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Published on July 23, 2008; 10.1104/pp.108.124776

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Received June 15, 2008
Accepted July 17, 2008

Functional characterization of the Arabidopsis thaliana AtSUC2 Suc/H+ symporter by tissue-specific complementation reveals an essential role in phloem loading but not in longdistance transport

Avinash C. Srivastava , Savita Ganesan , Ihab O. Ismail , and Brian G. Ayre *

University of North Texas, Department of Biological Sciences, P.O. Box 305220, Denton, TX 76203 5220

* Corresponding author; email: bgayre{at}unt.edu.

AtSUC2 (At1g22710) encodes a phloem-localized Suc/H+ symporter necessary for efficient sucrose transport from source tissues to sink tissues in Arabidopsis. AtSUC2 is highly expressed in the collection phloem of mature leaves and its function in phloem loading is well established. AtSUC2, however, is also expressed strongly in the transport phloem where its role is more ambiguous, and has been implicated in mediating both efflux and retrieval to and from flanking tissues via the apoplast. To characterize the role of AtSUC2 in controlling carbon partitioning along the phloem path, AtSUC2 cDNA was expressed from tissue-specific promoters in an Atsuc2 mutant background. Sucrose transport in this mutant is highly compromised, as indicated by stunted growth and accumulation of large quantities of sugar and starch in vegetative tissues. Expression of AtSUC2 cDNA from 2 kb of AtSUC2 promoter was sufficient to restore growth and carbon partitioning to near wild type levels. The GALACTINOL SYNTHASE promoter of Cucumis melo (CmGAS1) confers expression only in the minor veins of mature leaves, and not in the transport phloem of larger leaf-veins and stems. Mutant plants expressing AtSUC2 cDNA from the CmGAS1 promoter had intermediate growth and accumulated sugar and starch, but otherwise had normal morphology. These characteristics support a role for AtSUC2 in retrieval but not efflux along the transport phloem, and show that the only vital function of AtSUC2 in photoassimilate distribution is phloem loading. In addition, the Atsuc2 mutant plants, although debilitated, do grow, and AtSUC2-independent modes of phloem transport are discussed, including an entirely symplastic pathway from mesophyll cells to sink tissues.




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