Current understanding of heat shock protein-mediated responses to heat stress in rice

Rice, the most important source of calories for humans is prone to severe yield loss due to changing climate including heat stress. Additionally, rice encounters biotic stresses in conjunction with heat stress, which exacerbates the adverse effects, and exponentially increase such losses. Several investigations have identified biotic and heat stress-related quantitative trait loci (QTLs) that may contribute to improved tolerance to these stresses. However, a significant knowledge gap exists in identifying the genomic regions imparting tolerance against combined biotic and heat stress. Hereby, we are presenting a conceptual meta-analysis identifying genomic regions that may be promising candidates for enhancing combined biotic and heat stress tolerance in rice. Fourteen common genomic regions were identified along chromosomes 1, 2, 3, 4, 6, 10 and 12, which harbored 1265 genes related to heat stress and defense responses in rice. Further, the meta expression analysis revealed 24 differentially expressed genes (DEGs) involved in calcium-mediated stress signaling including transcription factors Myb, bHLH, ROS signaling, molecular chaperones HSP110 and pathogenesis related proteins. Additionally, we also proposed a hypothetical model based on GO and MapMan analysis representing the pathways intersecting heat and biotic stresses. These DEGs can be potential candidate genes for improving tolerance to combined biotic and heat stress in rice. We present a framework highlighting plausible connecting links (QTLs/genes) between rice response to heat stress and different biotic factors associated with yield, that can be extended to other crops.

Around the intensity, frequency, duration, accumulated temperature, and even extremes of high-temperature events, nine selected temperature-related indices were used to explore the space and time changes of rice heat stress in Jiangxi province, southeast China. Several statistical methods including Mann-Kendall trend test (M-K test) and principal component analysis (PCA) were used in this study, and main results were listed as follows: (1) The changes in the intensity indices for high-temperature events were more significant, it was mainly embodied in that more than 80% of stations had positive trends. (2) R-mode PCA was applied to the multiannual average values of nine selected indices of whole stations, and the results showed that the higher hazard for rice heat stress could be mainly detected in the middle and northeast area of Jiangxi. (3) S-mode PCA was applied to the integrated heat stress index series, and the results demonstrated that Jiangxi could be divided into four sub-regions with different variability in rice heat stress. However, all the sub-regions are dominated by increasing tendencies in rice heat stress since 1990. (4) Further analysis indicated that the western north Pacific sub-tropical high (WPSH) had the significant dominant influence on the rice heat stress in Jiangxi province.

Rice is considered as a major source of food for most people worldwide. Global rice production is under threat as a result of adverse effects of heat and drought stress. It is therefore necessary to safeguard its sustainable production. This study was carried out to characterize differentially expressed genes (DEGs) under both drought and heat stresses in rice. 1001 DEGs were determined to show up-regulation under heat and drought stresses, while 1690 DEGs were commonly down-regulated. Functional classification analysis generated 22 and 37 related gene groups from the commonly up-regulated and down-regulated genes respectively. Functional characterization revealed 38.1% and 45.2% DEGs annotating for Biological Process, 53.2% and 59.2% DEGs were for Cellular Components and 54.8% and 52.4% DEGs were for Molecular Function in the up and down commonly regulated DEGs respectively. KEGG analysis demonstrated that most of the up-regulated DEGs were particularly enriched in metabolic pathways and in the biosynthesis of secondary metabolites, while the down regulated genes were mostly enriched in pathways involving ribosomes, and in purine and pyrimidine metabolisms. These results could be helpful in further analysis and understanding of heat and drought stress tolerance in rice.

