Mg‐dechelatase is involved in the formation of photosystem II but not in chlorophyll degradation in Chlamydomonas reinhardtii

Abstract

Summary The STAY‐GREEN (SGR) gene encodes Mg‐dechelatase which catalyzes the conversion of chlorophyll (Chl) a to pheophytin (Pheo) a. This reaction is the first and most important regulatory step in the Chl degradation pathway. Conversely, Pheo a is an indispensable molecule in photosystem (PS) II, suggesting the involvement of SGR in the formation of PSII. To investigate the physiological functions of SGR, we isolated Chlamydomonas sgr mutants by screening an insertion‐mutant library. The sgr mutants had reduced maximum quantum efficiency of PSII (Fv/Fm) and reduced Pheo a levels. These phenotypes were complemented by the introduction of the Chlamydomonas SGR gene. Blue Native polyacrylamide gel electrophoresis and immunoblotting analysis showed that although PSII levels were reduced in the sgr mutants, PSI and light‐harvesting Chl a/b complex levels were unaffected. Under nitrogen starvation conditions, Chl degradation proceeded in the sgr mutants as in the wild type, indicating that ChlamydomonasSGR is not required for Chl degradation and primarily contributes to the formation of PSII. In contrast, in the Arabidopsis sgr triple mutant (sgr1 sgr2 sgrL), which completely lacks SGR activity, PSII was synthesized normally. These results suggest that the Arabidopsis SGR participates in Chl degradation while the ChlamydomonasSGR participates in PSII formation despite having the same catalytic property.

Arabidopsis CHLOROPHYLLASE 1 protects young leaves from long-term photodamage by facilitating FtsH-mediated D1 degradation in photosystem II repair

“…Chl breakdown in senescent leaves begins with the magnesium dechelation of Chl a and the production of Phein a by SGR1/2. Phein a is dephytylated to Pheide a by PPH/CLD1 and further degraded to phyllobilins via the PAO/phyllobilin pathway (Kr€ autler and H€ ortensteiner, 2014;Chen et al, 2019). In green leaves, Chlide a can be further degraded by the PAO/phyllobilin pathway via conversion to Pheide a by SGRL (Chen et al, 2019).…”

“…Phein a is dephytylated to Pheide a by PPH/CLD1 and further degraded to phyllobilins via the PAO/phyllobilin pathway (Kr€ autler and H€ ortensteiner, 2014;Chen et al, 2019). In green leaves, Chlide a can be further degraded by the PAO/phyllobilin pathway via conversion to Pheide a by SGRL (Chen et al, 2019). Chl, chlorophyll; Chlide, chlorophyllide; Phein, pheophytin; Pheide, pheophorbide; pFCC, primary fluorescent Chl catabolite; NCCs, non-fluorescent Chl catabolites; CLH, chlorophyllase; CLD1, Chl dephytylase 1; CHLG, Chl synthase; CAO, Chlide a oxygenase; NYC1, non-yellow coloring 1 (Chl b reductase); HCAR, 7-hydroxymethyl Chl a reductase; PPH, pheophytinase; SGR, STAY-GREEN (Chl a Mgdechelatase); SGRL, STAY-GREEN-LIKE (Chlide/Chl a Mg-dechelatase); PAO, pheophorbide a oxygenase.…”

Roles of stay-green (SGR) homologs during chlorophyll degradation in green plants

“…Remarkably, Phein a was an essential molecule in photosystem (PS) II, so SGRs might affect the formation of PSII. Despite the SGRs in Arabidopsis and Chlamydomonas reinhardtii having the same catalytic property, the physiological functions of SGRs were diverse (Chen et al 2018 ). For example, SGRs in Arabidopsis took part in Chl degradation while SGRs in Chlamydomonas reinhardtii participated in PSII formation (Chen et al 2018 ).…”

“…Despite the SGRs in Arabidopsis and Chlamydomonas reinhardtii having the same catalytic property, the physiological functions of SGRs were diverse (Chen et al 2018 ). For example, SGRs in Arabidopsis took part in Chl degradation while SGRs in Chlamydomonas reinhardtii participated in PSII formation (Chen et al 2018 ). Although the recent studies have demonstrated that SGRs are able to remove Mg from Chl a to initiate its breakdown, little is known about the domain basis of its functionality.…”

The rise and fall of the photoinhibition-related energy dissipation qI

“…Total protein extracts and thylakoids were isolated as described in (Ramundo et al, 2013). Blue native polyacrylamide gel electrophoresis (BN-PAGE) was performed according to (Chen et al, 2019; Wittig et al, 2006). The resolving gel of BN-PAGE used was 4.1-14% T (w/v), while the stacking gel was 3.6% T (w/v), where T stands for the total concentration of acrylamide and bisacrylamide monomers.…”