BEL1-like transcription factors interact with Knotted1 types to regulate numerous developmental processes. In potato (Solanum tuberosum), the BEL1 transcription factor St BEL5 and its protein partner POTH1 regulate tuber formation by mediating hormone levels in the stolon tip. The accumulation of St BEL5 RNA increases in response to short-day photoperiods, inductive for tuber formation. RNA detection methods and heterografting experiments demonstrate that BEL5 transcripts are present in phloem cells and move across a graft union to localize in stolon tips, the site of tuber induction. This movement of RNA to stolon tips is correlated with enhanced tuber production. Overexpression of BEL5 transcripts that include the untranslated sequences of the BEL5 transcript endows transgenic lines with the capacity to overcome the inhibitory effects of long days on tuber formation. Addition of the untranslated regions leads to preferential accumulation of the BEL5 RNA in stolon tips under short-day conditions. Using a leaf-specific promoter, the movement of BEL5 RNA to stolon tips was facilitated by a short-day photoperiod, and this movement was correlated with enhanced tuber production. These results implicate the transcripts of St BEL5 in a long-distance signaling pathway that are delivered to the target organ via the phloem stream.

Using the yeast (Saccharomyces cerevisiae) two-hybrid system and a potato (Solanum tuberosum) KNOX protein, designated POTH1, as bait, we have identified seven distinct interacting proteins from a stolon library of potato. All seven cDNAs are members of the BEL1-like family of transcription factors. Among these proteins, there are at least four regions of high sequence conservation including the homeodomain, the proline-tyrosine-proline three-amino acid loop extension, the SKY box, and a 120-amino acid region upstream from the homeodomain. Through deletion analysis, we identified a proteinbinding domain present in the carboxy end of the KNOX domain of POTH1. The protein-binding domain in the BEL1 protein is located in the amino-terminal one-half of the 120-residue conserved region of the BELs. RNA-blot analysis showed differential patterns of RNA accumulation for the BELs in various potato organs. The level of StBEL5 mRNA increased in response to a short-day photoperiod in both leaves and stolons. Similar to sense mutants of POTH1, transgenic lines that overexpressed StBEL5 exhibited enhanced tuber formation even under noninductive conditions. Unlike POTH1 sense lines, however, these BEL lines did not exhibit the extreme leaf and stem morphology characteristic of KNOX overexpressers and displayed a more rapid rate of growth than control plants. Both StBEL5 and POTH1 sense lines exhibited an increase in cytokinin levels in shoot tips. StBEL5 lines also exhibited a decrease in the levels of GA 20-oxidase1 mRNA in stolon tips from long-day plants. Our results demonstrate an interaction between KNOX and BEL1-like transcription factors of potato that may potentially regulate processes of development.The primary developmental events of plants originate from apical meristems (Clark, 1997; Kerstetter and Hake, 1997). Many of these events are controlled at the molecular level by transcription factors (TFs). TFs are proteins that act as developmental switches by binding to the DNA (or to other proteins that bind to the DNA) of specific target genes to modulate their expression. An important family of TFs involved in regulating the developmental events in apical meristems is the knox (knotted-like homeobox) gene family (Reiser et al., 2000). Knox genes belong to the group of TFs known as the three-amino acid loop extension (TALE) superclass (Bü rglin, 1997). These TFs are distinguished by a very high level of sequence conservation in the DNA-binding region, designated the homeodomain, and consisting of three ␣-helices similar to the bacterial helix-loop-helix motif (Kerstetter et al., 1994). The third helix, the recognition helix, is involved in DNA binding (Mann and Chan, 1996). TALE TFs contain a TALE (Pro-Tyr-Pro) between helices I and II in the homeodomain that has been implicated in protein interactions (Passner et al., 1999). There are numerous TFs from plants and animals in the TALE superclass, and the two main groups in plants are the KNOX and BEL types (Bü rglin, 1997). Related genes in animal systems play an im...

SummaryTwo interacting three amino acid loop extension (TALE) proteins of potato, StBEL5 (Solanum tuberosom BEL 5) and POTH1 (potato homeobox 1), mediate developmental processes by regulating phytohormone levels. Overexpression of either partner alone increased tuber yields by lowering gibberellin (GA) levels and increasing cytokinins. Gel shift assays demonstrated that StBEL5 and POTH1 bind to the regulatory region of ga20 oxidase1 (ga20ox1 ) from potato, a gene encoding a key enzyme in the GA biosynthetic pathway. In tandem, StBEL5 and POTH1 had a greater binding af®nity for the ga20ox1 promoter than either protein alone. The StBEL5±POTH1 heterodimer bound speci®cally to a composite 10-bp sequence, containing two TGAC cores. Using a transcription assay, StBEL5 and POTH1 suppressed the activity of the ga20ox1 promoter by more than 50%. Dominant negative constructs containing the protein-binding domains of StBEL5 or POTH1 blocked the repression activity of StBEL5 or POTH1, respectively. The mutated ga20ox1 promoter that could be bound by the StBEL5 or POTH1 proteins individually but not by the StBEL5±POTH1 heterodimer also abolished the repression activity of StBEL5, POTH1, and the StBEL5±POTH1 heterodimer. These results indicate that the tandem interaction of StBEL5 and POTH1 is essential for regulation of the expression of their target gene.

