Tie-dyed2 Functions with Tie-dyed1 to Promote Carbohydrate Export from Maize Leaves

R. Frank Baker and David M. Braun ^*

Department of Biology, 208 Mueller Lab, Pennsylvania State University, University Park, PA 16802

^* Corresponding author; email: dbraun{at}psu.edu.

Regulation of carbon partitioning is essential for plant growthand development. To gain insight into genes controlling carbonallocation in leaves, we identified mutants that hyperaccumulatecarbohydrates. tie-dyed2 (tdy2) is a recessive mutant of maize(Zea mays) with variegated, nonclonal, chlorotic leaf sectorscontaining excess starch and soluble sugars. Consistent witha defect in carbon export, we found that a byproduct of functionalchloroplasts, likely a sugar, induces tdy2 phenotypic expression.Based on the phenotypic similarities between tdy2 and two othermaize mutants with leaf carbon accumulation defects, tie-dyed1(tdy1) and sucrose export defective1 (sxd1), we investigatedwhether Tdy2 functioned in the same pathway as Tdy1 or Sxd1.Cytological and genetic studies demonstrate that Tdy2 and Sxd1function independently. However, in tdy1/+; tdy2/+ F₁ plants,we observed a moderate chlorotic sectored phenotype suggestingthat the two genes are dosage sensitive and have a related function.This type of genetic interaction is referred to as second sitenon-complementation (SSNC) and has often, though not exclusively,been found in cases where the two encoded proteins physicallyinteract. Moreover, tdy1; tdy2 double mutants display a synergisticinteraction supporting this hypothesis. Additionally, we determinedthat cell walls of chlorotic leaf tissues in tdy mutants containincreased cellulose; thus, tdy mutants potentially representenhanced feedstocks for biofuels production. From our phenotypicand genetic characterizations, we propose a model whereby TDY1and TDY2 function together in a single genetic pathway, possiblyin homo- and heteromeric complexes, to promote carbon exportfrom leaves.