Thioredoxin h2 and o1 Show Different Subcellular Localizations and Redox-Active Functions, and Are Extrachloroplastic Factors Influencing Photosynthetic Performance in Fluctuating Light

Abstract

Arabidopsis contains eight different h-type thioredoxins (Trx) being distributed in different cell organelles. Although Trx h2 is deemed to be confined to mitochondria, its subcellular localization and function are discussed controversially. Here, cell fractionation studies were used to clarify this question, showing Trx h2 protein to be exclusively localized in microsomes rather than mitochondria. Furthermore, Arabidopsis trxo1, trxh2 and trxo1h2 mutants were analyzed to compare the role of Trx h2 with mitochondrial Trx o1. Under medium light, trxo1 and trxo1h2 showed impaired growth, while trxh2 was similar to wild type. In line with this, trxo1 and trxo1h2 clustered differently from wild type with respect to nocturnal metabolite profiles, revealing a decrease in ascorbate and glutathione redox states. Under fluctuating light, these genotypic differences were attenuated. Instead, the trxo1h2 double mutant showed an improved NADPH redox balance, compared to wild type, accompanied by increased photosynthetic efficiency, specifically in the high-light phases. Conclusively, Trx h2 and Trx o1 are differentially localized in microsomes and mitochondria, respectively, which is associated with different redox-active functions and effects on plant growth in constant light, while there is a joint role of both Trxs in regulating NADPH redox balance and photosynthetic performance in fluctuating light.

Simultaneous adjustments of major mitochondrial pathways through redox regulation of dihydrolipoamide dehydrogenase (mtLPD1)

“…As another possibility we assumed parallel regulatory impact on the three pathways through mtLPD1 via a metabolic signal in response to changes in the redox states of subcellular compartments involving TRX o1 and h2. The latter hypothesis is congruent with alterations in the pyrimidine nucleotide amounts observed previously (Reinholdt et al, 2019;daFonseica-Pereira et al, 2020) and the recent finding of considerable shifts in the redox couples of ascorbate and glutathione in the TRX mutants, especially trxo1/h2 (Calderón et al, 2018;Hou et al, 2021).…”

“…However, an increase in the stoichiometry of photorespiratory CO 2 release was seen in both single TRX mutants during oxygen-dependent gas exchange measurements (Reinholdt et al, 2019a; daFonseica-Pereira et al, 2020). To answer the question, if TRX o1 and h2 are redundant or fulfil specific roles, a mutant bearing a combined deletion of TRX o1 and h2 was generated ( trxo1h2 ; Hou et al, 2021) and specifically analyzed regarding photorespiration as well as mitochondrial metabolism during this study. For this purpose, trxo1h2 was grown next to the wild type in ambient air (400 ppm CO 2 ) to growth stage 5.1 (Boyes et al, 2001) and used for gas exchange measurements.…”

“…TRX o1 and h2 , respectively, were shown to contribute to the regulation of the activation state of mtLPD1 in vitro and, perhaps, in vivo (Reinholdt et al, 2019a; daFonseica-Pereira et al, 2020). However, since TRX h2 was reported to reside to the microsomal fraction rather than to mitochondria (Hou et al, 2021), the impact of TRX h2 deficiency on mtLPD1 is likely indirect, presumably via modulation of the mitochondrial redox state. Interestingly, a study of E. coli LPD revealed its inhibition by NADH at physiological concentrations (Kim et al, 2008), making this molecule a prime candidate for further regulation of mtLPD1 in response to altered mitochondrial pathway fluxes, especially photorespiration.…”

“…It is worth mentioning that similar observations were reported with TRX h2, a member of the largest TRX family in Arabidopsis (daFonseica-Pereira et al, 2020). Although a few reports suggested mitochondrial localization of TRX h2, a recent study revealed its localization to the microsomal fraction (Hou et al, 2021).…”

“…It is worth mentioning that similar observations were reported with TRX h2 , a member of the largest TRX family in Arabidopsis (daFonseica-Pereira et al, 2020). Although a few reports suggested mitochondrial localization of TRX h2 , a recent study revealed its localization to the microsomal fraction (Hou et al, 2021). Nevertheless, there is experimental evidence that TRX h2 deficiency impacts mitochondrial photorespiratory metabolism, while the underlying mechanism remained elusive to date (daFonseica-Pereira et al, 2020; 2021).…”