Reproductive-Stage Heat Stress in Cereals: Impact, Plant Responses and Strategies for Tolerance Improvement

Abstract

Reproductive-stage heat stress (RSHS) poses a major constraint to cereal crop production by damaging main plant reproductive structures and hampering reproductive processes, including pollen and stigma viability, pollination, fertilization, grain setting and grain filling. Despite this well-recognized fact, research on crop heat stress (HS) is relatively recent compared to other abiotic stresses, such as drought and salinity, and in particular, RSHS studies in cereals are considerably few in comparison with seedling-stage and vegetative-stage-centered studies. Meanwhile, climate change-exacerbated HS, independently or synergistically with drought, will have huge implications on crop performance and future global food security. Fortunately, due to their sedentary nature, crop plants have evolved complex and diverse transient and long-term mechanisms to perceive, transduce, respond and adapt to HS at the molecular, cell, physiological and whole plant levels. Therefore, uncovering the molecular and physiological mechanisms governing plant response and tolerance to RSHS facilitates the designing of effective strategies to improve HS tolerance in cereal crops. In this review, we update our understanding of several aspects of RSHS in cereals, particularly impacts on physiological processes and yield; HS signal perception and transduction; and transcriptional regulation by heat shock factors and heat stress-responsive genes. We also discuss the epigenetic, post-translational modification and HS memory mechanisms modulating plant HS tolerance. Moreover, we offer a critical set of strategies (encompassing genomics and plant breeding, transgenesis, omics and agronomy) that could accelerate the development of RSHS-resilient cereal crop cultivars. We underline that a judicious combination of all of these strategies offers the best foot forward in RSHS tolerance improvement in cereals. Further, we highlight critical shortcomings to RSHS tolerance investigations in cereals and propositions for their circumvention, as well as some knowledge gaps, which should guide future research priorities. Overall, our review furthers our understanding of HS tolerance in plants and supports the rational designing of RSHS-tolerant cereal crop cultivars for the warming climate.

Keywords: HS improvement strategies; HS response mechanisms; cereal crops; epigenetic regulation; heat stress (HS); phytohormonal regulation; reproductive stage.

Figure 1

Molecular regulatory networks underpinning plant HS response and HS memory. HS alters plasma membrane (PM) fluidity, thereby initiating a lipid signalling pathway and inducting Ca²⁺ channels. Other PM-embedded receptors, such as the CNGCs, RBOHs, etc., perceive HS signals. Elevated cytosolic Ca²⁺ ions evoke the Ca²⁺ signalling pathways, mediated by the calmodulins (CaMs), CBKs or CDPKs (see text for details). The CaM interacts with CBK3 and CaM-binding protein phosphatase 7 (PP7) to activate HSFA1 by phosphorylation and enhanced regulation, respectively. The HSFA1 regulates the induction of several other HSFs, including HSFA2, dehydration-responsive element-binding 2A (DREB2A), HSFA7s, and micro-RNAs (miRNAs, e.g., miR398) that initiate the expression of HSPs and other heat stress-inducible genes, which eventually orchestrates physiological responses precipitating thermotolerance acquisition. Meanwhile, the ROS (H₂O₂) sensed by the RBOHs, together with nitric oxide (NO) and metabolic H₂0₂ generated from chloroplasts, mitochondria, etc., trigger ROS build-up, which initiates the ROS signalling pathway, and ultimately alters the expression of HSPs and other HS-inducible genes, via the activation of monomeric HSFs (inactive form in the cytoplasm), which yield oligomeric HSFs (active form which is translocated into the nucleus) to activate HSP gene expression. The PM-sensed HS signals, or ROS burst emanating from the endoplasmic reticulum (ER)-generated HS also activate the cytoplasmic protein response (CPR) and unfolded protein response (UPR) mechanisms. The UPR network is largely mediated by the inositol-requiring protein-1 (IRE1)-bZIP60 mRNA and the site-1/site-2 proteases (S1P/S2P)-bZIP28 pathways. Upon ER stress, IRE1 catalyzes the splicing of the bZIP60 mRNA into a truncated bZIP60 (s) variant that encodes a bZIP60 protein lacking a transmembrane domain, which allows for its translocation from the ER into the nucleus, where it will activate HS-responsive genes expression. Under benign conditions, BiP (an ER-embedded chaperone) maintains bZIP28 at the ER. However, ER stress causes the relocation of bZIP28 to the Golgi apparatus, where it undergoes proteolytic processing, consequently triggering nuclear relocation of its cytoplasm-facing domain, where it ultimately evokes HS-responsive genes expression. Meanwhile, histone modification (HM) and chromatin remodelling (CR) also stimulate HS-inducible gene expression. In addition, HM, CR, ONSEN and some transposons modulate plant HS memory (HSM). Heat-intolerant 4 (HIT4) is a central regulator of CR in response to HS and facilitates nucleosome dispersion leading to the release of transcriptional gene silencing. HSFA2-derived HM offers transient HSM that lasts for a few days and facilitates rapid gene reactivation, whereas CR, ONSEN, and transposons-mediated HSM last for a longer period and provides long-term adaptation to plant HS. Note: Dashed lines and question marks signify links yet to be confirmed. Other abbreviations mentioned in the diagram: GPRCRs, G-protein coupled receptors; PLC, phospholipase C; IP3, inositol triphosphate; CSD1, copper/zinc superoxide dismutase 1 (CSD1); DDM1, DNA methylation 1; MOM1, Morpheus’ molecule 1.

