XBAT31 regulates reproductive thermotolerance through controlling the accumulation of HSFB2a/B2b under heat stress conditions.

Heat shock transcription factors (HSFs) play a crucial role in heat stress tolerance in vegetative tissues. However, their involvement in reproductive tissues and their post-translational modifications are not well understood. In this study, we identify the E3 ligase XB3 ORTHOLOG 1 IN ARABIDOPSIS THALIANA (XBAT31) as a key player in the ubiquitination and degradation of HSFB2a/B2b. Our results show that the xbat31 mutant exhibits a higher percentage of unfertile siliques and decreased expression of HSPs in flowers under heat stress conditions compared to the wild type. Conversely, the hsfb2a hsfb2b double mutant displays improved reproductive thermotolerance. We find that XBAT31 interacts with HSFB2a/B2b and mediates their ubiquitination. Furthermore, HSFB2a/B2b ubiquitination is reduced in the xbat31-1 mutant, resulting in higher accumulation of HSFB2a/B2b in flowers under heat stress conditions. Overexpression of HSFB2a or HSFB2b leads to an increase in unfertile siliques under heat stress conditions. Thus, our results dissect the important role of the XBAT31-HSFB2a/B2b module in conferring reproductive thermotolerance in plants.

The FtsH-Inactive Protein FtsHi5 Is Required for Chloroplast Development and Protein Accumulation in Chloroplasts at Low Ambient Temperature in Arabidopsis.

Chloroplasts are indispensable for higher plants. The growth and development of plants are very sensitive to environmental temperature changes, and chloroplast development is also regulated by adverse environmental temperatures. However, the molecular mechanism of how plants coordinate chloroplast development and environmental temperature changes remains largely unknown. Here, a temperature-conditioned chloroplast development defective mutant thermo-sensitive mutant in leaf color 2 (tsl2) of Arabidopsis was obtained through a forward genetic screening. The tsl2 mutant showed a weak yellowish phenotype at normal growth temperature (22°C), and the phenotype was more pronounced at low growth temperature (16°C) and largely rescued at high growth temperature (29°C). Bulk Segregant Analysis (BSA) revealed that TSL2 encodes FtsH-Inactive Protein 5 (FtsHi5). Genetic complementation analysis confirmed that complemented expression of FtsHi5 rescued the chlorophyll content and thylakoid development defects observed in tsl2 mutants at 16°C. Quantitative mass spectrometry analysis with Tandem Mass Tag (TMT) isobaric labeling revealed broad changes in the chloroplast proteome of tsl2 mutant plants at low temperature, which is agreed with the impaired chloroplast biogenesis and function in tsl2 plants. Together, our data demonstrates that FtsHi5/TSL2 plays an important role in chloroplast development and protein accumulation in chloroplasts, especially at low environmental temperatures in Arabidopsis.

Protein Quality Control in Plant Organelles: Current Progress and Future Perspectives.

The endoplasmic reticulum, chloroplasts, and mitochondria are major plant organelles for protein synthesis, photosynthesis, metabolism, and energy production. Protein homeostasis in these organelles, maintained by a balance between protein synthesis and degradation, is essential for cell functions during plant growth, development, and stress resistance. Nucleus-encoded chloroplast- and mitochondrion-targeted proteins and ER-resident proteins are imported from the cytosol and undergo modification and maturation within their respective organelles. Protein folding is an error-prone process that is influenced by both developmental signals and environmental cues; a number of mechanisms have evolved to ensure efficient import and proper folding and maturation of proteins in plant organelles. Misfolded or damaged proteins with nonnative conformations are subject to degradation via complementary or competing pathways: intraorganelle proteases, the organelle-associated ubiquitin-proteasome system, and the selective autophagy of partial or entire organelles. When proteins in nonnative conformations accumulate, the organelle-specific unfolded protein response operates to restore protein homeostasis by reducing protein folding demand, increasing protein folding capacity, and enhancing components involved in proteasome-associated protein degradation and autophagy. This review summarizes recent progress on the understanding of protein quality control in the ER, chloroplasts, and mitochondria in plants, with a focus on common mechanisms shared by these organelles during protein homeostasis.

Chromatin remodeling factor CHR18 interacts with replication protein RPA1A to regulate the DNA replication stress response in Arabidopsis.

DNA replication is a fundamental process for the faithful transmission of genetic information in all living organisms. Many endogenous and environmental signals impede fork progression during DNA synthesis, which induces replication errors and DNA replication stress. Chromatin remodeling factors regulate nucleosome occupancy and the histone composition of the nucleosome in chromatin; however, whether chromatin remodeling factors are involved in the DNA replication stress response in plants is unknown. We reveal that chromatin remodeling factor CHR18 plays important roles in DNA replication stress in Arabidopsis thaliana by interacting with the DNA replication protein RPA1A. According to the genetic analysis, the loss of function of either CHR18 or RPA1A confers a high sensitivity to DNA replication stress in Arabidopsis. CHR18 interacts with RPA1A in both yeast cells and tobacco epidermal cells. The coexpression of RPA1A and CHR18 enhances the accumulation of CHR18 in nuclear foci in plants. CHR18 is a typical nuclear-localized chromatin remodeling factor with ATPase activity. Our results demonstrate that during DNA synthesis in plants, RPA1A interacts with CHR18 and recruits CHR18 to nuclear foci to resolve DNA replication stress, which is important for cell propagation and root growth in Arabidopsis plants.

A Novel QTL qTGW3 Encodes the GSK3/SHAGGY-Like Kinase OsGSK5/OsSK41 that Interacts with OsARF4 to Negatively Regulate Grain Size and Weight in Rice.

Grain size and shape are important determinants of grain weight and yield in rice. Here, we report a new major quantitative trait locus (QTL), qTGW3, that controls grain size and weight in rice. This locus, qTGW3, encodes OsSK41 (also known as OsGSK5), a member of the GLYCOGEN SYNTHASE KINASE 3/SHAGGY-like family. Rice near-isogenic lines carrying the loss-of-function allele of OsSK41 have increased grain length and weight. We demonstrate that OsSK41 interacts with and phosphorylates AUXIN RESPONSE FACTOR 4 (OsARF4). Co-expression of OsSK41 with OsARF4 increases the accumulation of OsARF4 in rice protoplasts. Loss of function of OsARF4 results in larger rice grains. RNA-sequencing analysis suggests that OsARF4 and OsSK41 repress the expression of a common set of downstream genes, including some auxin-responsive genes, during rice grain development. The loss-of-function form of OsSK41 at qTGW3 represents a rare allele that has not been extensively utilized in rice breeding. Suppression of OsSK41 function by either targeted gene editing or QTL pyramiding enhances rice grain size and weight. Thus, our study reveals the important role of OsSK41 in rice grain development and provides new candidate genes for genetic improvement of grain yield in rice and perhaps in other cereal crops.