BioDesign Research / 2022 / Article / Fig 2

Review Article

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

Figure 2

A schematic representation of sensing and signalling cascades associated with various abiotic stresses. (a) Heat: heat induces misfolding of proteins that bind with HEAT SHOCK PROTEINS (HSPs), releasing HEAT SHOCK FACTORS (HSFs), which are then free to mediate heat-responsive gene expression. Misfolded proteins caused by heat stress can also activate the unfolded protein response (UPR) signalling pathway in the endoplasmic reticulum (ER). The ER-associated UPR signalling pathway has two arms, one involving two ER membrane–associated TFs, bZIP17, and bZIP28, and the other involving the RNA-splicing factor IRE1 and its target bZIP60 mRNA. When unfolded proteins attach to the luminal domain of IRE1, they dimerize (or oligomerize) and activate RNase activity, which cleaves bZIP60(u) mRNA, resulting in a spliced form of bZIP60. The spliced variant’s translation creates of active bZIP60 TF protein, which transport to the nucleus activates the stress-responsive genes. When BiP is separated from the ER-anchored transcription factors bZIP28/17, they are mobilised to the Golgi and delivered to the nucleus. bZIP28/17 binds to ER stress response elements in the nucleus to increase the transcription of UPR genes. Phytochrome-mediated signalling may detect warm temperatures. Heat-induced conversion of PhyB from the active Pfr form to the inactive Pr form frees PIF4 from Pfr inhibition, resulting in the activation of heat-responsive genes. When exposed to heat, ELF3 undergoes a phase change and aggregates, losing its capacity to suppress transcription of heat-responsive genes. Heat-induced replacement of H2A.Z by H2A in nucleosomes at specific genes enhances chromatin accessibility for transcription. Heat-induced Ca2+ spikes and ROS accumulation detect changes in membrane lipid fluidity. (b) Cold: calcium ion channels may contribute to cold-induced Ca2+ spikes by detecting altered membrane fluidity. In rice, OsCOLD1 is required for cold-induced Ca2+ increases. Cold stress activates the MEKK1–MKK2–MPK4 module (yellow box), linked to Ca2+ signalling via protein–protein interactions between CRLK1 and MEKK1. Additionally, cold induces the release of SnRK2.6, which results in the production of CBFs, which control the transcription of cold-responsive genes. Cold-induced Ca2+ signalling can directly activate the CBF regulon via the CAMTAs. (c) Drought and salinity both induce hyperosmotic stress, which is thought to alter the tension of the bilipid membrane, which may be recognised by Ca2+ channels leading to the induction of Ca2+ spikes. Both ABA-dependent and ABA-independent signalling are initiated in response to hyperosmotic stress. Additionally, stress-induced H2O2 is likely recognised by the Leucine-rich repeat receptor-like kinase (LRR-RLK) gene HPCA1 and, more particularly, by GUARD CELL HYDROGEN PEROXIDE RESISTANT1 (GHR1) in guard cells to produce Ca2+ signals via Ca2+ channel activation. This Ca2+ signal is sent to guard cells by protein kinases CPKs, which phosphorylate ABA-response effectors such as SLOW ANION CHANNEL-ASSOCIATED 1. (SLAC1), potentially enhancing stomatal closure in response to osmotic stress sensing. The signalling network demonstrates the critical functions of protein phosphorylation, calcium signalling, and ABA signalling in response to hyperosmotic stress. Salinity stress degrades the integrity of the cell wall, which may be detected by the LRX–RALF–FER module. Ca2+ stimulates the SOS3–SOS2–SOS1 pathway, which is responsible for maintaining cellular Na+ homeostasis.