Figure 2

Phytohormonal signalling networks mediating plant HS response. The ABA signalling pathway evokes the ABA-mediated HS response through activation of downstream HSFs (e.g., HSFA6b), which then stimulate HSPs expression. The expressed HSPs invigorate photosynthesis and protein homeostasis under HS conditions, consequently leading to improved HS tolerance. Additionally, ABA mediates HS tolerance in a nitric-oxide (NO)-dependent manner, whereby the respiratory burst oxidase homologs (RBOHs), H₂O₂ and NO-transduced HS signals regulate osmolytes accumulation and antioxidant enzyme activities. In addition, ABA modulates the levels of carbohydrates and energy status via accelerated transport and enhanced metabolism of sucrose to enhance plant HS tolerance. Meanwhile, sucrose alone may act as a regulatory signal and/or provide energy, thereby contributing to HS tolerance. However, ABA has been shown to inhibit brassinosteroids (BRs) functioning in HS response. BRs activate HSPs through BR-controlled transcription factors such as Brassinosteroid Insensitive 1 (BRI1), EMS-Suppressor 1 (BES1), Brassinazole Resistant 1 (BRZ1) and phytochrome interacting factors (PIFs, e.g., PIF4, PIF7, etc.). BES1 may also evoke the heat shock response pathway mediated by ABA-repressed PP2 C-type phosphatases. BZR1 and PIFs, as regulated by BRs, play critical roles in auxin-mediated thermomophogenesis, by regulating the expression of auxin-biosynthesis genes, via auxin-responsive factors (ARFs). The YUCCA8 (YUC8) and Transport Inhibitor Response 1 (TIR) also play central regulatory roles in this BR-Auxin signalling pathway. Meanwhile, the chromatin-modifying enzyme Histone Deacetylase 9 (HDA9) mediates histone deacetylation at YUC8 nucleosomes to promote H2A.Z depletion, which then allows for the binding of the transcriptional regulator, e.g., PIF4 to YUC8 promoter. Conversely, light/thermo- receptors Crytochrome 1 (CRY1) and Phytochrome B (PhyB) suppress the HS-induced activation of PIFs and the expression of auxin-responsive genes. In addition, BR-mediated HS tolerance has pointed to a crosstalk that possibly exists among BRs with ABA and SA signalling pathways, through these hormones sharing similar transcriptional targets. Salicylic acid (SA) is known to trigger HSFs, which then activate HSPs to restore protein homeostasis, thereby inducing HS tolerance. Additionally, SA increases reactive oxygen species (especially H₂O₂) accumulation, which triggers the activation of antioxidant enzymes, consequently improving HS tolerance. Furthermore, SA enhances proline biosynthesis resulting in increased antioxidant enzyme activities and maintenance of photosynthetic activity under HS conditions. Exogenously applied jasmonic acid (JA) increases the accumulation of jasmonates such as methyl jasmonate, 12-oxophytodienoic acid and JA-isoleucine (JA-Ile), which activate the antioxidant defense system, consequently leading to HS tolerance. Meanwhile, JA and SA have exhibited cross-linkage, while JA and ET are antagonistic in their HS response regulation. Ethylene (ET)-mediated signalling involves ET modulating the transcript levels of HSFs (e.g., HSFA1a and HSFA2a, b, etc., which then stimulate ET-signalling-related genes (e.g., Ethylene Insensitive 2, Ethylene Insensitive-Like 1, Ethylene Insensitive-Like 2, etc.). Meanwhile, auxin plays a critical role in HS-induced thermomorphogenesis. Note: Arrows depict positive regulation while lines with blunt ends indicate inhibition or negative regulation. Other abbreviations mentioned in the diagram: HSFs, heat shock factors; SOD, superoxide dismutase; POD, peroxidases; CAT, catalase.

Figure 3

A multi-pronged approach for reproductive-stage HS tolerance improvement in cereals. Driven by advances in genome sequencing and assembly technologies, combined conventional and modern plant breeding approaches remain the primary way through which complex quantitative traits, such as HS, can be improved. Meanwhile, the advent of omics technologies has helped to inform and galvanize genomics and plant breeding programs. Biotechnological interventions such as transgenesis considerably circumvent some of the shortcomings of the recombination breeding methods, while genome-editing techniques such as CRISPR-Cas9 significantly shorten breeding cycles andreduceg breeding costs. These endeavors are supported by other disciplines such as machine learning, bioinformatics and data analytics, robotics, crop modelling and decision support systems. In addition, agronomic options remain relevant in alleviating H+D stress effects on crops in the field. Abbreviations: HS, heat stress; GAB, genomics assisted breeding; GWAS, genome-wide association studies; GS, genomic selection; SB. Speed breeding; DHB, doubled-haploid breeding; HBB, haplotype-based breeding; FFB, fast-forward breeding; SSD, single-seed descent; CRISPR-Cas9, clustered regularly interspaced palindromic repeats (CRISPR)–CRISPR-associated protein 9; ZFNs, zinc-finger nucleases; TALENs, transcription activator-like effector nucleases; CA, conservation agriculture; H+D, combined heat and drought.