Drought is one of the major constraints to rain-fed agricultural production, especially under climate change conditions. Plants evolved an array of adaptive strategies that perceive stress stimuli and respond to these stress signals through specific mechanisms. Abscisic acid (ABA) is a premier signal for plants to respond to drought and plays a critical role in plant growth and development. ABA triggers a variety of physiological processes such as stomatal closure, root system modulation, organizing soil microbial communities, activation of transcriptional and post-transcriptional gene expression, and metabolic alterations. Thus, understanding the mechanisms of ABA-mediated drought responses in plants is critical for ensuring crop yield and global food security. In this review, we highlighted how plants adjust ABA perception, transcriptional levels of ABA- and drought-related genes, and regulation of metabolic pathways to alter drought stress responses at both cellular and the whole plant level. Understanding the synergetic role of drought and ABA will strengthen our knowledge to develop stress-resilient crops through integrated advanced biotechnology approaches. This review will elaborate on ABA-mediated drought responses at genetic, biochemical, and molecular levels in plants, which is critical for advancement in stress biology research.

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Mechanisms of Abscisic Acid-Mediated Drought Stress Responses in Plants

Root system architecture (RSA) is an important developmental and agronomic trait that is regulated by various physical factors such as nutrients, water, microbes, gravity, and soil compaction as well as hormone-mediated pathways. Phytohormones act as internal mediators between soil and RSA to influence various events of root development, starting from organogenesis to the formation of higher order lateral roots (LRs) through diverse mechanisms. Apart from interaction with the external cues, root development also relies on the complex web of interaction among phytohormones to exhibit synergistic or antagonistic effects to improve crop performance. However, there are considerable gaps in understanding the interaction of these hormonal networks during various aspects of root development. In this review, we elucidate the role of different hormones to modulate a common phenotypic output, such as RSA in Arabidopsis and crop plants, and discuss future perspectives to channel vast information on root development to modulate RSA components.

Understanding the Intricate Web of Phytohormone Signalling in Modulating Root System Architecture.

By 2050, the world population will increase to 10 billion which urged global demand for food production to double. Plant disease and land drought will make the situation more dire, and safer and environment-friendly materials are thus considered as a new countermeasure. The rice blast fungus, Magnaporthe oryzae, causes one of the most destructive diseases of cultivated rice worldwide that seriously threatens rice production. Unfortunately, traditional breeding nor chemical approaches along control it well. Nowadays, nanotechnology stands as a new weapon against these mounting challenges and silica nanoparticles (SiO2 NPs) have been considered as potential new safer agrochemicals recently but the systematically studies remain limited, especially in rice.Salicylic acid (SA) is a key plant hormone essential for establishing plant resistance to several pathogens and its further affected a special form of induced resistance, the systemic acquired resistance (SAR), which considered as an important aspect of plant innate immunity from the locally induced disease resistance to the whole plant. Here we showed that SiO2 NPs could stimulate plant immunity to protect rice against M. oryzae through foliar treatment that significantly decreased disease severity by nearly 70% within an appropriate concentration range. Excessive concentration of foliar treatment led to disordered intake and abnormal SA responsive genes expressions which weaken the plant resistance and even aggravated the disease. Importantly, this SA-dependent fungal resistance could achieve better results with root treatment through a SAR manner with no phytotoxicity since the orderly and moderate absorption. What's more, root treatment with SiO2 NPs could also promote root development which was better to deal with drought.Taken together, our findings not only revealed SiO2 NPs as a potential effective and safe strategy to protect rice against biotic and abiotic stresses, but also identify root treatment for the appropriate application method since it seems not causing negative effects and even have promotion on root development. 

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Silica nanoparticles protect rice against biotic and abiotic stresses

Salt stress is a major threat to crop quality and yield. Most experiments on salt stress-related genes have been conducted at the laboratory or greenhouse scale. Consequently, there is a lack of research demonstrating the merit of exploring these genes in field crops. Here, we found that the R2R3-MYB transcription factor SiMYB19 from foxtail millet is expressed mainly in the roots and is induced by various abiotic stressors such as salt, drought, low nitrogen, and abscisic acid. SiMYB19 is tentatively localized to the nucleus and activates transcription. It enhances salt tolerance in transgenic rice at the germination and seedling stages. SiMYB19 overexpression increased shoot height, grain yield, and salt tolerance in field- and salt pond-grown transgenic rice. SiMYB19 overexpression promotes abscisic acid (ABA) accumulation in transgenic rice and upregulates the ABA synthesis gene OsNCED3 and the ABA signal transduction pathway-related genes OsPK1 and OsABF2. Thus, SiMYB19 improves salt tolerance in transgenic rice by regulating ABA synthesis and signal transduction. Using rice heterologous expression analysis, the present study introduced a novel candidate gene for improving salt tolerance and increasing yield in crops grown in saline-alkali soil.

