More Like this

1. Editorial: Structure and function of chloroplasts, Volume III

   https://doi.org/10.3389/fpls.2023.1145680  

   Gao, Hongbo; McCormick, Alistair J.; Roston, Rebecca L.; Lu, Yan (March 2023, Frontiers in Plant Science)

   Chloroplasts are endosymbiotic organelles derived from cyanobacteria. They have a double envelope membrane, including the outer envelope and the inner envelope. A complex membrane system, thylakoids, exists inside the chloroplast. It is the site of the light-dependent reactions of photosynthesis. The stroma is the main site of the carbon fixation reactions. Although photosynthesis is a very complicated process with many proteins involved, there are many other important processes that occur in chloroplasts, including the regulation of photosynthesis, the biogenesis and maintenance of the structures, carbohydrate, lipid, tetrapyrrole, amino acid, and isoprenoid metabolism, production of some phytohormones, production of specialized metabolites, regulation of redox, and interactions with other parts of the cell (Sabater, 2018). During evolution, most of the cyanobacterial genes were lost and many of them were transferred into the nuclear genome. A majority of chloroplast proteins are nuclear-encoded and possess an N-terminal transit peptide which helps the protein to be targeted into chloroplasts. Chloroplasts have their own highly reduced genome which works coordinately with the nuclear genome for the biogenesis and function of chloroplasts (Liebers et al., 2022). This Research Topic presents studies covering different aspects of chloroplast function, including photosynthesis, biogenesis, structure, and maintenance. These works push the frontiers of chloroplast research further in the field of plant biology. 

   more » « less
2. Correlated retrograde and developmental regulons implicate multiple retrograde signals as coordinators of chloroplast development in maize

   https://doi.org/10.1093/plcell/koac276  

   Kendrick, Rennie; Chotewutmontri, Prakitchai; Belcher, Susan; Barkan, Alice (September 2022, The Plant Cell)

   Abstract Signals emanating from chloroplasts influence nuclear gene expression, but roles of retrograde signals during chloroplast development are unclear. To address this gap, we analyzed transcriptomes of non-photosynthetic maize mutants and compared them to transcriptomes of stages of normal leaf development. The transcriptomes of two albino mutants lacking plastid ribosomes resembled transcriptomes at very early stages of normal leaf development, whereas the transcriptomes of two chlorotic mutants with thylakoid targeting or plastid transcription defects resembled those at a slightly later stage. We identified ∼2,700 differentially expressed genes, which fall into six major categories based on the polarity and mutant-specificity of the change. Downregulated genes were generally expressed late in normal development and were enriched in photosynthesis genes, whereas upregulated genes act early and were enriched for functions in chloroplast biogenesis and cytosolic translation. We showed further that target-of-rapamycin (TOR) signaling was elevated in mutants lacking plastid ribosomes and declined in concert with plastid ribosome buildup during normal leaf development. Our results implicate three plastid signals as coordinators of photosynthetic differentiation. One signal requires plastid ribosomes and activates photosynthesis genes. A second signal reflects attainment of chloroplast maturity and represses chloroplast biogenesis genes. A third signal, the consumption of nutrients by developing chloroplasts, represses TOR, promoting termination of cell proliferation during leaf development. 

   more » « less
3. Anterograde signaling controls plastid transcription via sigma factors separately from nuclear photosynthesis genes

   https://doi.org/10.1038/s41467-022-35080-0  

   Hwang, Youra; Han, Soeun; Yoo, Chan Yul; Hong, Liu; You, Chenjiang; Le, Brandon H.; Shi, Hui; Zhong, Shangwei; Hoecker, Ute; Chen, Xuemei; et al (December 2022, Nature Communications)

   Abstract Light initiates chloroplast biogenesis inArabidopsisby eliminating PHYTOCHROME-INTERACTING transcription FACTORs (PIFs), which in turn de-represses nuclear photosynthesis genes, and synchronously, generates a nucleus-to-plastid (anterograde) signal that activates the plastid-encoded bacterial-type RNA polymerase (PEP) to transcribe plastid photosynthesis genes. However, the identity of the anterograde signal remains frustratingly elusive. The main challenge has been the difficulty to distinguish regulators from the plethora of necessary components for plastid transcription and other essential chloroplast functions, such as photosynthesis. Here, we show that the genome-wide induction of nuclear photosynthesis genes is insufficient to activate the PEP. PEP inhibition is imposed redundantly by multiple PIFs and requires PIF3’s activator activity. Among the nuclear-encoded components of the PEP holoenzyme, we identify four light-inducible, PIF-repressed sigma factors as anterograde signals. Together, our results elucidate that light-dependent inhibition of PIFs activates plastid photosynthesis genes via sigma factors as anterograde signals in parallel with the induction of nuclear photosynthesis genes. 

   more » « less
4. Phosphorylation at serine-260 of Toc33 is essential for chloroplast biogenesis

   https://doi.org/10.1126/sciadv.adu4054  

   Chien, Yuan-Chi; Yoon, Gyeong Mee (March 2025, Science Advances)

   Chloroplast biogenesis, essential for photosynthesis, depends on the import of nuclear-encoded proteins through the translocon at the outer envelope of chloroplasts (TOC) complexes. Despite its importance, the mechanisms regulating this process remain largely elusive. We identify serine-260 (S260) as a critical phosphorylation site in Toc33, a core TOC component. This phosphorylation stabilizes Toc33 by preventing its ubiquitination and degradation. Constitutive triple response 1 (CTR1), a negative regulator of ethylene signaling, and its paralog RAF-like kinase are involved in phosphorylating Toc33. Disruption of Toc33 phosphorylation impairs its stability and photosynthetic protein import, consequently affecting chloroplast structural integrity and biogenesis. Our findings underscore the essential role of TOC phosphorylation in chloroplast biogenesis and reveal an unexpected regulatory network involving RAF-like kinases in organelle development. 

   more » « less
5. Membrane association of active genes organizes the chloroplast nucleoid structure

   https://doi.org/10.1073/pnas.2309244121  

   Palomar, V Miguel; Cho, Yoonjin; Fujii, Sho; Rothi, M Hafiz; Jaksich, Sarah; Min, Ji-Hee; Schlachter, Adriana N; Wang, Joyful; Liu, Zhengde; Wierzbicki, Andrzej T (July 2024, Proceedings of the National Academy of Sciences)

   DNA is organized into chromatin-like structures that support the maintenance and regulation of genomes. A unique and poorly understood form of DNA organization exists in chloroplasts, which are organelles of endosymbiotic origin responsible for photosynthesis. Chloroplast genomes, together with associated proteins, form membrane-less structures known as nucleoids. The internal arrangement of the nucleoid, molecular mechanisms of DNA organization, and connections between nucleoid structure and gene expression remain mostly unknown. We show thatArabidopsis thalianachloroplast nucleoids have a unique sequence-specific organization driven by DNA binding to the thylakoid membranes. DNA associated with the membranes has high protein occupancy, has reduced DNA accessibility, and is highly transcribed. In contrast, genes with low levels of transcription are further away from the membranes, have lower protein occupancy, and have higher DNA accessibility. Membrane association of active genes relies on the pattern of transcription and proper chloroplast development. We propose a speculative model that transcription organizes the chloroplast nucleoid into a transcriptionally active membrane-associated core and a less active periphery. 

   more » « less