Ectopic expression of Rubisco subunits in maize mesophyll cells does not overcome barriers to cell type-specific accumulation

Authors

Katia Wostrikoff, Université Pierre et Marie Curie, ParisFollow
Aimee Clark, Boyce Thompson Institute for Plant Research, Ithaca, NYFollow
Shirley Sato, University of Nebraska - LincolnFollow
Thomas E. Clemente, University of Nebraska - LincolnFollow
David Stern, Boyce Thompson Institute for Plant Research, Ithaca, NYFollow

Date of this Version

6-2012

Citation

Plant Physiology Preview. Published on June 28, 2012, as DOI:10.1104/pp.112.195677

Comments

Copyright 2012 by the American Society of Plant Biologists.
OPEN ACCESS ARTICLE Free via Open Access: OA http://www.plantphysiol.org/content/early/2012/06/28/pp.112.195677.full.pdf+html

Abstract

In Zea mays, ribulose bisphosphate carboxylase (Rubisco) accumulates in bundle sheath but not mesophyll chloroplasts, but the mechanisms that underlie cell-type specific expression are poorly understood. To explore the coordinated expression of the chloroplast rbcL gene, which encodes the Rubisco large subunit (LS), and the two nuclear RBCS genes which encode the small subunit (SS), RNAi was used to reduce RBCS expression. This resulted in Rubisco deficiency, and was correlated with translational repression of rbcL. Thus, as in C3 plants, LS synthesis depends on the presence of its assembly partner SS. To test the hypothesis that the previously documented transcriptional repression of RBCS in mesophyll cells is responsible for repressing LS synthesis in mesophyll chloroplasts, a ubiquitin promoter-driven RBCS gene was expressed in both bundle sheath and mesophyll cells. This did not lead to Rubisco accumulation in the mesophyll, suggesting that LS synthesis is impeded even in the presence of ectopic SS expression. To attempt to bypass this putative mechanism, a ubiquitin promoter-driven nuclear version of the rbcL gene was created, encoding an epitope-tagged LS, which was expressed in the presence or absence of the Ubi-RBCS construct. Both transgenes were robustly expressed, and the tagged LS was readily incorporated into Rubisco complexes. However, neither immunolocalization nor biochemical approaches revealed significant accumulation of Rubisco in mesophyll cells, suggesting a continuing cell type-specific impairment of its assembly or stability. We conclude that additional cell type-specific factors limit Rubisco expression to bundle sheath chloroplasts.

Includes supplemental data (Table 1).