Strigolactones are a recently discovered class of plant hormone involved in branching, leaf senescence, root development, and plant-microbe interactions. They are carotenoid-derived lactones, synthesized in the roots and transported acropetally to modulate axillary bud outgrowth (i.e., branching). However, a receptor for strigolactones has not been identified. We have identified the DAD2 gene from petunia, an ortholog of the rice and Arabidopsis D14 genes, and present evidence for its roles in strigolactone perception and signaling. DAD2 acts in the strigolactone pathway, and the dad2 mutant is insensitive to the strigolactone analog GR24. The crystal structure of DAD2 reveals an α/β hydrolase fold containing a canonical catalytic triad with a large internal cavity capable of accommodating strigolactones. In the presence of GR24 DAD2 interacts with PhMAX2A, a central component of strigolactone signaling, in a GR24 concentration-dependent manner. DAD2 can hydrolyze GR24, with mutants of the catalytic triad abolishing both this activity and the ability of DAD2 to interact with PhMAX2A. The hydrolysis products can neither stimulate the protein-protein interaction nor modulate branching. These observations suggest that DAD2 acts to bind the mobile strigolactone signal and then interacts with PhMAX2A during catalysis to initiate an SCF-mediated signal transduction pathway.

Ethylene is the major effector of ripening in many fleshy fruits. In apples (Malus x domestica) the addition of ethylene causes a climacteric burst of respiration, an increase in aroma, and softening of the flesh. We have generated a transgenic line of 'Royal Gala' apple that produces no detectable levels of ethylene using antisense ACC OXIDASE, resulting in apples with no ethyleneinduced ripening attributes. In response to external ethylene these antisense fruits undergo a normal climacteric burst and produced increasing concentrations of ester, polypropanoid, and terpene volatile compounds over an 8-d period. A total of 186 candidate genes that might be involved in the production of these compounds were mined from expressed sequence tags databases and full sequence obtained. Expression patterns of 179 of these were assessed using a 15,720 oligonucleotide apple microarray. Based on sequence similarity and gene expression patterns we identified 17 candidate genes that are likely to be ethylene control points for aroma production in apple. While many of the biosynthetic steps in these pathways were represented by gene families containing two or more genes, expression patterns revealed that only a single member is typically regulated by ethylene. Only certain points within the aroma biosynthesis pathways were regulated by ethylene. Often the first step, and in all pathways the last steps, contained enzymes that were ethylene regulated. This analysis suggests that the initial and final enzymatic steps with the biosynthetic pathways are important transcriptional regulation points for aroma production in apple.

Background: Apple fruit develop over a period of 150 days from anthesis to fully ripe. An array representing approximately 13000 genes (15726 oligonucleotides of 45-55 bases) designed from apple ESTs has been used to study gene expression over eight time points during fruit development. This analysis of gene expression lays the groundwork for a molecular understanding of fruit growth and development in apple.

The Arabidopsis genes CONSTANS-LIKE 1 (COL1) and CONSTANS-LIKE 2 (COL2) are predicted to encode zinc finger proteins with approximately 67% amino acid identity to the protein encoded by the flowering-time gene CONSTANS (CO). We show that the circadian clock regulates expression of COL1 and COL2 with a peak in transcript levels around dawn. We analyzed transgenic plants misexpressing COL1, COL2 and CO. Unlike CO, altered expression of COL1 and COL2 in transgenic plants had little effect on flowering time. However, analysis of circadian phenotypes in the transgenic plants showed that over-expression of COL1 can shorten the period of two distinct circadian rhythms. Experiments with the highest COL1 over-expressing line indicate that its circadian defects are fluence rate-dependent, suggesting an effect on a light input pathway(s).

Five cDNAs for genes differentially expressed during fruit development of kiwifruit (Actinidia deliciosa var. deliciosa cv. Hayward) were isolated from a library made from young fruit, 8-10 days after anthesis. One gene (pKIWI503) has low levels of expression in young fruit but is induced late in fruit development and during fruit ripening, and has some homology to plant metallothionein-like proteins. The other four genes are highly expressed in young fruit with reduced expression in the later stages of fruit development. pKIWI504 has strong homology to plant metallothionein-like proteins and pKIWI505 exhibits homology to the beta-subunit of the mitochondrial ATP synthase gene. The two other genes (pKIWI501 and 502) encode proteins with no significant homology to other known sequences.