactions amongst barley homeodomain proteins encoded by the Three Amino acid Loop Extension (TALE) superfamily. The class I KNOX protein BKN3 is shown to homodimerise and to associate with proteins encoded by the class I and II Knox genes BKn-1 and BKn-7. Furthermore, JUBEL1 and JUBEL2, two BELL1 homologous proteins, are identified and characterised as interacting partners of BKN3. Differences in the requirements of BKN3 derivatives for interactions with KNOX and JUBEL proteins imply the involvement of overlapping but slightly different domains. This set of results is an example for interactions amongst different classes of plant TALE homeodomain proteins, as previously described for related animal proteins. Apparently identical spatial and temporal expression patterns of BKn-1, BKn-3, BKn-7, JuBel1 and JuBel2, as determined by in situ hybridisation, are compatible with possible interactions of their protein products in planta. Contradictory to the common model, that the transcriptional down-regulation of certain class 1 Knox-genes is the prerequisite for organ differentiation, transcripts of all five genes were, similar to Tkn1 and Tkn2/LeT6 of tomato, detected in incipient and immature leaves as well as in meristematic tissues. A characteristic phenotype is induced by the overexpression of JuBel2 in transgenic tobacco plants.

Function of KNOX Homeodomain Proteins in Plant Development.

KNOX homeodomain proteins are encoded by knotted1-like homeobox (knox) genes that constitute a gene family in plants. Similar to the animal homeodomain proteins, KNOX proteins are considered to be key transcriptional regulators that control the expression of genes involved in plant organogenesis at the shoot apical meristem. Therefore, in order to understand the developmental processes in plants, it is important to elucidate the molecular mechanisms underlying KNOX protein regulation of gene expression. In this review, we discuss the structural features of KNOX proteins and the mechanisms by which they interact with and regulate target gene expression.

Mutations that cause amino acid substitutions at the invariant positions in homeodomain of OSH3 KNOX protein suggest artificial selection during rice domestication.

KNOX homeodomain (HD) proteins encoded by KNOTTED1-like homeobox genes (KNOX genes) are considered to work as important regulators for plant developmental and morphogenetic events. We found that OSH3, one of the KNOX genes isolated from a cultivar of Oryza sativa (Nipponbare), encodes a novel HD, which has two amino acid substitutions at invariant positions. Sequence analysis of OSH3 from various domesticated and wild species of rice has revealed that these substitutions are distributed only in Japonica and Javanica type of O. sativa, two groups of domesticated rice in Asia. Surprisingly, nucleotide sequences in the first intron are almost conserved in the rice strains that have the substitutions at the invariant amino acids. Overexpression studies revealed that these invariant amino acids are critical for the function of OSH3 in vivo. The facts that these substitutions occurred specifically at the functionally important amino acids and the sequences are conserved in intron where neutral mutations accumulate suggest the substitutions at the invariant positions of OSH3 have been fixed by artificial selections during domestication. Based on these observations, we hypothesize that OSH3 is responsible for one of the traits that are selectively introduced during the domestication of most of Japonica and a part of Javanica type of rice.

Disruption of auxin transport is associated with aberrant leaf development in maize

Despite recent progress, the mechanisms governing shoot morphogenesis in higher plants are only partially understood. Classical physiological studies have suggested that gradients of the plant growth regulator auxin may play a role in controlling tissue differentiation in shoots. More recent molecular genetic studies have also identified knotted1 like homeobox (knox) genes as important regulators of shoot development. The maize (Zea mays L.) mutant rough sheath2 (rs2) displays ectopic expression of at least three knox genes and consequently conditions a range of shoot and leaf phenotypes, including aberrant vascular development, ligular displacements, and dwarfism (R. Schneeberger, M. Tsiantis, M. Freeling, J.A. Langdale [1998] Development 125: 2857-2865). In this report, we show that rs2 mutants also display decreased polar auxin transport in the shoot. We also demonstrate that germination of wild-type maize seedlings on agents known to inhibit polar auxin transport mimics aspects of the rs2 mutant phenotype. The phenotype elaborated in inhibitor-treated plants is not correlated with ectopic KNOX protein accumulation.

Gnarley1 is a dominant mutation in the knox4 homeobox gene affecting cell shape and identity.

Maize leaves have a stereotypical pattern of cell types organized into discrete domains. These domains are altered by mutations in knotted1 (kn1) and knox (for kn1-like homeobox) genes. Gnarley (Gn1) is a dominant maize mutant that exhibits many of the phenotypic characteristics of the kn1 family of mutants. Gn1 is unique because it changes parameters of cell growth in the basal-most region of the leaf, the sheath, resulting in dramatically altered sheath morphology. The strongly expressive allele Gn1-R also gives rise to a floral phenotype in which ectopic carpels form. Introgression studies showed that the severity of the Gn1-conferred phenotype is strongly influenced by genetic background. Gn1 maps to knox4, and knox4 is ectopically expressed in plants with the Gn1-conferred phenotype. Immunolocalization experiments showed that the KNOX protein accumulates at the base of Gn1 leaves in a pattern that is spatially and temporally correlated with appearance of the mutant phenotype. We further demonstrate that Gn1 is knox4 by correlating loss of the mutant phenotype with insertion of a Mutator transposon into knox4.

ROUGH SHEATH2: a Myb protein that represses knox homeobox genes in maize lateral organ primordia.

The regulation of members of the knotted1-like homeobox (knox) gene family is required for the normal initiation and development of lateral organs. The maize rough sheath2 (rs2) gene, which encodes a Myb-domain protein, is expressed in lateral organ primordia and their initials. Mutations in the rs2 gene permit ectopic expression of knox genes in leaf and floral primordia, causing a variety of developmental defects. Ectopic KNOX protein accumulation in rs2 mutants occurs in a subset of the normal rs2-expressing cells. This variegated accumulation of KNOX proteins in rs2 mutants suggests that rs2 represses knox expression through epigenetic means.

Phylogenetic relationships and evolution of the KNOTTED class of plant homeodomain proteins.

Knotted-like (KNOX) proteins constitute a group of homeodomain proteins involved in pattern formation in developing tissues of angiosperms and other green plants. We conducted phylogenetic analyses of nucleotide and amino acid sequences of all known KNOX proteins in order to examine their evolution. Our analyses reveal two groups of KNOX proteins, classes I and II. Dicot and monocot sequences occur in both classes, indicating that the protein classes arose prior to the origin of the monocots. A conifer (Picea) sequence is nested within class I, suggesting that there are likely to be other copies of KNOX genes in this and other conifers. The orthology of several grass genes (including Zea Kn1, ZMKN1) is strongly supported by phylogenetic and synteny analyses. However, no compelling evidence supports the hypothesis of orthology previously proposed for several dicot genes and ZMKN1. Analysis of expression patterns suggests that the ancestral KNOX gene was expressed in all plant parts and that the propensity to be downregulated in roots and leaves evolved in the class I genes.