A membrane-associated thioredoxin required for plant growth moves from cell to cell, suggestive of a role in intercellular communication

Abstract

Thioredoxins (Trxs) are small ubiquitous regulatory disulfide proteins. Plants have an unusually complex complement of Trxs composed of six well-defined types (Trxs f , m , x , y , h , and o ) that reside in different cell compartments and function in an array of processes. The extraplastidic h type consists of multiple members that in general have resisted isolation of a specific phenotype. In analyzing mutant lines in Arabidopsis thaliana , we identified a phenotype of dwarf plants with short roots and small yellowish leaves for AtTrx h 9 (henceforth, Trx h 9), a member of the Arabidopsis Trx h family. Trx h 9 was found to be associated with the plasma membrane and to move from cell to cell. Controls conducted in conjunction with the localization of Trx h 9 uncovered another h -type Trx in mitochondria (Trx h 2) and a Trx in plastids earlier described as a cytosolic form in tomato. Analysis of Trx h 9 revealed a 17-amino acid N-terminal extension in which the second Gly (Gly 2 ) and fourth cysteine (Cys 4 ) were highly conserved. Mutagenesis experiments demonstrated that Gly 2 was required for membrane binding, possibly via myristoylation. Both Gly 2 and Cys 4 were needed for movement, the latter seemingly for protein structure and palmitoylation. A three-dimensional model was consistent with these predictions as well as with earlier evidence showing that a poplar ortholog is reduced by a glutaredoxin rather than NADP-thioredoxin reductase. In demonstrating the membrane location and intercellular mobility of Trx h 9, the present results extend the known boundaries of Trx and suggest a role in cell-to-cell communication.