The objective of this work was to study the effect of foliar boric acid (BA) or sodium borate (SB) sprays on rice plants subjected to high daytime temperature. The established treatments were the following: i) seedlings with heat stress (40 °C) and treated with foliar SB sprays at three rates 25, 50 or 100 mg of SB L−1, ii) seedlings with heat stress and with foliar BA sprays at three rates 25, 50 or 100 mg of BA L−1, iii) seedlings without heat stress (28 °C) and foliar boron (B) sprays (absolute control-AB) and iv) seedlings with heat stress and any foliar B compounds treatments (HT), resulting a total of eight group of treatments (AB, HT, SB25, SB50, SB100, BA25, BA50 and BA100). The results showed that, in heat stress condition, foliar BA (25, 50 or 100 mg L−1, respectively) or SB (50 mg L−1) sprays significantly increased the values of net photosynthesis compared to untreated plants (19.7 µmol CO2 m−2 s−1 with B 14.4 µmol CO2 m−2 s−1 without B). The foliar B compounds sprays caused an increase on photochemical efficiency of PSII (0.76 with B vs. 0.72 without B) and lower oxidative damage (expressed as leaf malondialdehyde and proline production) in seedlings under heat stress. The results in the present study allow concluding that the use of B (BA or SB) can be considered as an agronomic strategy to mitigate the negative impact of heat stress in rice when high daytime stress periods are expected in rice areas.

OsFKBP20-1b, a plant-specific cyclophilin protein, has been implicated to regulate pre-mRNA splicing under stress conditions in rice. Here, we demonstrated that OsFKBP20-1b is SUMOylated in a reconstituted SUMOylation system in E.coli and in planta, and that the SUMOylation-coupled regulation was associated with enhanced protein stability using a less SUMOylated OsFKBP20-1b mutant (5KR_OsFKBP20-1b). Furthermore, OsFKBP20-1b directly interacted with OsSUMO1 and OsSUMO2 in the nucleus and cytoplasm, whereas the less SUMOylated 5KR_OsFKBP20-1b mutant had an impaired interaction with OsSUMO1 and 2 in the cytoplasm but not in the nucleus. Under heat stress, the abundance of an OsFKBP20-1b-GFP fusion protein was substantially increased in the nuclear speckles and cytoplasmic foci, whereas the heat-responsiveness was remarkably diminished in the presence of the less SUMOylated 5KR_OsFKBP20-1b-GFP mutant. The accumulation of endogenous SUMOylated OsFKBP20-1b was enhanced by heat stress in planta. Moreover, 5KR_OsFKBP20-1b was not sufficiently associated with the U snRNAs in the nucleus as a spliceosome component. A protoplast transfection assay indicated that the low SUMOylation level of 5KR_OsFKBP20-1b led to inaccurate alternative splicing and transcription under heat stress. Thus, our results suggest that OsFKBP20-1b is post-translationally regulated by SUMOylation, and the modification is crucial for proper RNA processing in response to heat stress in rice.

Heat exposure alters the expression of SOD, POD, APX and CAT isozymes and mitigates low cadmium toxicity in seedlings of sensitive and tolerant rice cultivars

Heat stress is a present day hot topic in the world as it throws great challenges before the scientific world by adversely affecting the crop plants and their yield, the need for resilience in all aspects of the crop and resilient varietal identification and improvement are the need of the hour. Here in this review, plant responses to heat stress morpho-anatomical and biochemical changes along the phenology were reported. Importance of physiological parameters in identifying heat tolerant varieties is a necessary prerequisite and is reliable and superior to all the screening procedures. The importance of the photorespiration and its role in final yield loss, as it has interwoven metabolic links with carbon and nitrogen metabolisms are specially focussed on evolutionary aspects. The changes in the hormonal ratio with phenology and molecular responses to heat stress, mechanism of heat tolerance and genetic improvement for heat-stress tolerance, fertigation role in tolerance is discussed.