Potato (Solanum tuberosum) homeobox 1 (POTH1) is a class I homeobox gene isolated from an early-stage tuber cDNA library. The RNA expression pattern of POTH1, unlike that of most other class I knotted-like homeobox genes, is widespread in the cells of both indeterminate and differentiated tissues. Using in situ hybridization, POTH1 transcripts were detected in meristematic cells, leaf primordia, and the vascular procambium of the young stem. Overexpression of POTH1 produced dwarf plants with altered leaf morphology. Leaves were reduced in size and displayed a "mouse-ear" phenotype. The mid-vein was less prominent, resulting in a palmate venation pattern. The overall plant height of overexpression lines was reduced due to a decrease in internode length. Levels of intermediates in the gibberellin (GA) biosynthetic pathway were altered, and the bioactive GA, GA 1 , was reduced by one-half in sense mutants. Accumulation of mRNA for GA 20-oxidase1, a key biosynthetic enzyme, decreased in overexpression lines. In vitro tuberization was enhanced under both short-and long-day photoperiods in several POTH1 overexpression lines. Sense lines produced more tubers at a faster rate than controls. These results imply that POTH1 mediates the development of potato by acting as a negative regulator of GA biosynthesis.Homeobox genes, a family of transcription factors highly conserved in animals, plants, and yeast (Chan et al., 1998), are implicated in the control of cell fate. The Antennapedia homeobox gene in fruitfly (Drosophila melanogaster), for example, specifies leg identity while inhibiting the formation of an antenna (Mann and Chan, 1996). Ectopic expression of the eyeless gene in the wing imaginal disc tissue of fruitfly embryos causes a normal eye to form on the wings (Halder et al., 1995). The first plant homeobox gene to be discovered was knotted1 (kn1) from maize (Zea mays; Vollbrecht et al., 1991). Dominant gain-offunction mutations of Kn1 formed knot-like structures along lateral veins. These knots were composed of cells that continued to divide rather than differentiate normally (Vollbrecht et al., 1991;Smith et al., 1992), indicating that kn1 is involved in regulating cell fate (Clark et al., 1996; Kerstetter et al., 1997; Chan et al., 1998).Knotted-like homeobox (knox) genes have been isolated from several plant species (for review, see Reiser et al., 2000) and can be divided into two classes based on expression patterns and sequence similarity (Kerstetter et al., 1994). Class I knox genes have high similarity to the kn1 homeodomain and generally have a meristem-specific mRNA expression pattern. Class II knox genes usually have a more widespread expression pattern. Knox genes are members of the three amino acid loop extension (TALE) superclass of homeobox genes (Bü rglin, 1997). The TALE superclass includes members from plants, animals, and fungi and is characterized by the addition of three amino acids, Pro-Tyr-Pro (PYP), between helices 1 and 2 of the homeodomain. Knox genes share conserved regions outside of...

Plant lipoxygenases (LOXs) are a functionally diverse class of dioxygenases implicated in physiological processes such as growth, senescence, and stress-related responses. LOXs incorporate oxygen into their fatty acid substrates and produce hydroperoxide fatty acids that are precursors of jasmonic acid and related compounds. Here, we report the involvement of the tuber-associated LOXs, designated the Lox1 class, in the control of tuber growth. RNA hybridization analysis showed that the accumulation of Lox1 class transcripts was restricted to developing tubers, stolons, and roots and that mRNA accumulation correlated positively with tuber initiation and growth. In situ hybridization showed that Lox1 class transcripts accumulated in the apical and subapical regions of the newly formed tuber, specifically in the vascular tissue of the perimedullary region, the site of the most active cell growth during tuber enlargement. Suppression mutants produced by expressing antisense coding sequence of a specific tuber LOX, designated POTLX-1 , exhibited a significant reduction in LOX activity in stolons and tubers. The suppression of LOX activity correlated with reduced tuber yield, decreased average tuber size, and a disruption of tuber formation. Our results indicate that the pathway initiated by the expression of the Lox1 class genes of potato is involved in the regulation of tuber enlargement. INTRODUCTIONTuber formation in potatoes is a complex developmental process that requires the interaction of environmental, biochemical, and genetic factors. Several important biological processes, such as carbon partitioning, signal transduction, and meristem determination are involved (Ewing and Struik, 1992). Under conditions of a short-day photoperiod and cool temperature, a transmissible signal is activated that initiates cell division and expansion and a change in the orientation of cell growth in the subapical region of the stolon tip (Ewing and Struik, 1992; Xu et al., 1998a). In this signal transduction pathway, perception of the appropriate environmental cues occurs in leaves and is mediated by phytochrome and gibberellins (van den Berg et al., 1995; Jackson and Prat, 1996; Jackson et al., 1996). van den Berg et al. (1996) detected at least 10 quanitative trait loci that control the ability to tuberize under long days, but none of these genes has been identified definitively.Tuber development at the stolon tip consists of biochemical and morphological processes. Both processes are controlled by differential gene expression (Hannapel, 1991; Bachem et al., 1996; Macleod et al., 1999), and most of the research in this area has focused on the biochemical processes, including starch synthesis (Abel et al., 1996;Preiss, 1996; Geigenberger et al., 1998) and storage protein accumulation (Mignery et al., 1984; Hendriks et al., 1991;Suh et al., 1991). Much less is known about the morphological controls of tuberization, although it is clear that phytohormones play a prominent role (Koda et al., 1991; Xu et al., 1998b;Sergeeva et ...