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

Abstract

A major question in plant physiology is how the large amount of sucrose made in leaves is transported to the rest of the plant. Although physiological, biochemical, and anatomical investigations have been performed in this field, to date there have been very few genetic studies. Using a reverse genetic screen, we have identified mutant Arabidopsis plants containing transferred DNA insertions in the gene encoding a phloem-specific sucrose transporter, SUC2. SUC2 is thought to function in loading sugar from the apoplast into the conducting sieve tubes. In the homozygous state, these mutations resulted in stunted growth, retarded development, and sterility. The source leaves of mutant plants contained a great excess of starch, and radiolabeled sugar failed to be transported efficiently to roots and inflorescences. These 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.

Genetic Control of Carbon Partitioning in Grasses: Roles of Sucrose Transporters and Tie-dyed Loci in Phloem Loading

“…Presumably, these phenotypes result from a reduction in assimilates delivered to sink tissues due to the failure to export Suc from source leaves. These phenotypes are similar to those reported in dicot plants containing mutations in SUT genes that are responsible for phloem loading (Riesmeier et al, 1994;Bü rkle et al, 1998;Gottwald et al, 2000;Hackel et al, 2006;Srivastava et al, 2008). Hence, it appears, at least in maize, that SUT1 function is essential for phloem loading of Suc.…”

“…AtSUC2 has a biochemical affinity for Suc consistent with a role in phloem loading from the apoplast (Chandran et al, 2003). Definitive proof of a function in phloem loading came from analyses of T-DNA insertion mutations in AtSUC2 (Gottwald et al, 2000;Srivastava et al, 2008). Mutant plants have reduced Suc export from leaves, chlorotic leaves that accumulate carbohydrates, diminished shoot growth, and delayed flowering.…”

Photosynthetic sucrose acts as cotyledon-derived long-distance signal to control root growth during early seedling development in Arabidopsis

“…As expected (22,23), a fraction of the segregating progeny of heterozygous (SUC2/suc2) plants grown on sucrose-free medium in the light showed a strong retardation of root growth (Fig. 8 A and B).…”

Two splice variants of the IDD14 transcription factor competitively form nonfunctional heterodimers which may regulate starch metabolism

“…The 35S:IDD14α phenotypes, such as growth retardation and pale green leaves with downward curling (Supplementary Fig. S2), are frequently observed in plants with disturbed sugar metabolism 12,13 , suggesting that the IDD14 gene may have metabolic roots. In support of this view, several IDD members have been implicated in sugar and starch metabolism in Arabidopsis and maize 14,15 .…”