Two Japanese rice cultivars with different heat-tolerance, Hinohikari (sensitive) and Nikomaru (tolerant), were grown in pots inside open-top chambers and exposed to ambient CO2 (400 µmol mol−1) or elevated CO2 (550 µmol mol−1) from the beginning of the tillering stage to maturity. The study was conducted in Nagasaki, in the Kyushu region of Japan, where heat stress on rice has become increasingly evident. Although elevated CO2 significantly improved the net photosynthesis and whole-plant growth of the cultivars, there were no significant effects on grain yield, which in turn reduced harvest index. In both cultivars, adverse effects occurred with elevated CO2, such as reductions in spikelet fertility and grain appearance quality, which are typical manifestations of heat stress in rice. During the flowering period, the air temperature was high that spikelet fertility was reduced even under ambient CO2 conditions for both cultivars. These results suggest that, under high-temperature conditions, elevated CO2 could induce or exacerbate the manifestations of heat stress in rice. Because transpiration rate in the flag leaf was significantly reduced by the exposure to elevated CO2, it is possible that elevated CO2 increased plant temperature via a reduction in transpiration during flowering period, although we did not detect significance of the increase in leaf and panicle temperature. To ensure a more confident conclusion, further studies focusing on the effects of elevated CO2 on the determinants of spikelet fertility and grain appearance quality with other cultivars in different year are required.

Global warming is a serious problem, with significant negative impacts on agricultural productivity. To better understand plant anatomical adaptation mechanisms as responses to heat stress, improved basic knowledge is required. This research studied the physiological and anatomical responses of Khao Dawk Mali 105 (KDML105) to artificial heat stress. Dehusked seeds were sterilized and cultured on Murashige and Skoog (MS) medium, supplemented with 3 mg/L 2,4-Dichlorophenoxyacetic acid (2,4-D) for callus induction. The cultures were maintained at 25 °C and 35 °C for 4 weeks, while the other culture was treated with heat shock at 42 °C for 1 week before further incubation at 25 °C for 3 weeks. Results revealed that elevated temperatures (35 °C and 42 °C) adversely impacted seedling growth. Plant height, root length, leaf number per plant, fresh and dry weight, chlorophyll a, chlorophyll b and total chlorophyll content decreased after heat stress treatment, while malondialdehyde (MDA) and electrolyte leakage percentage significantly increased, compared to the control. Heat stress induced ROS accumulation, leading to lipid peroxidation and membrane instability. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) results also confirmed negative correlations between MDA, electrolyte leakage and other parameters. MDA content and electrolyte leakage are effective indicators of heat stress in rice. Surface anatomical responses of rice seedlings to heat stress were studied but significant alterations were not observed, and heat stress had no significant negative effects on KDML105 calli. Size and mass of calli increased because heat stress stimulated gene expression that induced thermotolerance. Our results provide useful information for rice breeding and heat stress tolerance programs to benefit long-term global food security.

Identifying candidate genes and patterns of heat-stress response in rice using a genome-wide association study and transcriptome analyses

Heat stress is an example of a severe abiotic stress that plants can suffer in the field, causing a significant detrimental effect on their growth and productivity. Understanding the mechanism of plant response to heat stress is important for improving the productivity of crop plants under global warming. We used a microarray dataset that is deposited in the public database to evaluate plant responses to heat stress, and we selected the top 10 genes that are highly expressed under heat stress in rice. Two genes, OsSHSP1 (Os03g16030) and OsSHSP2 (Os01g04380), were selected for further study. These genes were highly induced in response to salt and drought but not in response to cold. In addition, OsSHSP1 and OsSHSP2 gene transcripts were induced under abscisic acid and salicylic acid but not under jasmonic acid and ethylene. Subcellular localization of proteins of 35S::OsSHSP1 were associated with the cytosol, whereas those of and 35S::OsSHSP2 were associated with the cytosol and nucleus. Heterogeneous overexpression of both genes exhibited higher germination rates than those of wild-type plants under the salt treatment, but not under heat or drought stress, supporting a hypothesis regarding functional specialization of members of small heat-shock protein family over evolutionary time. The network of both genes harboring nine sHSPs as well as at least 13 other chaperone genes might support the idea of a role for sHSPs in the chaperone network. Our findings might provide clues to shed light on the molecular functions of OsSHSP1 and OsSHSP2 in response to abiotic stresses, especially heat stress.

