Physiological performance of transplastomic tobacco plants overexpressing aquaporin AQP1 in chloroplast membranes.

Alicia Fernández-San Millán,Miquel Nadal,María Ancín,Inmaculada Farran,Jon Veramendi,Cyril Douthe,Luis Larraya,Iker Aranjuelo,Jaume Flexas

doi:10.1093/jxb/ery148

Abstract

The leaf mesophyll CO2 conductance and the concentration of CO2 within the chloroplast are major factors affecting photosynthetic performance. Previous studies have shown that the aquaporin NtAQP1 (which localizes to the plasma membrane and chloroplast inner envelope membrane) is involved in CO2 permeability in the chloroplast. Levels of NtAQP1 in plants genetically engineered to overexpress the protein correlated positively with leaf mesophyll CO2 conductance and photosynthetic rate. In these studies, the nuclear transformation method used led to changes in NtAQP1 levels in the plasma membrane and the chloroplast inner envelope membrane. In the present work, NtAQP1 levels were increased up to 16-fold in the chloroplast membranes alone by the overexpression of NtAQP1 from the plastid genome. Despite the high NtAQP1 levels achieved, transplastomic plants showed lower photosynthetic rates than wild-type plants. This result was associated with lower Rubisco maximum carboxylation rate and ribulose 1,5-bisphosphate regeneration. Transplastomic plants showed reduced mesophyll CO2 conductance but no changes in chloroplast CO2 concentration. The absence of differences in chloroplast CO2 concentration was associated with the lower CO2 fixation activity of the transplastomic plants. These findings suggest that non-functional pores of recombinant NtAQP1 may be produced in the chloroplast inner envelope membrane.

Highlights

It is predicted that future increases in the human population will require a 30% increase in crop yield rates (Edgerton, 2009)
Generation of tobacco transplastomic plants and determination of homoplasmy Tobacco plants expressing NtAQP1 from the plastid genome were obtained by biolistic bombardment of the leaves with the engineered pAF vector (Fernández-San Millán et al, 2008), which inserted the transgenes between the trnI and trnA regions of the plastid genome (Fig. 1A)
The present study shows that NtAQP1 can be overexpressed from the plastid genome and that it localizes to the chloroplast membranes

Summary

Introduction

It is predicted that future increases in the human population will require a 30% increase in crop yield rates (Edgerton, 2009). Other studies have shown that the gm can change quickly in response to varying environmental conditions, such as leaf temperature (Bernacchi et al, 2002), water stress (Galmés et al, 2007), blue light (Loreto et al, 2009), and the external CO2 concentration (Flexas et al, 2007a). This rapid modification of gm points to the existence of additional components, some of them probably proteins, controlling the conductance of the mesophyll to CO2 diffusion. Proteins forming pore-like structures, such as aquaporins (AQPs), might help explain how these rapid variations in gm occur

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