Alleviation of Photoinhibition by Co-ordination of Chlororespiration and Cyclic Electron Flow Mediated by NDH under Heat Stressed Condition in Tobacco.

Qinghua Li,Zheng-Ju Yao,Hualing Mi

doi:10.3389/fpls.2016.00285

Abstract

With increase of temperature, Fo gradually rose in both WT and the mutant inactivated in the type 1 NAD(P)H dehydrogenase (NDH), a double mutant disrupted the genes of ndhJ and ndhK (ΔndhJK) or a triple mutant disrupted the genes of ndhC, ndhJ, and ndhK (ΔndhCJK). The temperature threshold of Fo rise was about 3–5°C lower in the mutants than in WT, indicating ΔndhJK and ΔndhCJK were more sensitive to elevated temperature. The Fo rise after the threshold was slower and the reached maximal level was lower in the mutants than in WT, implying the chlororespiratory pathway was suppressed when NDH was inactivated. Meanwhile, the maximum quantum efficiency of photosystem II (PS II) (Fv/Fm) decreased to a similar extent below 50°C in WT and mutants. However, the decline was sharper in WT when temperature rose above 55°C, indicating a down regulation of PS II photochemical activity by the chlororespiratory pathway in response to elevated temperature. On the other hand, in the presence of n-propyl gallate, an inhibitor of plastid terminal oxidase (PTOX), the less evident increase in Fo while the more decrease in Fv/Fm in ΔndhCJK than in WT after incubation at 50°C for 6 h suggest the increased sensitivity to heat stress when both NDH and chlororespiratory pathways are suppressed. Moreover, the net photosynthetic rate and photo-efficiency decreased more significantly in ΔndhJK than in WT under the heat stressed conditions. Compared to the light-oxidation of P700, the difference in the dark-reduction of P700+ between WT and ndhJK disruptant was much less under the heat stressed conditions, implying significantly enhanced cyclic electron flow in light and the competition for electron from PQ between PTOX and photosystem I in the dark at the elevated temperature. Heat-stimulated expression of both NdhK and PTOX significantly increased in WT, while the expression of PTOX was less in ΔndhJK than in WT. Meanwhile, the amount of active form of Rubisco activase decreased much more in the mutant. The results suggest that chlororespiration and cyclic electron flow mediated by NDH may coordinate to alleviate the over-reduction of stroma, thus to keep operation of CO2 assimilation at certain extent under heat stress condition.

Highlights

The concept of chlororespiration was used to describe the respiratory electron transport pathway within the chloroplast (Bennoun, 1982)
Further evidence supporting the operation of chlororespiration includes: (1) genes with high sequence homology to those encoding the subunits of mitochondrial complex I have been found in chloroplasts (Ohyama et al, 1986; Shinozaki et al, 1986); the ndh genes encode at least 15 subunits (Ndh-A-O), among which NdhA–NdhK are plastid-encoded and the rest (NdhL–NdhO) are nuclear-encoded (Ifuku et al, 2011), and (2) a protein designated plastid terminal oxidase (PTOX) with sequence homology to alternative oxidases of plant mitochondria has been identified in chloroplasts of Arabidopsis thaliana (Carol et al, 1999; Wu et al, 1999)
The rise of Fo at the elevated temperature was not attributed to the detachment of LHC photosystem II (II) from photosystem II (PS II) complex and the inactivation of PS II photochemical reaction (Schreiber and Armond, 1978), because the photochemical efficiency of PS II (Fv/Fm) did not significantly decrease after exposing wild type (WT) and ndhKJ or ndhCKJ to 50◦C (Yao et al, 2001; Figure 1C)

Summary

Introduction

The concept of chlororespiration was used to describe the respiratory electron transport pathway within the chloroplast (Bennoun, 1982). Evidence shows that chlororespiration protects plant against environmental stresses such as heat or high light conditions (Quiles, 2006; Diaz et al, 2007), chilling stress (Ivanov et al, 2012; Segura and Quiles, 2015), drought stress (Ibanez et al, 2010; Paredes and Jose Quiles, 2013). Both the cyclic electron flow around PS I and chlororespiration function during photosynthesis under changing environmental conditions (Rumeau et al, 2007). How the cyclic electron flow around PS I and chlororespiration are coordinated in their protective roles still remains to be further investigated

Methods

Results

Conclusion