Arabidopsis KNOXI Proteins Activate Cytokinin Biosynthesis

Abstract

Plant architecture is shaped through the continuous formation of organs by meristems. Class I KNOTTED1-like homeobox (KNOXI) genes are expressed in the shoot apical meristem (SAM) and are required for SAM maintenance. KNOXI proteins and cytokinin, a plant hormone intimately associated with the regulation of cell division, share overlapping roles, such as meristem maintenance and repression of senescence, but their mechanistic and hierarchical relationship have yet to be defined. Here, we show that activation of three different KNOXI proteins using an inducible system resulted in a rapid increase in mRNA levels of the cytokinin biosynthesis gene isopentenyl transferase 7 (AtIPT7) and in the activation of ARR5, a cytokinin response factor. We further demonstrate a rapid and dramatic increase in cytokinin levels following activation of the KNOXI protein SHOOT MERISTEMLESS (STM). Application of exogenous cytokinin or expression of a cytokinin biosynthesis gene through the STM promoter partially rescued the stm mutant. We conclude that activation of cytokinin biosynthesis mediates KNOXI function in meristem maintenance. KNOXI proteins emerge as central regulators of hormone levels in plant meristems.

Transcriptomic Analyses Indicate That Maize Ligule Development Recapitulates Gene Expression Patterns That Occur during Lateral Organ Initiation

“…A gene with similarity to ARABIDOPSIS THALIANA RESPONSE REGULATOR9, a type-A response regulator and negative regulator of CK signaling, was also upregulated at the blade-sheath boundary (Supplemental Figure 2E; Kiba et al, 2003;To et al, 2004). Lateral organ founder cells exhibit low ratios of CK/auxin (Jasinski et al, 2005;Yanai et al, 2005). Our data suggest that reduced CK levels in preligule cells may promote cell expansion and ligule outgrowth in a manner similar to leaf initiation.…”

KNOX genes: versatile regulators of plant development and diversity

“…CKs activate cell division, whereas GAs promote cell elongation, which is a cell differentiation process -as is lignin deposition -and KNOX proteins regulate the balance between GAs and CKs in meristem and leaf founder cells. In the SAM, KNOX proteins raise CK levels by activating the transcription of ISOPENTENYL TRANSFEREASE7 (IPT7) and lower GA levels by directly inhibiting the biosynthetic gene GA 20-oxidase1 (GA20ox1) and activating the catabolic gene GA 2-oxidase1 (GA2ox1) (Bolduc and Hake, 2009;Jasinski et al, 2005;Sakamoto et al, 2001;Yanai et al, 2005). Genetic analyses demonstrate that this high CK:low GA ratio is important for KNOX function -to prevent cell differentiation and thus maintain pluripotent cell fate in the SAM.…”

“…Genetic analyses demonstrate that this high CK:low GA ratio is important for KNOX function -to prevent cell differentiation and thus maintain pluripotent cell fate in the SAM. For example, stm mutant phenotypes are suppressed by elevating CK biosynthesis and are enhanced by either elevating GA activity or by reducing CK biosynthesis, and ultimately the combination of elevated GAs and reduced CKs is sufficient to phenocopy stm mutant phenotypes in wild-type Arabidopsis (Jasinski et al, 2005;Yanai et al, 2005). Conversely, the absence of KNOX expression in lateral organ founder cells likely correlates with a low CK:high GA ratio and with the competence to differentiate (Jasinski et al, 2005;Yanai et al, 2005).…”

GeBP and GeBP-Like Proteins Are Noncanonical Leucine-Zipper Transcription Factors That Regulate Cytokinin Response in Arabidopsis

“…Although the double mutants were indistinguishable from wild types, the triple mutant gebp-1 gpl1-1 gpl2-1 was slightly paler than the wild types when grown on Murashige and Skoog (MS) medium (see below). As the founding member GeBP acts downstream of KNOXI genes (Curaba et al, 2003) whose main role is to control hormonal pathways, especially GA and cytokinin path- ways (Sakamoto et al, 2001;Chen et al, 2004;Jasinski et al, 2005;Yanai et al, 2005), the triple mutant was grown in the presence of different hormones, including GAs, cytokinins, abscisic acid, auxins, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, jasmonic acid, and brassinosteroids. When grown on MS medium supplemented with various concentrations of each of these hormones, the triple mutant was indistinguishable from the wild types (data not shown) except with cytokinin hormones (Fig.…”

“…GeBP is predicted to play a role in hormonal pathways on the basis of the following observations: (1) the GeBP protein binds the cisregulatory element of the GLABROUS1 gene, a myb-gene regulated by GA and cytokinin hormones (Perazza et al, 1998;Gan et al, 2007) involved in epidermis cell determination (Oppenheimer et al, 1991); and (2) transcript levels of GeBP are positively regulated by BRE-VIPEDICELLUS (BP; Curaba et al, 2004), a gene of the KNOTTED1 homeodomain (KNOX) family that positively regulates the cytokinin pathway in the shoot apical meristem (SAM; Jasinski et al, 2005;Yanai et al, 2005). Hormones such as cytokinins, GAs, and auxin are involved in the establishment of the balance between the production of organs from the flanks of the SAM and indeterminate growth at its center.…”