Uncoupled Expression of Nuclear and Plastid Photosynthesis-Associated Genes Contributes to Cell Death in a Lesion Mimic Mutant

N-Terminal Acetylation Stabilizes SIGMA FACTOR BINDING PROTEIN1 Involved in Salicylic Acid-Primed Cell Death

N-terminal (Nt) acetylation (NTA) is an ample and irreversible cotranslational protein modification catalyzed by ribosome-associated Nt-acetyltransferases. NTA on specific proteins can act as a degradation signal (called an Ac/N-degron) for proteolysis in yeast and mammals. However, in plants, the biological relevance of NTA remains largely unexplored. In this study, we reveal that Arabidopsis (Arabidopsis thaliana) SIGMA FACTOR-BINDING PROTEIN1 (SIB1), a transcription coregulator and a positive regulator of salicylic acid-primed cell death, undergoes an absolute NTA on the initiator Met; Nt-acetyltransferase B (NatB) partly contributes to this modification. While NTA results in destabilization of certain target proteins, our genetic and biochemical analyses revealed that plant NatB-involved NTA instead renders SIB1 more stable. Given that the ubiquitin/proteasome system stimulates SIB1 degradation, it seems that the NTA-conferred stability ensures the timely expression of SIB1-dependent genes, mostly related to immune responses. Taking our findings together, here we report a noncanonical NTA-driven protein stabilization in land plants.

The Arabidopsis SAFEGUARD1 suppresses singlet oxygen-induced stress responses by protecting grana margins

Singlet oxygen (¹O₂), the major reactive oxygen species (ROS) produced in chloroplasts, has been demonstrated recently to be a highly versatile signal that induces various stress responses. In the fluorescent (flu) mutant, its release causes seedling lethality and inhibits mature plant growth. However, these drastic phenotypes are suppressed when EXECUTER1 (EX1) is absent in the flu ex1 double mutant. We identified SAFEGUARD1 (SAFE1) in a screen of ethyl methanesulfonate (EMS) mutagenized flu ex1 plants for suppressor mutants with a flu-like phenotype. In flu ex1 safe1, all ¹O₂-induced responses, including transcriptional rewiring of nuclear gene expression, return to levels, such as, or even higher than, those in flu Without SAFE1, grana margins (GMs) of chloroplast thylakoids (Thys) are specifically damaged upon ¹O₂ generation and associate with plastoglobules (PGs). SAFE1 is localized in the chloroplast stroma, and release of ¹O₂ induces SAFE1 degradation via chloroplast-originated vesicles. Our paper demonstrates that flu-produced ¹O₂ triggers an EX1-independent signaling pathway and proves that SAFE1 suppresses this signaling pathway by protecting GMs.

Cyclophilin OsCYP20-2 with a novel variant integrates defense and cell elongation for chilling response in rice

Coordinating stress defense and plant growth is a survival strategy for adaptation to different environments that contains a series of processes, such as, cell growth, division and differentiation. However, little is known about the coordination mechanism for protein conformation change. A cyclophilin OsCYP20-2 with a variant interacts with SLENDER RICE1 (SLR1) and OsFSD2 in the nucleus and chloroplasts, respectively, to integrate chilling tolerance and cell elongation in rice (Oryza sativa) (FSD2, Fe-superoxide dismutase 2). Mass spectrum assay showed that OsNuCYP20-2 localized at the nucleus (nuclear located OsCYP20-2) was a new variant of OsCYP20-2 that truncated 71 amino-acid residues in N-terminal. The loss-of function OsCYP20-2 mutant showed sensitivity to chilling stress with accumulation of extra reactive oxygen species (ROS). In chloroplasts, the full-length OsCYP20-2 promotes OsFSD2 forming homodimers which enhance its activity, eliminating the accumulation of ROS under chilling stress. However, the mutant had shorter epidermal cells in comparison with wild-type Hwayoung (HY). In the nucleus, OsCYP20-2 caused conformation change of SLR1 to promote its degradation for cell elongation. Our data reveal a cyclophilin with a variant with dual-localization in chloroplasts and the nucleus, which mediate chilling tolerance and cell elongation.

