Redox regulation of PGRL1 at the onset of low light intensity.

PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) regulates photosystem I cyclic electron flow which transiently activates non-photochemical quenching at the onset of light. Here, we show that a disulfide-based mechanism of PGRL1 regulated this process in vivo at the onset of low light levels. We found that PGRL1 regulation depended on active formation of key regulatory disulfides in the dark, and that PGR5 was required for this activity. The disulfide state of PGRL1 was modulated in plants by counteracting reductive and oxidative components and reached a balanced state that depended on the light level. We propose that the redox regulation of PGRL1 fine-tunes a timely activation of photosynthesis at the onset of low light.

Artificial intelligence enabled preliminary diagnosis for COVID-19 from voice cues and questionnaires.

The COVID-19 outbreak was announced as a global pandemic by the World Health Organization in March 2020 and has affected a growing number of people in the past few months. In this context, advanced artificial intelligence techniques are brought to the forefront as a response to the ongoing fight toward reducing the impact of this global health crisis. In this study, potential use-cases of intelligent speech analysis for COVID-19 identification are being developed. By analyzing speech recordings from COVID-19 positive and negative patients, we constructed audio- and symptomatic-based models to automatically categorize the health state of patients, whether they are COVID-19 positive or not. For this purpose, many acoustic features were established, and various machine learning algorithms are being utilized. Experiments show that an average accuracy of 80% was obtained estimating COVID-19 positive or negative, derived from multiple cough and vowel /a/ recordings, and an average accuracy of 83% was obtained estimating COVID-19 positive or negative patients by evaluating six symptomatic questions. We hope that this study can foster an extremely fast, low-cost, and convenient way to automatically detect the COVID-19 disease.

Knockdown of the Arabidopsis thaliana chloroplast protein disulfide isomerase 6 results in reduced levels of photoinhibition and increased D1 synthesis in high light.

A chloroplast protein disulfide isomerase (PDI) was previously proposed to regulate translation of the unicellular green alga Chlamydomonas reinhardtii chloroplast psbA mRNA, encoding the D1 protein, in response to light. Here we show that AtPDI6, one of 13 Arabidopsis thaliana PDI genes, also plays a role in the chloroplast. We found that AtPDI6 is targeted and localized to the chloroplast. Interestingly, AtPDI6 knockdown plants displayed higher resistance to photoinhibition than wild-type plants when exposed to a tenfold increase in light intensity. The AtPDI6 knockdown plants also displayed a higher rate of D1 synthesis under a similar light intensity. The increased resistance to photoinhibition may not be rationalized by changes in antenna or non-photochemical quenching. Thus, the increased D1 synthesis rate, which may result in a larger proportion of active D1 under light stress, may led to the decrease in photoinhibition. These results suggest that, although the D1 synthesis rates observed in wild-type plants under high light intensities are elevated, repair can potentially occur faster. The findings implicate AtPDI6 as an attenuator of D1 synthesis, modulating photoinhibition in a light-regulated manner.

A chloroplast light-regulated oxidative sensor for moderate light intensity in Arabidopsis.

The transition from dark to light involves marked changes in the redox reactions of photosynthetic electron transport and in chloroplast stromal enzyme activity even under mild light and growth conditions. Thus, it is not surprising that redox regulation is used to dynamically adjust and coordinate the stromal and thylakoid compartments. While oxidation of regulatory proteins is necessary for the regulation, the identity and the mechanism of action of the oxidizing pathway are still unresolved. Here, we studied the oxidation of a thylakoid-associated atypical thioredoxin-type protein, ACHT1, in the Arabidopsis thaliana chloroplast. We found that after a brief period of net reduction in plants illuminated with moderate light intensity, a significant oxidation reaction of ACHT1 arises and counterbalances its reduction. Interestingly, ACHT1 oxidation is driven by 2-Cys peroxiredoxin (Prx), which in turn eliminates peroxides. The ACHT1 and 2-Cys Prx reaction characteristics in plants further indicated that ACHT1 oxidation is linked with changes in the photosynthetic production of peroxides. Our findings that plants with altered redox poise of the ACHT1 and 2-Cys Prx pathway show higher nonphotochemical quenching and lower photosynthetic electron transport infer a feedback regulatory role for this pathway.

A small family of chloroplast atypical thioredoxins.

The reduction and the formation of regulatory disulfide bonds serve as a key signaling element in chloroplasts. Members of the thioredoxin (Trx) superfamily of oxidoreductases play a major role in these processes. We have characterized a small family of plant-specific Trxs in Arabidopsis (Arabidopsis thaliana) that are rich in cysteine and histidine residues and are typified by a variable noncanonical redox active site. We found that the redox midpoint potential of three selected family members is significantly less reducing than that of the classic Trxs. Assays of subcellular localization demonstrated that all proteins are localized to the chloroplast. Selected members showed high activity, contingent on a dithiol electron donor, toward the chloroplast 2-cysteine peroxiredoxin A and poor activity toward the chloroplast NADP-malate dehydrogenase. The expression profile of the family members suggests that they have distinct roles. The intermediate redox midpoint potential value of the atypical Trxs might imply adaptability to function in modulating the redox state of chloroplast proteins with regulatory disulfides.

Dual targeting of the protein disulfide isomerase RB60 to the chloroplast and the endoplasmic reticulum.

RB60 is an atypical protein disulfide isomerase (PDI) that functions as a member of a redox regulatory protein complex controlling translation in the chloroplast of Chlamydomonas reinhardtii, but also contains a C-terminal endoplasmic reticulum (ER) retention signal, -KDEL. Here, we show by fluorescence microscopy that RB60 resides in the chloroplast but also outside of the chloroplast colocalized with BiP, an ER marker protein. RB60 accumulates in microsomes that exhibit a typical ER magnesium-shift, and cotranslationally translocates into ER microsomes. The first 50-aa leader of RB60 is sufficient for both chloroplast and ER targeting. The leader is cleaved upon translocation into the ER, whereas it remains intact after import to the chloroplast. The leader sequence also contains an acidic domain that appears necessary for the protein's association with the thylakoid membranes. Based on these and additional results, we propose that the dual localization of RB60 occurs via the two conserved transport mechanisms, to the chloroplast and to the ER, that the chloroplast RB60 most likely carries an additional function in the ER, and that its mode of transport, including the differential cleavage of its N terminus, plays an important role in its suborganellar localization and organellar-specific function.