Chloroplast thioredoxin systems dynamically regulate photosynthesis in plants.

Lauri Nikkanen,Eevi Rintamäki

doi:10.1042/bcj20180707

Abstract

Photosynthesis is a highly regulated process in photoautotrophic cells. The main goal of the regulation is to keep the basic photosynthetic reactions, i.e. capturing light energy, conversion into chemical energy and production of carbohydrates, in balance. The rationale behind the evolution of strong regulation mechanisms is to keep photosynthesis functional under all conditions encountered by sessile plants during their lifetimes. The regulatory mechanisms may, however, also impair photosynthetic efficiency by overriding the photosynthetic reactions in controlled environments like crop fields or bioreactors, where light energy could be used for production of sugars instead of dissipation as heat and down-regulation of carbon fixation. The plant chloroplast has a high number of regulatory proteins called thioredoxins (TRX), which control the function of chloroplasts from biogenesis and assembly of chloroplast machinery to light and carbon fixation reactions as well as photoprotective mechanisms. Here, we review the current knowledge of regulation of photosynthesis by chloroplast TRXs and assess the prospect of improving plant photosynthetic efficiency by modification of chloroplast thioredoxin systems.

Highlights

In nature, light intensity is constantly changing in plant growth habitats, including both seasonal alteration of daily light period and daily fluctuation of light intensity due to cloudiness and other environmental factors
NADPH-dependent chloroplast thioredoxin reductase (NTRC) has been found to be an inefficient reductant of TRX targets in the CBB cycle in comparison with TRXf and TRXm in vitro, but in vivo, deficiency of NTRC impairs the reduction in the enzymes in CBB cycle, photosynthetic electron transfer and leaf growth to much greater extent than deficiency of TRXf or TRXm [26,28,29,30,34]. These studies demonstrate that NTRC has a specific function in the regulatory network of the chloroplast that cannot be compensated by other TRXs
NTRC overexpression enhances reduction in the PQ pool in darkness, increases the magnitude of pmf and photosystem I (PSI) yield in comparison with WT in low light and upon increases in light intensity, and enhances the acidification of the lumen under all light intensities in an NADH dehydrogenase-like complex (NDH)-dependent manner [12]. These observations suggest that in WT plants, NTRC-dependent activation of NDH at dark–light and under fluctuating light releases redox pressure in thylakoid membranes by inducing dissipation of light energy as heat by non-photochemical quenching (NPQ) until the CBB cycle is ready to exploit the electrons in carbon fixation

Summary

Introduction

Light intensity is constantly changing in plant growth habitats, including both seasonal alteration of daily light period and daily fluctuation of light intensity due to cloudiness and other environmental factors.

Results

Conclusion