VENOSA4, a Human dNTPase SAMHD1 Homolog, Contributes to Chloroplast Development and Abiotic Stress Tolerance

Chloroplast biogenesis depends on an extensive interplay between the nucleus, cytosol, and chloroplasts, involving regulatory nucleus-encoded chloroplast proteins, as well as nucleocytosolic photoreceptors such as phytochromes (phys) and other extrachloroplastic factors. However, this whole process is only partially understood. Here, we describe the role of VENOSA4 (VEN4) in chloroplast development and acclimation to adverse growth conditions. A 35S:VEN4-eGFP fusion protein localizes to the nucleus in Arabidopsis (Arabidopsis thaliana) protoplasts, and VEN4 homologs are present in a wide range of eukaryotes including humans, where the corresponding homolog (SAMHD1) cleaves dNTPs. Defective photosynthesis in ven4 seedlings results from reduced accumulation of photosynthetic proteins and appears to be caused by a reduction in the translational capacity of chloroplasts. The negative effect of the ven4 mutation on photosynthesis can be phenotypically suppressed by germinating seeds in the presence of excess dCTP or a pool of dNTPs, implying that VEN4, like human SAMHD1, is involved in dNTP catabolism. Moreover, VEN4 activity is also required for optimal responses to cold and salt stresses. In conclusion, our work emphasizes the importance of the nucleocytosolic compartment and the fine-tuning of dNTP levels for chloroplast translation and development.

Comparative analysis of codon usage patterns in chloroplast genomes of six Euphorbiaceae species

Euphorbiaceae plants are important as suppliers of biodiesel. In the current study, the codon usage patterns and sources of variance in chloroplast genome sequences of six different Euphorbiaceae plant species have been systematically analyzed. Our results revealed that the chloroplast genomes of six Euphorbiaceae plant species were biased towards A/T bases and A/T-ending codons, followed by detection of 17 identical high-frequency codons including GCT, TGT, GAT, GAA, TTT, GGA, CAT, AAA, TTA, AAT, CCT, CAA, AGA, TCT, ACT, TAT and TAA. It was found that mutation pressure was a minor factor affecting the variation of codon usage, however, natural selection played a significant role. Comparative analysis of codon usage frequencies of six Euphorbiaceae plant species with four model organisms reflected that Arabidopsis thaliana, Populus trichocarpa, and Saccharomyces cerevisiae should be considered as suitable exogenous expression receptor systems for chloroplast genes of six Euphorbiaceae plant species. Furthermore, it is optimal to choose Saccharomyces cerevisiae as the exogenous expression receptor. The outcome of the present study might provide important reference information for further understanding the codon usage patterns of chloroplast genomes in other plant species.

H2O2 as a Feedback Signal on Dual-Located WHIRLY1 Associates with Leaf Senescence in Arabidopsis

Leaf senescence, either as a natural stage of development or as an induced process under stress conditions, incorporates multiple intricate signaling pathways. At the cellular level, retrograde signals have been considered as important players during the initiation and progression of senescence in both animals and plants. The plant-specific single-strand DNA-binding protein WHIRLY1 (WHY1), a repressor of leaf natural senescence, is dually located in both nucleus and plastids. Despite many years of studies, the myth about its dual location and the underlying functional implications remain elusive. Here, we provide further evidence in Arabidopsis showing that alteration in WHY1 allocation between the nucleus and chloroplast causes perturbation in H₂O₂ homeostasis, resulting in adverse plant senescence phenotypes. The knockout of WHY1 increased H₂O₂ content at 37 days post-germination, coincident with an early leaf senescence phenotype, which can be rescued by ectopic expression of the nuclear isoform (nWHY1), but not by the plastid isoform (pWHY1). Instead, accumulated pWHY1 greatly provoked H₂O₂ in cells. On the other hand, exogenous H₂O₂ treatment induced a substantial plastid accumulation of WHY1 proteins and at the same time reduced the nuclear isoforms. This H₂O₂-induced loss of nucleus WHY1 isoform was accompanied by enhanced enrichments of histone H3 lysine 9 acetylation (H3K9ac) and recruitment of RNA polymerase II (RNAP II) globally, and specifically at the promoter of the senescence-related transcription factor WRKY53, which in turn activated WRKY53 transcription and led to a senescence phenotype. Thus, the distribution of WHY1 organelle isoforms and the feedback of H₂O₂ intervene in a circularly integrated regulatory network during plant senescence in Arabidopsis.

Crop Improvement Through Temperature Resilience

Abnormal environmental temperature affects plant growth and threatens crop production. Understanding temperature signal sensing and the balance between defense and development in plants lays the foundation for improvement of temperature resilience. Here, we summarize the current understanding of cold signal perception/transduction as well as heat stress response. Dissection of plant responses to different levels of cold stresses (chilling and freezing) illustrates their common and distinct signaling pathways. Axillary bud differentiation in response to chilling is presented as an example of the trade-off between defense and development. Vernalization is a cold-dependent development adjustment mediated by O-GlcNAcylation and phosphorylation to sense long-term cold. Recent progress on major quantitative trait loci genes for heat tolerance has been summarized. Molecular mechanisms in utilizing temperature-sensitive sterility in super hybrid breeding in China are revealed. The way to improve crop temperature resilience using integrative knowledge of omics as well as systemic and synthetic biology, especially the molecular module program, is summarized.