Genetic evidence for the in planta role of phloem-specific plasma membranesucrose transporters

A major question in plant physiology is how the large amount of sucrose mad e in leaves is transported to the rest of the plant. Although physiological , biochemical. and anatomical investigations have been performed in this fi eld. to date there have been very few genetic studies. Using a reverse gene tic screen, we have identified mutant Arabidopsis plants containing transfe rred DNA insertions in the gene encoding a phloem-specific sucrose transpor ter, SUC2. SUC2 is thought to function in loading sugar from the apoplast i nto the conducting sieve tu bes. In the homozygous state, these mutations r esulted in stunted growth, retarded development, and sterility. The source leaves of mutant plants contained a great excess of starch, and radiolabele d sugar failed to be transported efficiently to roots and inflorescences. T hese data provide genetic: proof that apoplastic phloem loading is critical for growth, development, and reproduction in Arabidopsis and that SUC2 is at least partially responsible for this step.