The major low-molecular-weight heat shock protein in chloroplasts shows antigenic conservation among diverse higher plant species

Physiological and Proteomic Analysis Responsive Mechanisms for Salt Stress in Oat

Oat is considered as a moderately salt-tolerant crop that can be used to improve saline and alkaline soils. Previous studies have focused on short-term salt stress exposure, and the molecular mechanisms of salt tolerance in oat have not yet been elucidated. In this study, the salt-tolerant oat cultivar Vao-9 and the salt-sensitive oat cultivar Bai5 were treated with 6 days of 0 and 150 mmol L^-1 salt stress (nNaCl:nNa₂SO₄ = 1:1). Label-Free technology was then used to analyze the differentially expressed proteins in leaves under 0 and 150 mmol L^-1 salt stress. The obtained results indicated that total of 2,631 proteins were identified by mass spectrometry in the four samples. The salt-tolerant cultivar Vao-9 mainly enhances its carbohydrate and energy metabolism through the pentose and glucuronate interconversions, and carbon fixation pathways in prokaryotes, thereby reducing the damage caused by salt stress. In addition, the down-regulation of ribosomes expression and the up-regulated expression of HSPs and CRT are all through the regulation of protein synthesis in response to salt stress. However, GABA metabolism presents a different synthesis pattern in Bai5 and Vao-9. The main KEGG function of differential expressed protein (DEP) in Bai5 is classified into protein processing in the endoplasmic reticulum, estrogen signaling pathway, antigen processing and presentation, longevity regulating pathway-multiple species, arginine and proline metabolism, beta-alanine metabolism, vitamin B6 metabolism, salmonella infection, chloroalkane and chloroalkene degradation, and limonene and pinene degradation. Moreover, the main KEGG functions of DEP in Vao-9 are classified as ribosome and carbon fixation pathways in prokaryotes, pentose and glucuronate interconversions, GABA ergic synapse, and taurine and hypotaurine metabolism. The results obtained in this study provide an important basis for further research on the underlying mechanisms of salt response and tolerance in oat and other plant species.

Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants

Increases in ambient temperatures have been a severe threat to crop production in many countries around the world under climate change. Chloroplasts serve as metabolic centers and play a key role in physiological adaptive processes to heat stress. In addition to expressing heat shock proteins that protect proteins from heat-induced damage, metabolic reprogramming occurs during adaptive physiological processes in chloroplasts. Heat stress leads to inhibition of plant photosynthetic activity by damaging key components functioning in a variety of metabolic processes, with concomitant reductions in biomass production and crop yield. In this review article, we will focus on events through extensive and transient metabolic reprogramming in response to heat stress, which included chlorophyll breakdown, generation of reactive oxygen species (ROS), antioxidant defense, protein turnover, and metabolic alterations with carbon assimilation. Such diverse metabolic reprogramming in chloroplasts is required for systemic acquired acclimation to heat stress in plants.

Clonal mosaic analysis of EMPTY PERICARP2 reveals nonredundant functions of the duplicated HEAT SHOCK FACTOR BINDING PROTEINs during maize shoot development

The paralogous maize proteins EMPTY PERICARP2 (EMP2) and HEAT SHOCK FACTOR BINDING PROTEIN2 (HSBP2) each contain a single recognizable motif: the coiled-coil domain. EMP2 and HSBP2 accumulate differentially during maize development and heat stress. Previous analyses revealed that EMP2 is required for regulation of heat shock protein (hsp) gene expression and also for embryo morphogenesis. Developmentally abnormal emp2 mutant embryos are aborted during early embryogenesis. To analyze EMP2 function during postembryonic stages, plants mosaic for sectors of emp2 mutant tissue were constructed. Clonal sectors of emp2 mutant tissue revealed multiple defects during maize vegetative shoot development, but these sector phenotypes are not correlated with aberrant hsp gene regulation. Furthermore, equivalent phenotypes are observed in emp2 sectored plants grown under heat stress and nonstress conditions. Thus, the function of EMP2 during regulation of the heat stress response can be separated from its role in plant development. The discovery of emp2 mutant phenotypes in postembryonic shoots reveals that the duplicate genes emp2 and hsbp2 encode nonredundant functions throughout maize development. Distinct developmental phenotypes correlated with the developmental timing, position, and tissue layer of emp2 mutant sectors, suggesting that EMP2 has evolved diverse developmental functions in the maize shoot.

Poly(A) tail length of a heat shock protein RNA is increased by severe heat stress, but intron splicing is unaffected

The small heat shock proteins (sHSPs) are induced in all eukaryotes in response to high temperature stress, but are most abundant among members of the plant kingdom where they accumulate in multiple subcellular compartments. We have analyzed the expression of the chloroplast-localized sHSP from Arabidopsis thaliana, HSP21, and characterized the structure of the gene encoding this protein to facilitate future genetic studies on the function of HSP21 in the heat shock response. HSP21 is encoded in Arabidopsis by a single gene whose coding region is interrupted by a single intron. Previous studies have shown that intron processing is disrupted by severe, abrupt heat stress but is protected by pretreatments that induce thermotolerance. The processing of the HSP21 transcript was investigated in response to an abrupt heat stress regime and a gradual heat stress regime, the latter of which is known to confer thermotolerance in plants. Under abrupt stress conditions the HSP21 transcript is somewhat longer than under gradual heat stress conditions. However, the molecular basis for the size difference is not impaired intron splicing, but rather a difference in the length of the poly(A) tail depending on the heat stress regime. The results suggest that an increase in poly(A) tail length may be a generalized response to severe, abrupt heat stress and that poly(A) tail metabolism may be one of numerous cellular processes normally protected in thermotolerant cells from the otherwise damaging effects of high temperature stress.