The Ca2+/calmodulin (CaM) signaling pathway mediates the heat stress (HS) response and acquisition of thermotolerance in plants. We showed that the rice CaM1-1 isoform can interpret a Ca2+ signature difference in amplitude, frequency, and temporal–spatial properties in regulating transcription of nucleoplasmic small heat-shock protein gene (sHSPC/N) during HS. Ca2+ and A23187 treatments under HS generated an intense and sustained increase in [Ca2+]cyt and accelerated the expression of CaM1-1 and sHSPC/N genes, which suggests that HS-induced apoplastic Ca2+ influx was responsible for the [Ca2+]cyt transient and downstream HS signaling. Here, we discuss an emerging paradigm in the oscillation regulation of CaM1-1 expression during HS and highlight the areas that need further investigation.

Heat stress is an important factor that has a negative impact on rice (Oryza sativa) production. To alleviate this problem, it is necessary to extensively understand the genetic basis of heat tolerance and adaptability to heat stress in rice. Here, we report the molecular mechanism underlying heat acclimation memory that confers long-term acquired thermotolerance (LAT) in this monocot plant. Our results showed that a positive feedback loop formed by two heat-inducible genes, HEAT SHOCK PROTEIN101 (HSP101) and HEAT STRESS-ASSOCIATED 32-KD PROTEIN (HSA32), at the posttranscriptional level prolongs the effect of heat acclimation in rice seedlings. The interplay between HSP101 and HSA32 also affects basal thermotolerance of rice seeds. These findings are similar to those reported for the dicot plant Arabidopsis (Arabidopsis thaliana), suggesting a conserved function in plant heat stress response. Comparison between two rice cultivars, japonica Nipponbare and indica N22 showed opposite performance in basal thermotolerance and LAT assays. 'N22' seedlings have a higher basal thermotolerance level than cv Nipponbare and vice versa at the LAT level, indicating that these two types of thermotolerance can be decoupled. The HSP101 and HSA32 protein levels were substantially higher in cv Nipponbare than in cv N22 after a long recovery following heat acclimation treatment, at least partly explaining the difference in the LAT phenotype. Our results point out the complexity of thermotolerance diversity in rice cultivars, which may need to be taken into consideration when breeding for heat tolerance for different climate scenarios.

Productivity of rice, world's most important cereal is threatened by high temperature stress, intensified by climate change. Development of heat stress-tolerant varieties is one of the best strategies to maintain its productivity. However, heat stress tolerance is a multigenic trait and the candidate genes are poorly known. Therefore, we aimed to identify quantitative trait loci (QTL) for vegetative stage tolerance to heat stress in rice and the corresponding candidate genes. We used genotyping-by-sequencing to generate single nucleotide polymorphic (SNP) markers and genotype 150 F8 recombinant inbred lines (RILs) obtained by crossing heat tolerant “N22” and heat susceptible “IR64” varieties. A linkage map was constructed using 4,074 high quality SNP markers that corresponded to 1,638 recombinationally unique events in this mapping population. Six QTL for root length and two for shoot length under control conditions with 2.1–12% effect were identified. One QTL rlht5.1 was identified for “root length under heat stress,” with 20.4% effect. Four QTL were identified for “root length under heat stress as percent of control” that explained the total phenotypic variation from 5.2 to 8.6%. Three QTL with 5.3–10.2% effect were identified for “shoot length under heat stress,” and seven QTL with 6.6–19% effect were identified for “shoot length under heat stress expressed as percentage of control.” Among the QTL identified six were overlapping between those identified using shoot traits and root traits: two were overlapping between QTL identified for “shoot length under heat stress” and “root length expressed as percentage of control” and two QTL for “shoot length as percentage of control” were overlapping a QTL each for “root length as percentage of control” and “shoot length under heat stress.” Genes coding 1,037 potential transcripts were identified based on their location in 10 QTL regions for vegetative stage heat stress tolerance. Among these, 213 transcript annotations were reported to be connected to stress tolerance in previous research in the literature. These putative candidate genes included transcription factors, chaperone proteins (e.g., alpha-crystallin family heat shock protein 20 and DNAJ homolog heat shock protein), proteases, protein kinases, phospholipases, and proteins related to disease resistance and defense and several novel proteins currently annotated as expressed and hypothetical proteins.

Comparative transcriptomics of indica and japonica rice roots under heat stress reveals the crucial role of OsMAPK3 in heat response.