Flower initiation is a major determinant of plant reproductive success and one of the challenges for plant biotechnology is to develop ways to control this process at will. In many annual plants, like Arabidopsis, flower initiation marks the end of vegetative growth, and in plants that live longer than a year considerable resources are diverted to the production of flowers. For these reasons, flowering is a process that needs to be tightly controlled by integrating endogenous signals, related to factors such as plant age and metabolic status, with environmental signals like daylength, nutrient status, and temperature. The genes that integrate these signals, the flowering-time genes, act as promoters or repressors of flowering. The flowering-time genes act on the flower- meristem- identity genes whose role is to change the identity of young primordia, and thus their fate from a shoot/leaf-primordium fate to a flower primordium fate. In Arabidopsis, the principal flower- meristem- identity gene, LFY, has a central position in being the only gene whose expression is absolutely necessary for normal flower initiation. Little is known about the functions of the different flowering-time genes and how they transduce the flowering signal to LFY. However, it has been suggested that some of these genes are involved in the partitioning or metabolism/sensing of compounds that might play a part in the endogenous plant signal(s), such as the plant hormone gibberellin, and sucrose (metabolic status). We are characterizing the gibberellin and sucrose metabolism in various late flowering Arabidopsis mutants in order to test this hypothesis. Furthermore, we are investigating the role of the flowering-time genes in the flowering of hybrid aspen trees by isolating and characterizing aspen homologues of the Arabidopsis genes and by expressing the Arabidopsis and aspen homologues both constitutively and inducibly in transgenic aspen trees. With the help of the Populus EST-project we are also identifying genes that are upregulated during the juvenility to maturity transition in aspen. This will help us to answer questions about how some trees are able to maintain a juvenile state with only vegetative growth for several decades before forming their first flower.

In a related project we are studying the signals that control the initiation pattern of primordia formed from the apical meristem. Based on the effects of surgical removal and laser ablation of primordia, it has been suggested that the newly formed primordia, and the shoot apical meristem itself, produce inhibitory fields that prevent new primordia from being initiated in their vicinity. In other words, it is postulated that these inhibitory fields determine the plants phyllotaxy. The nature of these fields is unknown, but the plant hormone IAA has been proposed as a likely agent of their establishment. We are testing various candidate constituents of these fields by changing the metabolism of different plant growth regulators locally in the newly formed primordia or in the apical meristem itself. To achieve this we are utilizing tissue-specific promoters to drive the expression of genes involved in the metabolism or sensing of IAA, cytokinins, gibberellins and sucrose. Such transformations give us the possibility to both up- and down-regulate the activity of these growth regulators in the relevant tissues, and then investigate any changes in the phyllotactic patterns that are created in the transgenic plants expressing the constructs. We will be using transgenic Arabidopsis, tobacco and aspen for these studies. In addition, these constructs will allow us to answer questions regarding the sites of action of growth regulators involved in controlling floral induction and floral initiation. For instance, do gibberellins control Arabidopsis flowering by acting on targets in leaves or in the shoot apical region?

 

Nilsson, O., Crozier, A., Schmülling, T., Sandberg, G. & Olsson, O. () Indole-3-acetic acid homeostasis in transgenic tobacco plants expressing the Agrobacterium rhizogenes rolB gene. Plant Journal 3, 681-689.

 

Weigel, D. & Nilsson, O. () A developmental switch sufficient for flower initiation in diverse plants. Nature 377, 495-500.

 

Nilsson, O., Moritz, T., Sundberg, B., Sandberg, G. & Olsson, O. () Expression of the Agrobacterium rhizogenes rolC gene in a deciduous forest tree alters growth and development and leads to stem fasciation. Plant Physiol. 112, 493-502.

 

Nilsson, O., Little, C. H. A., Sandberg, G. & Olsson, O. () Expression of two heterologous promoters, Agrobacterium rhizogenes rolC and cauliflower mosaic virus 35S, in the stem of transgenic hybrid aspen plants during the annual cycle of growth and dormancy. Plant Mol. Biol. 31, 887-895.

 

Lee, I., Wolfe, D. S., Nilsson, O. & Weigel, D. () A LEAFY co-regulator encoded by UNUSUAL FLORAL ORGANS. Curr. Biol. 7, 95-104.

 

Blàzquez, M.A., Green, R., Nilsson, O., Sussman, M. & Weigel, D. () Gibberellins promote flowering of Arabidopsis by activating the LEAFY promoter. Plant Cell 10, 791-800.

 

Nilsson, O., Lee, I., Blàzquez, M.A. & Weigel, D. () Flowering-time genes modulate the response to LEAFY activity. Genetics 150, 403-410.

 

Nilsson, O., Wu, E., Wolfe, D.S. & Weigel, D. () Genetic ablation of flowers in transgenic Arabidopsis. Plant Journal 15, 799-804.

 

Parcy, F., Nilsson, O. , Busch, M. A., Lee, I. & Weigel, D. () A genetic framework for floral patterning. Nature 395, 561-566.