ALTERED GROWTH AND WOOD CHARACTERISTICS IN TRANSGENIC HYBRID ASPEN EXPRESSING AGROBACTERIUM-TUMEFACIENS T-DNA INDOLEACETIC-ACID BIOSYNTHETIC GENES

A key regulator of cambial growth is the plant hormone indoleacetic ac id (IAA). Here we report on altered wood characteristics and growth pa tterns in transgenic hybrid aspen (Populus tremula L. x Populus tremul oides Michx.) expressing Agrobacterium tumefaciens T-DNA IAA-biosynthe tic iaaM and iaaH genes. Eighteen lines simultaneously expressing both genes were regenerated. Of these, four lines, verified to be transgen ic by northern blot analysis, were selected and raised under controlle d growth conditions. All four lines were affected in their growth patt erns, including alterations in height and stem diameter growth, intern ode elongation, leaf enlargement, and degree of apical dominance. Two transgenic lines, showing the most distinct phenotypic deviation from the wild type, were characterized in more detail for free and conjugat ed IAA levels and for wood characteristics. Both lines showed an alter ed IAA balance, particularly in mature leaves and roots where IAA leve ls were elevated. They also exhibited changes in wood anatomy, most no tably a reduction in vessel size, an increase in vessel density, and c hanges in ray development. Thus, the recent development of techniques for gene transfer to forest trees enabled us to investigate the influe nce of an altered IAA balance on xylem development in an intact experi mental system. In addition, the results demonstrate the possibility of manipulating wood properties in a forest tree through controlled chan ges of IAA concentration and distribution.