Proteins affecting thylakoid morphology - the key to understanding vesicle transport in chloroplasts?

Structural and mechanistic basis for the regulation of the chloroplast signal recognition particle by (p)ppGpp

The alarmones (p)ppGpp play a critical role in chloroplasts by acting as signalling molecules that regulate gene expression, protein synthesis and chloroplast (cp) development, particularly in response to stress and nutrient availability. However, the underlying molecular mechanisms are still poorly understood. Here, we show that (p)ppGpp binds to the GTPase-containing NG domains of the chloroplast signal recognition particle (SRP) and its receptor, preventing their GTP-dependent association through a competitive mechanism. The structure of (cp)FtsY bound to ppGpp reveals that the alarmone employs the same binding mode as its GDP counterpart and hinders chloroplast SRP:FtsY complex formation via its pyrophosphate moiety. Consequently, (p)ppGpp also inhibits the mutual stimulation of the two GTPases present in the (cp)SRP54:FtsY complex. Taken together, our findings provide the first description of how the alarmones (p)ppGpp may regulate the SRP-dependent protein trafficking pathway in plants.

Overexpression of RtSYP121 confers cadmium colerance by promoting vesicle trafficking, maintaining ion homeostasis, and alleviating photosynthetic inhibition in Arabidopsis

Cadmium (Cd) is a toxic heavy metal in soil that seriously threatens crop production, food security, and human health. Syntaxins, a prototype family of Soluble N-ethyl-maleimide-associated protein receptors (SNAREs) involved in vesicle trafficking, are implicated in resistance to abiotic stresses, including Cd stress, but the molecular mechanisms underlying the involvement of syntaxins in Cd tolerance in plants are unclear. In this study, we isolated and functionally characterized the syntaxin gene RtSYP121 from Reaumuria trigyna to evaluate its potential for phytoremediation. RtSYP121 resides in the plasma membrane. The transcriptional level of RtSYP121 was strongly increased by salt, drought, and Cd stress. Overexpression of RtSYP121 significantly enhanced the Cd tolerance of transgenic Arabidopsis. The Cd tolerance of transgenic plants mainly depended on elevated vesicle trafficking, which increased the content of K⁺ and Ca²⁺ and thus decreased the accumulation of Cd²⁺ by regulating the delivery or activity of ion transporters, channels, and pumps. Moreover, overexpression of RtSYP121 in Arabidopsis ameliorated Cd stress-induced phytotoxic effects, including growth inhibition, ROS burst, photosynthetic impairment, and cell death. Therefore, we suggest that RtSYP121 plays multiple roles in the plant response to Cd stress by promoting vesicle trafficking, maintaining ion homeostasis, and alleviating photosynthetic inhibition.

A Chromosome-level Assembly of A Wild Castor Genome Provides New Insights into the Adaptive Evolution in A Tropical Desert

Wild castor grows in the high-altitude tropical desert of the African Plateau, a region known for high ultraviolet radiation, strong light, and extremely dry condition. To investigate the potential genetic basis of adaptation to both highland and tropical deserts, we generated a chromosome-level genome sequence assembly of the wild castor accession WT05, with a genome size of 316 Mb, a scaffold N50 of 31.93 Mb, and a contig N50 of 8.96 Mb, respectively. Compared with cultivated castor and other Euphorbiaceae species, the wild castor exhibits positive selection and gene family expansion for genes involved in DNA repair, photosynthesis, and abiotic stress responses. Genetic variations associated with positive selection were identified in several key genes, such as LIG1, DDB2, and RECG1, involved in nucleotide excision repair. Moreover, a study of genomic diversity among wild and cultivated accessions revealed genomic regions containing selection signatures associated with the adaptation to extreme environments. The identification of the genes and alleles with selection signatures provides insights into the genetic mechanisms underlying the adaptation of wild castor to the high-altitude tropical desert and would facilitate direct improvement of modern castor varieties.

Chloroplast vesicle transport

Photosynthesis is a well-known process that has been intensively investigated, but less is known about the biogenesis of the thylakoid membrane that harbors the photosynthetic machinery. Thylakoid membranes are constituted by several components, the major ones being proteins and lipids. However, neither of these two are produced in the thylakoid membranes themselves but are targeted there by different mechanisms. The interior of the chloroplast, the stroma, is an aqueous compartment that prevents spontaneous transport of single lipids and/or membrane proteins due to their hydrophobicities. Thylakoid targeted proteins are encoded either in the nucleus or plastid, and thus some cross the envelope membrane before entering one of the identified thylakoid targeting pathways. However, the pathway for all thylakoid proteins is not known. Lipids are produced at the envelope membrane and have been proposed to reach the thylakoid membrane by different means: invaginations of the envelope membrane, direct contact sites between these membranes, or through vesicles. Vesicles have been observed in chloroplasts but not much is yet known about the mechanism or regulation of their formation. The question of whether proteins can also make use of vesicles as one mechanism of transport remains to be answered. Here we discuss the presence of vesicles in chloroplasts and their potential role in transporting lipids and proteins. We additionally discuss what is known about the proteins involved in the vesicle transport and the gaps in knowledge that remain to be filled.

Secretory COPII Protein SEC31B Is Required for Pollen Wall Development

The pollen wall protects pollen grains from abiotic and biotic stresses. During pollen wall development, tapetal cells play a vital role by secreting proteins, signals, and pollen wall material to ensure microspore development. But the regulatory mechanism underlying the secretory pathway of the tapetum is largely unknown. Here, we characterize the essential role of the Arabidopsis (Arabidopsis thaliana) COPII protein SECRETORY31B (SEC31B) in pollen wall development and the secretory activity of tapetal cells. The sporophyte-controlled atsec31b mutant exhibits severe pollen and seed abortion. Transmission electron microscopy observation indicates that pollen exine formation in the atsec31b mutant is disrupted significantly. AtSEC31B is a functional COPII protein revealed by endoplasmic reticulum (ER) exit site localization, interaction with AtSEC13A, and retarded ER-Golgi protein trafficking in the atsec31b mutant. A genetic tapetum-specific rescue assay indicates that AtSEC31B functions primarily in the tapetum. Moreover, deletion of AtSEC31B interrupted the formation of the ER-derived tapetosome and altered the location of the ATP-BINDING CASSETTE TRANSPORTER9 protein in the tapetum. Therefore, this work demonstrates that AtSEC31B plays a vital role in pollen wall development by regulating the secretory pathway of the tapetal cells.