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SiMYB19 from Foxtail Millet (Setaria italica) Confers Transgenic Rice Tolerance to High Salt Stress in the Field

As a result of global warming, plants are subjected to ever-increasing abiotic stresses including heat and drought. Drought stress frequently co-occurs with heat stress as a result of water evaporation. These stressors have adverse effects on crop production, which in turn affects human food security. Rice is a major food resource grown widely in crop-producing regions throughout the world. However, increasingly common heat and drought stresses in growth regions can have negative impacts on seedling morphogenesis, reproductive organ establishment, overall yield, and quality. This review centers on responses to heat and drought stress in rice. Current knowledge of molecular regulation mechanisms is summarized. We focus on approaches to cope with heat and drought stress, both at the genetic level and from an agricultural practice perspective. This review establishes a basis for improving rice stress tolerance, grain quality, and yield for human benefit. 

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Enhancement of Heat and Drought Stress Tolerance in Rice by Genetic Manipulation: A Systematic Review

Some of MADS-box transcription factors (TFs) have been shown to play essential roles in the adaptation of plant to abiotic stress. Still, the mechanisms that MADS-box proteins regulate plant stress response are not fully understood. Here, a stress-responsive MADS-box TF OsMADS23 from rice conferring the osmotic stress tolerance in plants is reported. Overexpression of OsMADS23 remarkably enhanced, but knockout of the gene greatly reduced the drought and salt tolerance in rice plants. Further, OsMADS23 was shown to promote the biosynthesis of endogenous ABA and proline by activating the transcription of target genes OsNCED2, OsNCED3, OsNCED4 and OsP5CR that are key components for ABA and proline biosynthesis, respectively. Then, the convincing evidence showed that the OsNCED2-knockout mutants had lower ABA levels and exhibited higher sensitivity to drought and oxidative stress than wild type, which is similar to osmads23 mutant. Interestingly, the SnRK2-type protein kinase SAPK9 was found to physically interact with and phosphorylate OsMADS23, and thus increase its stability and transcriptional activity. Furthermore, the activation of OsMADS23 by SAPK9-mediated phosphorylation is dependent on ABA in plants. Collectively, these findings establish a mechanism that OsMADS23 functions as a positive regulator in response to osmotic stress by regulating ABA biosynthesis, and provide a new strategy for improving drought and salt tolerance in rice.

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OsMADS23 phosphorylated by SAPK9 confers drought and salt tolerance by regulating ABA biosynthesis in rice.

FPF1 (flowering-promoting factor 1) is one of the important family involved in the genetic control of flowering time in plant. Until now, limited knowledge concerning FPF1 family in rice has been understood. As a homologue of AtFPF1, FPF1-like protein 4 of rice (OsFPFL4) is expressed in various tissues of plants. The functions of OsFPFL4 in rice were investigated by the reverse genetics approaches. Plants overexpressing OsFPFL4 have shorter primary root, more lateral roots and adventitious roots than wild type; however, RNA interference (RNAi) of OsFPFL4 significantly inhibits the growth of root system, and also delays the flowering time in rice. Interestingly, increased or repressed expression of OsFPFL4 leads to shrunken anthers and abnormal pollen grains. It is well recognized that auxin plays important roles in plant root and flower development, and the root elongation is also regulated by reactive oxygen species (ROS) homeostasis. Here, our results show that rice plants overexpressing OsFPFL4 accumulate more auxin in the shoot and root, whereas RNAi lines have less auxin than wild type. As expected, the transcript levels of genes responsible for auxin biosynthesis and polar transport are altered in these OsFPFL4 transgenic plants. As to ROS, slightly higher ROS levels were detected in overexpression root and inflorescence than the counterparts of wild type; however, the ROS levels were significantly increased in the RNAi lines, due to increased expression of ROS-producers and reduced expression of ROS-scavengers. Our results reveal that OsFPFL4 is involved in modulating the root and flower development by affecting auxin and ROS homeostasis in rice plants. OsFPFL4 controls auxin accumulation via affecting auxin biosynthesis and transport, and also modulates ROS homeostasis by balancing ROS producing and scavenging. Thus, auxin-mediated ROS production might play a role in regulating redox status, which controls plant root and flower development.

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Rongfeng Zeng

Papers

OsMADS23 phosphorylated by SAPK9 confers drought and salt tolerance by regulating ABA biosynthesis in rice.

OsFPFL4 is Involved in the Root and Flower Development by Affecting Auxin Levels and ROS Accumulation in Rice (Oryza sativa)