The Arabidopsis Spontaneous Cell Death1 gene, encoding a ζ-carotene desaturase essential for carotenoid biosynthesis, is involved in chloroplast development, photoprotection and retrograde signalling

Carotenoids, a class of natural pigments found in all photosynthetic organisms, are involved in a variety of physiologi-cal processes, including coloration, photoprotection, biosynthesis of abscisic acid (ABA) and chloroplast biogenesis.Although carotenoid biosynthesis has been well studied biochemically, the genetic basis of the pathway is not wellunderstood. Here, we report the characterization of two allelic Arabidopsis mutants, spontaneous cell death 1-1 (spcl-1)and spcl-2. The weak allele spcl-1 mutant showed characteristics of bleached leaves, accumulation of superoxide andmosaic cell death. The strong mutant allele spcl-2 caused a complete arrest of plant growth and development shortlyafter germination, leading to a seedling-lethal phenotype. Genetic and molecular analyses indicated that SPC1 encodesa putative ζ-carotene desaturase (ZDS) in the carotenoid biosynthesis pathway. Analysis of carotenoids revealed thatseveral major carotenoid compounds downstream of SPC1/ZDS were substantially reduced in spcl-1, suggesting thatSPC1 is a functional ZDS. Consistent with the downregulated expression of CAO and PORB, the chlorophyll contentwas decreased in spcl-1 plants. In addition, expression of Lhcb1.1, Lhcb1.4 and RbcS was absent in spcl-2, suggestingthe possible involvement of carotenoids in the plastid-to-nucleus retrograde signaling. The spcl-1 mutant also displaysan ABA-deficient phenotype that can be partially rescued by the externally supplied phytohormone. These results suggestthat SPC1/ZDS is essential for biosynthesis of carotenoids and plays a crucial role in plant growth and development.

Decreased glutathione reductase2 leads to early leaf senescence in Arabidopsis

Glutathione reductase（GR） catalyzes the reduction of glutathione disulfide（GSSG） to reduced glutathione（GSH）and participates in the ascorbate-glutathione cycle, which scavenges H_2O_2. Here, we report that chloroplastic/mitochondrial GR2 is an important regulator of leaf senescence. Seed development of the homozygous gr2 knockout mutant was blocked at the globular stage. Therefore, to investigate the function of GR2 in leaf senescence, we generated transgenic Arabidopsis plants with decreased GR2 using RNAi. The GR2 RNAi plants displayed early onset of age-dependent and darkand H2O2-induced leaf senescence, which was accompanied by the induction of the senescence-related marker genes SAG12 and SAG13. Furthermore, transcriptome analysis revealed that genes related to leaf senescence, oxidative stress, and phytohormone pathways were upregulated directly before senescence in RNAi plants. In addition, H2O2 accumulated to higher levels in RNAi plants than in wild-type plants and the levels of H_2O_2 peaked in RNAi plants directly before the early onset of leaf senescence. RNAi plants showed a greater decrease in GSH/GSSG levels than wild-type plants during leaf development. Our results suggest that GR2 plays an important role in leaf senescence by modulating H_2O_2 and glutathione signaling in Arabidopsis.

mTERF5 Acts as a Transcriptional Pausing Factor to Positively Regulate Transcription of Chloroplast psbEFLJ

RNA polymerase transcriptional pausing represents a major checkpoint in transcription in bacteria and metazoans,but it is unknown whether this phenomenon occurs in plant organelles.Here,we report that transcriptional pausing occurs in chloroplasts.We found that mTERF5 specifically and positively regulates the transcription of chloroplast psbEFLJ in Arabidopsis thaliana that encodes four key subunits of photosystem II.We found that mTERF5 causes the plastid-encoded RNA polymerase(PEP)complex to pause at psbEFLJ by binding to the+30 to+51 region of double-stranded DNA.Moreover,we revealed that mTERF5 interacts with pTAC6,an essential subunit of the PEP complex,although pTAC6 is not involved in the transcriptional pausing at psbEFLJ.We showed that mTERF5 recruits additional pTAC6 to the transcriptionally paused region of psbEFLJ,and the recruited pTAC6 proteins could be assembled into the PEP complex to regulate psbEFLJ transcription.Taken together,our findings shed light on the role of transcriptional pausing in chloroplast transcription in plants.

The Phytol Phosphorylation Pathway Is Essential for the Biosynthesis of Phylloquinone, which Is Required for Photosystem I Stability in Arabidopsis

Phytyl-diphosphate, which provides phytyl moieties as a common substrate in both tocopherol and phyllo- quinone biosynthesis, derives from de novo isoprenoid biosynthesis or a salvage pathway via phytol phos- phorylation. However, very little is known about the role and origin of the phytyl moiety for phylloquinone biosynthesis. Since VTE6, a phytyl-phosphate kinase, is a key enzyme for phytol phosphorylation, we char- acterized Arabidopsis vte6 mutants to gain insight into the roles of phytyl moieties in phylloquinone biosyn- thesis and of phylloquinone in photosystem I （PSI） biogenesis. The VTE6 knockout mutants vte6-1 and vte6-2 lacked detectable phylloquinone, whereas the phylloquinone content in the VTE6 knockdown mutant vte6-3 was 90% lower than that in wild-type. In vte6 mutants, PSI function was impaired and accu- mulation of the PSI complex was defective. The PSI core subunits PsaA/B were efficiently synthesized and assembled into the PSI complex in vte6-3. However, the degradation rate of PSI subunits in the assembled PSI complex was more rapid in vte6-3 than in wild-type. In vte6-3, PSI was more susceptible to high-light damage than in wild-type. Our results provide the first genetic evidence that the phytol phosphorylation pathway is essential for phylloquinone biosynthesis, and that phylloquinone is required for PSI complex stability.

Autophagy targets Hd1 for vacuolar degradation to regulate rice flowering

Flowering time(heading date)is a critical agronomic trait that determines the yield and regional adaptability of crops.Heading date 1(Hd1)is a central regulator of photoperiodic flowering in rice(Oryza sativa).However,how the homeostasis of Hd1 protein is achieved is poorly understood.Here,we report that the nuclear autophagy pathway mediates Hd1 degradation in the dark to regulate flowering.Loss of autophagy function results in an accumulation of Hd1 and delays flowering under both short-day and long-day conditions.In the dark,nucleus-localized Hd1 is recognized as a substrate for autophagy and is subjected to vacuolar degradation via the autophagy protein OsATG8.The Hd1-0sATG8 interaction is required for autophagic degradation of Hd1 in the dark.Our study reveals a new mechanism by which Hd1 protein homeostasis is regulated by autophagy to control rice flowering.Our study also indicates that the regulation of flowering by autophagic degradation of Hd1 orthologs may have arisen over the course of mesangiosperm evolution,which would have increased their flexibility and adaptability to the environment by modulating flowering time.