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Review
, 38 (5), 390-5

The Divergent Roles of STAYGREEN (SGR) Homologs in Chlorophyll Degradation

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Review

The Divergent Roles of STAYGREEN (SGR) Homologs in Chlorophyll Degradation

Yasuhito Sakuraba et al. Mol Cells.

Abstract

Degradation of chlorophyll (Chl) by Chl catabolic enzymes (CCEs) causes the loss of green color that typically occurs during senescence of leaves. In addition to CCEs, staygreen1 (SGR1) functions as a key regulator of Chl degradation. Although sgr1 mutants in many plant species exhibit a stay-green phenotype, the biochemical function of the SGR1 protein remains elusive. Many recent studies have examined the physiological and molecular roles of SGR1 and its homologs (SGR2 and SGR-LIKE) in Chl metabolism, finding that these proteins have different roles in different species. In this review, we summarize the recent studies on SGR and discuss the most likely functions of SGR homologs.

Keywords: SGR-LIKE (SGRL); STAYGREEN (SGR); abiotic stress; chlorophyll catabolic enzymes; chlorophyll degradation.

Figures

Fig. 1.
Fig. 1.
Phylogenetic tree and domain structure of SGR homologs in plants. (A) The phylogenic tree was constructed using MEGA 5.1 (http://www.megasoftware.net/megamac.php). The evolutionary relationship was inferred using the Neighbor-joining method. Numbers at branch points represent bootstrap values of 100 replicate trees. The accession numbers of SGRs and SGRLs in the GenBank, AGI (Arabidopsis Gene Index) or TGI (The Gene Index; TC) are as follows: At SGR1 (Arabidopsis thaliana), At4g22920; At SGR2, At4g11910; At SGRL, At1g44000; Ca SGR (pepper, Capsicum annuum), EU414631, AAY98500.1, ACB56586; Os SGR (rice, Oryza sativa), Os09g36200, AY850134; Os SGRL, Os04g59610, AK105982; Gm SGR1 (soybean, Glycine max), AY850141; Gm SGR2, AY850142; Gm SGRL, TC216309; Mt SGR (Medicago truncatula), BF633258; Mt SGRL, TC182595+GD185310; Nt SGR (tobacco, Nicotiana tabacum), ABY19382; Nt SGRL, TC129900; Sl SGR (tomato, Solanum lycopersicum), DQ100158; SI SGRL, TC118764; Zm SGR1 (maize, Zea mays), AY850136; Zm SGR2, AY850137; Zm SGRL, TC503941. (B) Domain structure of Arabidopsis and rice SGR homologs. Chloroplast transit peptides (Red line), the conserved SGR domains (Blue line), and the variable C-terminal regions (Green line) are shown.
Fig. 2.
Fig. 2.
Divergent functions of Arabidopsis SGR1, SGR2, and SGRL proteins. In presenescent leaves during vegetative growth, SGRL and NOL transcripts are much abundant than SGR1 and NYC1 transcripts. Thus, under abiotic/biotic stress conditions, SGRL and NOL are probably the main components of the SGRL-CCEs-LHCII complex. During leaf senescence, SGR1 forms a dynamic, multi-protein complex with CCEs on LHCII to activate Chl degradation. However, SGR2 negatively regulates Chl degradation. Like SGR1 and SGRL, SGR2 can interact with LHCII. However, SGR2 does not interact with most CCEs. Thus, SGR2 likely interrupts the formation of the SGR1-CCE or SGRL-CCE complexes, leading to adjustment of Chl degradation rate in senescing chloroplasts. SGR, STAYGREEN; SGRL, SGR-LIKE; NYC1, NON-YELLOW COLORING1; NOL, NYC1-LIKE; Chl, chlorophyll; CCE, Chl catabolic enzyme; LHCII, light-harvesting complex II.
Fig. 3.
Fig. 3.
Distinct responses of three SGR overexpressors to salt stress. WT (Col-0) and overexpressors of SGR1 (SGR1-OX), SGR2 (SGR2-OX), and SGRL (SGRL-OX) were grown for 3 weeks under long-day conditions (16-hour light/day), and then 200 mM NaCl solution was supplied for 10 days. SGR, STAYGREEN; DST, days of salt treatment.

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