Localization of small heat shock proteins to the higher plant endomembrane system

Three related gene families of low-molecular-weight (LMW) heat shock proteins (HSPs) have been characterized in plants. We describe a fourth LMW HSP family, represented by PsHSP22.7 from Pisum sativum and GmHSP22.0 from Glycine max, and demonstrate that this family of proteins is endomembrane localized. PsHSP22.7 and GmHSP22.0 are 76.7% identical at the amino acid level. Both proteins have amino-terminal signal peptides and carboxyl-terminal sequences characteristic of endoplasmic reticulum (ER) retention signals. The two proteins closely resemble class I cytoplasmic LMW HSPs, suggesting that they evolved from the cytoplasmic proteins through the addition of the signal peptide and ER retention motif. The endomembrane localization of these proteins was confirmed by cell fractionation. The polypeptide product of PsHSP22.7 mRNA was processed to a smaller-M(r) form by canine pancreatic microsomes; in vivo, GmHSP22.0 polysomal mRNA was found to be predominantly membrane bound. In vitro-processed PsHSP22.7 corresponded in mass and pI to one of two proteins detected in ER fractions from heat-stressed plants by using anti-PsHSP22.7 antibodies. Like other LMW HSPs, PsHSP22.7 was observed in higher-molecular-weight structures with apparent masses of between 80 and 240 kDa. The results reported here indicate that members of this new class of LMW HSPs are most likely resident ER proteins and may be similar in function to related LMW HSPs in the cytoplasm. Along with the HSP90 and HSP70 classes of HSPs, this is the third category of HSPs localized to the ER.

Localization of small heat shock proteins to the higher plant endomembrane system

Three related gene families of low-molecular-weight (LMW) heat shock proteins (HSPs) have been characterized in plants. We describe a fourth LMW HSP family, represented by PsHSP22.7 from Pisum sativum and GmHSP22.0 from Glycine max, and demonstrate that this family of proteins is endomembrane localized. PsHSP22.7 and GmHSP22.0 are 76.7% identical at the amino acid level. Both proteins have amino-terminal signal peptides and carboxyl-terminal sequences characteristic of endoplasmic reticulum (ER) retention signals. The two proteins closely resemble class I cytoplasmic LMW HSPs, suggesting that they evolved from the cytoplasmic proteins through the addition of the signal peptide and ER retention motif. The endomembrane localization of these proteins was confirmed by cell fractionation. The polypeptide product of PsHSP22.7 mRNA was processed to a smaller-M(r) form by canine pancreatic microsomes; in vivo, GmHSP22.0 polysomal mRNA was found to be predominantly membrane bound. In vitro-processed PsHSP22.7 corresponded in mass and pI to one of two proteins detected in ER fractions from heat-stressed plants by using anti-PsHSP22.7 antibodies. Like other LMW HSPs, PsHSP22.7 was observed in higher-molecular-weight structures with apparent masses of between 80 and 240 kDa. The results reported here indicate that members of this new class of LMW HSPs are most likely resident ER proteins and may be similar in function to related LMW HSPs in the cytoplasm. Along with the HSP90 and HSP70 classes of HSPs, this is the third category of HSPs localized to the ER.

Accumulation, stability, and localization of a major chloroplast heat-shock protein

Diverse higher plant species synthesize low molecular weight (LMW) heat shock proteins (HSPs) which localize to chloroplasts. These proteins are homologous to LMW HSPs found in the cytoplasm of all eukaryotes, a class of HSPs whose molecular mode of action is not understood. To obtain basic information concerning the role of chloroplast HSPs, we examined the accumulation, stability, tissue specificity, and intra-chloroplast localization of HSP21, the major LMW chloroplast HSP in pea. Intact pea plants were subjected to heat stress conditions which would be encountered in the natural environment and HSP21 mRNA and protein levels were measured in leaves and roots. HSP21 was not detected in leaves or roots before stress, but the mature, 21-kD protein accumulated in direct proportion to temperature and HSP21 mRNA levels in both tissues. All of the HSP21 in leaves was localized to chloroplasts; there was no evidence for its transport into other organelles. In chloroplast fractionation experiments, greater than 80% of HSP21 was recovered in the soluble chloroplast protein fraction. The half-life of HSP21 at control temperatures was 52 +/- 12 h, suggesting the protein's function is critical during recovery as well as during stress. We hypothesize that HSP21 functions in a catalytic fashion in both photosynthetic and nonphotosynthetic plastids.

Identification of heat shock protein hsp70 homologues in chloroplasts

Cytoplasmic members of the heat shock protein hsp70 family have recently been implicated in the transport of proteins to the endoplasmic reticulum and mitochondria. In addition, other hsp70 homologues have been found in the endoplasmic reticulum and mitochondria and, at least for the endoplasmic reticulum hsp70 homologue, may be involved in the proper folding and assembly of newly transported proteins. Since chloroplasts are an important site of protein transport in plant cells, we were interested in determining whether hsp70 proteins might be located in this organelle. By using immunoblotting techniques and two antibody preparations against hsp70 proteins, we have identified three chloroplastic proteins of approximately 70 kDa that are related to hsp70 proteins. One of these proteins was tightly associated with the outer envelope membrane and was not exposed at the outer surface of the chloroplasts. The other two were soluble proteins located in the stroma. Steady-state levels of the chloroplastic hsp70 homologues did not change after heat stress nor were any additional hsp70 homologues detected in chloroplasts isolated from heat-stressed plants. We discuss the possible functions of these hsp70 homologues in the transport of proteins into and within chloroplasts.