Arabidopsis plants harbouring a mutation in AtSUC2, encoding the predominant sucrose/proton symporter necessary for efficient phloem transport, are able to complete their life cycle and produce viable seed.
ABSTRACT AtSUC2 encodes a sucrose/proton symporter that localizes throughout the collection and transport phloem and is necessary for efficient transport of sucrose from source to sink tissues in Arabidopsis thaliana. Plants harbouring homozygous AtSUC2 null alleles accumulate sugar, starch, and anthocyanin in mature leaves, have severely delayed development and stunted growth and, in previous studies, failed to complete their life cycle by producing viable seed.
An AtSUC2 allele with a T-DNA insertion in the second intron was analysed. Full-length transcript from this allele is not produced, and a truncated protein translated from sequences upstream of the insertion site did not catalyse sucrose uptake into yeast, supporting the contention that this is a null allele. Mutant plants were grown in a growth chamber with a diurnal light/dark cycle, and growth patterns recorded.
This allele (SALK_038124, designated AtSUC2-4) has the hallmarks of previously described null alleles but, despite compromised carbon partitioning and growth, produces viable seeds. The onset of flowering was chronologically delayed but occurred at the same point in the plastochron index as wild type.
AtSUC2 is important for phloem loading and is therefore fundamental to phloem transport and plant productivity, but plants can complete their life cycle and produce viable seed in its absence. Arabidopsis appears to have mechanisms for mobilizing reduced carbon from the phloem into developing seeds independent of AtSUC2.
Article: Genetic evidence for the in planta role of phloem-specific plasma membrane sucrose transporters.[show abstract] [hide abstract]
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.Proceedings of the National Academy of Sciences 01/2001; 97(25):13979-84. · 9.68 Impact Factor
Article: Arabidopsis sucrose transporter AtSUC9. High-affinity transport activity, intragenic control of expression, and early flowering mutant phenotype.[show abstract] [hide abstract]
ABSTRACT: AtSUC9 (At5g06170), a sucrose (Suc) transporter from Arabidopsis (Arabidopsis thaliana) L. Heynh., was expressed in Xenopus (Xenopus laevis) oocytes, and transport activity was analyzed. Compared to all other Suc transporters, AtSUC9 had an ultrahigh affinity for Suc (K(0.5) = 0.066 +/- 0.025 mm). AtSUC9 showed low substrate specificity, similar to AtSUC2 (At1g22710), and transported a wide range of glucosides, including helicin, salicin, arbutin, maltose, fraxin, esculin, turanose, and alpha-methyl-d-glucose. The ability of AtSUC9 to transport 10 glucosides was compared directly with that of AtSUC2, HvSUT1 (from barley [Hordeum vulgare]), and ShSUT1 (from sugarcane [Saccharum hybrid]), and results indicate that type I and type II Suc transporters have different substrate specificities. AtSUC9 protein was localized to the plasma membrane by transient expression in onion (Allium cepa) epidermis. Using a whole-gene translational fusion to beta-glucuronidase, AtSUC9 expression was found in sink tissues throughout the shoots and in flowers. AtSUC9 expression in Arabidopsis was dependent on intragenic sequence, and this was found to also be true for AtSUC1 (At1g71880) but not AtSUC2. Plants containing mutations in Suc transporter gene AtSUC9 were found to have an early flowering phenotype under short-day conditions. The transport properties of AtSUC9 indicate that it is uniquely suited to provide cellular uptake of Suc at very low extracellular Suc concentrations. The mutant phenotype of atsuc9 alleles indicates that AtSUC9 activity leads to a delay in floral transition.Plant physiology 02/2007; 143(1):188-98. · 6.53 Impact Factor
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ABSTRACT: Sugars and amino acids are generated in plants by assimilation from inorganic forms. Assimilated forms cross multiple membranes on their way from production sites to storage or use locations. Specific transport systems are responsible for vacuolar uptake and release, for efflux from the cells, and for uptake into the vasculature. Detailed phylogenetic analyses suggest that only proton-coupled cotransporters involved in phloem loading have been identified to date, whereas systems for vacuolar transport and efflux still await identification. Novel imaging approaches may provide the means to characterize the cellular events and elucidate whole plant control of assimilate partitioning and allocation.Annual review of plant biology 02/2004; 55:341-72. · 25.96 Impact Factor