Regulatory Genes in Plant Development: Homeobox


The Knotted‐like homeobox (KNOX) genes play a profound role in shoot apical meristem organization, maintenance and leaf initiation. A MYB domain gene PHANTASTICA plays a role in regulating dorsal–ventral polarity in lateral organs and also downregulates a subset of KNOX genes.

Keywords: knotted‐like; homeobox genes; KNOX; PHANTASTICA; plant development

Figure 1.

An alignment of selected homeodomains from animals, plants and fungi shows that a three‐amino acid loop insertion (aa 24–26, TALE) typifies some fungal, KNOX and BEL1 proteins. Conserved residues are highlighted. The species names and Genbank accession numbers for the sequences are as follows: Drosophila melanogaster, DM‐ANTP : M20704, M14699, M14701; Saccharomyces cerevisiae, SC‐CUP9 : L36815, SC‐ORFYGL : Z72618, SC‐MATa2 : J01332, L00060; Coprinus cinereus CC‐A42b1‐1 : 729992; Zea mays ZM‐KN1 : X61308, X57672; Lycopersicon esculentum LE‐T6 : AF000141, LE‐T12 : AF000142; Arabidopsis thaliana AT‐BELL1 : U39944, AT‐GL2 : L32873, AT‐KNAT2 : U14175.

Figure 2.

Neighbour‐joining tree of plant homeobox proteins based on amino‐acid sequences from the conserved homeodomain. Numbers on branches indicate bootstrap values (100 replicates). Less than 50% bootstrap support is not shown. Arrow indicates the Acetabularia acetabulum knox gene which is basal to KNOX I and KNOX II. Accession number and sources for the sequences are as follows: Zea mays, : LG3: AAD13611, KN1: CAA43605.1, RS1: Q41853; Oryza sativa, OSH1: P46609, OSH15: BAA31688, OSH45: BAA08553; putative homeodomain protein, NP_915494, OCL1: CAG38614 ; Lycopersicon esculentum, TKN1: Q41330, LET6: O22299, LET12: O22300; Nicotiana tabacum, NTH15: BAA25546, NTH 20: BAA76904; Arabidopsis thaliana, STM: NP_176426, KNAT1: NP_192555, KNAT2: NP_177208, KNAT3: NP_001031938, KNAT4: NP_196667, KNAT5: NP_194932, KNAT6: NP_173752, BEL1: NP_198957, ATH1: NP_195024, BLH1: NP_001031490, BLH2: NP_195405, Athb‐7: S47137, ATHB‐12: NP_191748, ATHB13: AAF20996, ATHB‐3: NP_568309, HAT4: Q05466, HAT1: NP_193476, HAT3: NP_191598, PHV: NP_174337, PHB: NP_181018, REV: NP_200877, CORONA: AAW88440, ATHB‐8: NP_195014, ATML1: NP_193906, PDF2: NP_567274, GL2: NP_565223, ANL2: NP_567183; Malus x domestica, MDH1: AAF43095; Silene latifolia, SlHDL1: BAD27254, SlHDL2: BAD27255 ; Physcomitrella patens, MKN4: AAK62559, MKN2: AAK61308, MKN1‐3: AAK61309; Acetabularia acetabulum, AAKNOX: AAD51632.

Figure 3.

(a) The maize Knotted mutation causes alterations in leaf morphology – mutant leaves (on the left) have outgrowths on the blade compared with wild‐type (on the right). (b) The tomato Mouse ears mutation (on the right) causes extra orders of leaf complexity when compared with the unipinnate wild‐type leaf on the left. Both Knotted and Mouse ears are dominant gain‐of‐function mutations in the class I KNOX genes KNOTTED1 (Hake et al., ) and LET6 (Chen et al., ), respectively.

Figure 4.

Schematic representation of KNOX (a), BELL1 (b) and HD‐Zip (c) proteins. BELL, a 120‐amino‐acid region containing a putative amphipathic α‐helix; ELK, domain with a highly conserved series of Glu, Leu and Lys residues; HD, homeodomain; MEINOX, MEIS and KNOX domain; SKY, SKY box domain; START, lipid/sterol‐binding StAR‐related lipid transfer protein: domain. ZLZ, zipper loop zipper; ZIP, leucine zipper.



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Further Reading

Bürglin TR (1994) Guidebook to the Homeobox Genes. Oxford: Oxford University Press.

Hake S, Jackson D, Kerstetter R et al. (1993) How a leaf begins: homeobox genes and plant development. Molecular Biology of the Cell 4: 226A.

Mallory AC and Vaucheret H (2006) Functions of microRNAs and related small RNAs in plants. Nature Genetics 38: S31–36.

Scofield S and Murray JA (2006) KNOX Gene function in plant stem cell niches. Plant Molecular Biology 60: 929–946.

Sinha N (1999) Leaf development in angiosperms. Annual Reviews of Plant Physiology and Plant Molecular Biology 50: 419–446.

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Chung, Kook‐Hyun, and Sinha, Neelima R(Jan 2007) Regulatory Genes in Plant Development: Homeobox. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002073.pub2]