The KNAT2 Homeodomain Protein Interacts with Ethylene and Cytokinin Signaling

Abstract

Using a transgenic line that overexpresses a fusion of the KNAT2 (KNOTTED-like Arabidopsis) homeodomain protein and the hormone-binding domain of the glucocorticoid receptor (GR), we have investigated the possible relations between KNAT2 and various hormones. Upon activation of the KNAT2-GR fusion, we observed a delayed senescence of the leaves and a higher rate of shoot initiation, two processes that are also induced by cytokinins and inhibited by ethylene. Furthermore, the activation of the KNAT2-GR fusion induced lobing of the leaves. This feature was partially suppressed by treatment with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, or by the constitutive ethylene response ctr1 mutation. Conversely, some phenotypic traits of the ctr1 mutant were suppressed by the activation of the KNAT2-GR fusion. These data suggest that KNAT2 acts synergistically with cytokinins and antagonistically with ethylene. In the shoot apical meristem, the KNAT2 gene is expressed in the L3 layer and the rib zone. 1-Aminocyclopropane-1-carboxylic acid treatment restricted the KNAT2 expression domain in the shoot apical meristem and reduced the number of cells in the L3. The latter effect was suppressed by the activation of the KNAT2-GR construct. Conversely, the KNAT2 gene expression domain was enlarged in the ethylene-resistant etr1-1 mutant or in response to cytokinin treatment. These data suggest that ethylene and cytokinins act antagonistically in the meristem via KNAT2 to regulate the meristem activity.Plant organs are formed continuously during postembryonic development from groups of indeterminate meristematic cells. In Angiosperms like Arabidopsis, the cells within the shoot apical meristem (SAM) are distributed in three layers: L1, L2, and L3 (Clark, 1997; Barton, 1998). The cells derived from the L1 will preferentially form the epidermis, whereas L2 and L3 layers will give rise to the inner parts of the organs. The SAM is furthermore organized in three distinct zones: the peripheral zone, the central zone, and the rib zone. The central zone maintains a population of founder cells. Cells are recruited to the peripheral zone and the rib zone from the central zone, and generate the lateral organs and the inner parts of the stem, respectively. The family of KNOX (KNOTTED homeobox) genes plays a crucial role in the SAM: in Arabidopsis, the KNAT (KNOTTED-like in Arabidopsis) family comprises eight members (Serikawa et al., 1996;Reiser et al., 2000;Semiarti et al., 2001). Of these, four have been associated with meristem function. The best characterized is the STM (SHOOTMERISTEMLESS) gene, which is absolutely required for meristem maintenance (Long et al., 1996). More recently, loss-of-function mutants have been described for KNAT1 (Douglas et al., 2002;Venglat et al., 2002). The molecular characterization of the bp (brevipedicellus) mutant of Arabidopsis has revealed that BP encodes the KNAT1 protein. In addition to its role in meristem maintenance in redundancy with STM, BP plays a key role in regulating the inf...

Interaction ofKNAT6andKNAT2withBREVIPEDICELLUSandPENNYWISEinArabidopsisInflorescences

“…Arabidopsis thaliana plants were grown either on soil or in vitro as described by Hamant et al (2002). All mutants used in this study are in the Col-0 background.…”

A Role for KNAT Class II Genes in Root Development

“…Moreover, the KNAT class I genes BP/KNAT1, KNAT2, and KNAT6, and several of the BELL and OVATE genes are also expressed in the root. 21,23,24,30 A more detailed study of the expression of these genes will identify possible interaction partners for KNAT class II proteins in the root. This will bring us closer towards understanding the role of these proteins in root development.…”

An Ethylene-Induced Regulatory Module Delays Flower Senescence by Regulating Cytokinin Content

“…In petunia, driving the overexpression of the CTK biosynthesis gene IPT with the Senescenceassociated gene12 (SAG12) promoter resulted in a significant delay in flower senescence, accompanied by reduced ethylene sensitivity (Chang et al, 2003). In addition, it has been reported that the Arabidopsis KNOTTED-like homeodomain protein KNAT2 acts synergistically with CTKs and antagonistically with ethylene to delay senescence (Hamant et al, 2002). However, despite such observations exemplifying the antagonistic effects of ethylene and CTKs on flower senescence, the underlying molecular mechanisms and the many details of the genes in the associated regulatory networks are still largely unknown.…”