The Knotted-1 (Kn1) locus is defined by several dominant gain-of-function mutations that alter leaf development. Foci of cells along the lateral veins do not differentiate properly, but continue to divide, forming outpocketings or knots. The ligule, a fringe normally found at the junction of leaf blade and sheath, is often displaced and perpendicular to its normal position. The phenotype is manifested in all cell layers of the leaf blade, but is controlled by a subgroup of cells of the inner layer. Mutations result from the insertion of transposable elements or a tandem duplication. We show that the Kn1 gene encodes a homeodomain-containing protein, the first identified in the plant kingdom. Sequence comparisons strongly suggest that Kn1 acts as a transcription factor. Here we use the Kn1 homeobox to isolate other expressed homeobox genes in maize. The Kn1 homeobox may permit the isolation of genes that, like animal and fungal counterparts, regulate cell fate determination.

All living organisms rely on nutrients to sustain cell metabolism and energy production, which in turn need to be adjusted based on available resources. The evolutionarily conserved target of rapamycin (TOR) protein kinase is a central regulatory hub that connects environmental information about the quantity and quality of nutrients to developmental and metabolic processes in order to maintain cellular homeostasis. TOR is activated by both nitrogen and carbon metabolites and promotes energy-consuming processes such as cell division, mRNA translation, and anabolism in times of abundance while repressing nutrient remobilization through autophagy. In animals and yeasts, TOR acts antagonistically to the starvation-induced AMP-activated kinase (AMPK)/sucrose nonfermenting 1 (Snf1) kinase, called Snf1-related kinase 1 (SnRK1) in plants. This review summarizes the immense knowledge on the relationship between TOR signaling and nutrients in nonphotosynthetic organisms and presents recent findings in plants that illuminate the crucial role of this pathway in conveying nutrient-derived signals and regulating many aspects of metabolism and growth.

The homeobox of the knottedl (knl) gene was used to isolate 12 related sequences in maize. The homeodomains encoded by the knl-like genes are very similar, ranging from 55 to 89% amino acid identity. Differences outside the precisely conserved third helix allowed us to group the genes into two classes. The homeodomains of the seven class 1 genes share 73 to 89% identical residues with knl. The four class 2 genes share 55 to 58% identical residues with knl, although the conservation within the class is greater than 87%. Expression patterns were analyzed by RNA gel blot analysis. Class 1 genes were highly expressed in meristem-enriched tissues, such as the vegetative meristem and ear primordia. Expression was not detectable in leaves. The class 2 genes were expressed in all tissues, although one was abundantly expressed in roots. The genes were mapped using recombinant inbred populations. We determined that clusters of two to three linked genes are present on chromosomes 1 and 8; otherwise, the genes are distributed throughout the genome. Four pairs of genes, similar in both sequence and expression patterns, mapped within duplicated regions of the genome.

We have characterized the tomato (Lycopersicon esculentum Mill.) MADS box gene TM29 that shared a high amino acid sequence homology to the Arabidopsis SEP1, 2, and 3 (SEPALLATA1, 2, and 3) genes. TM29 showed similar expression profiles to SEP1, with accumulation of mRNA in the primordia of all four whorls of floral organs. In addition, TM29 mRNA was detected in inflorescence and vegetative meristems. To understand TM29 function, we produced transgenic tomato plants in which TM29 expression was down-regulated by either cosuppression or antisense techniques. These transgenic plants produced aberrant flowers with morphogenetic alterations in the organs of the inner three whorls. Petals and stamens were green rather than yellow, suggesting a partial conversion to a sepalloid identity. Stamens and ovaries were infertile, with the later developing into parthenocarpic fruit. Ectopic shoots with partially developed leaves and secondary flowers emerged from the fruit. These shoots resembled the primary transgenic flowers and continued to produce parthenocarpic fruit and additional ectopic shoots. Based on the temporal and spatial expression pattern and transgenic phenotypes, we propose that TM29 functions in floral organ development, fruit development, and maintenance of floral meristem identity in tomato.Flower development has been the subject of intensive studies over the last decade, particularly in the model plants Arabidopsis and snapdragon (Antirrhinum majus). These studies led to the formulation of the ABC model of floral organ identity, which explained the activities of three classes of genes in specifying the identity of floral organs (Weigel and Meyerowitz, 1994). This model has been supported by genetic and molecular data in a wide range of angiosperm species.According to the ABC model, expression of a class A gene specifies the formation of sepals (the first whorl organ); in combination with the class B genes expression, specifies petal formation. Expression of class B genes and a class C gene specifies stamen identity, whereas expression of C alone determines a carpel identity (Coen and Meyerowitz, 1991;Weigel and Meyerowitz, 1994). Most of the ABC genes belong to the MADS box family (Yanofsky et al., 1990;Jack et al., 1992;Mandel et al., 1992;Goto and Meyerowitz, 1994).Although the ectopic expressions of the ABC genes are sufficient to determine various floral organ identities within the floral meristem, they are insufficient to convert vegetative leaves to floral organs. This suggested that other regulators, in addition to the ABC genes, are required for floral organ specification. Recently, a group of three related MADS box genes SEP 1, 2, and 3 (SEPALLATA 1, 2, and 3) were shown to be necessary for the activity B and C class genes in the control of floral organ formation. First, the SEP1, SEP2, and SEP3 (formerly AGL2, AGL4, and AGL9) redundantly control the activities of the B and C organ identity genes in Arabidopsis because the triple mutant sep1sep2sep3 flower consists entirely of sepals. The sep1/2/...