Integrating Early Transcriptomic Responses to Rhizotoxins in Rice (Oryza sativa. L.) Reveals Key Regulators and a Potential Early Biomarker of Cadmium Toxicity

The role of priming and memory in rice environmental stress adaptation: Current knowledge and perspectives

Plant responses to abiotic stresses are dynamic, following the unpredictable changes of physical environmental parameters such as temperature, water and nutrients. Physiological and phenotypical responses to stress are intercalated by periods of recovery. An earlier stress can be remembered as 'stress memory' to mount a response within a generation or transgenerationally. The 'stress priming' phenomenon allows plants to respond quickly and more robustly to stressors to increase survival, and therefore has significant implications for agriculture. Although evidence for stress memory in various plant species is accumulating, understanding of the mechanisms implicated, especially for crops of agricultural interest, is in its infancy. Rice is a major food crop which is susceptible to abiotic stresses causing constraints on its cultivation and yield globally. Advancing the understanding of the stress response network will thus have a significant impact on rice sustainable production and global food security in the face of climate change. Therefore, this review highlights the effects of priming on rice abiotic stress tolerance and focuses on specific aspects of stress memory, its perpetuation and its regulation at epigenetic, transcriptional, metabolic as well as physiological levels. The open questions and future directions in this exciting research field are also laid out.

Comparative study of two indoor microbial volatile pollutants, 2-Methyl-1-butanol and 3-Methyl-1-butanol, on growth and antioxidant system of rice (Oryza sativa) seedlings

2-Methyl-1-butanol (2MB) and 3-Methyl-1-butanol (3MB) are microbial volatile organic compounds (VOCs) and found in indoor air. Here, we applied rice as a bioindicator to investigate the effects of these indoor microbial volatile pollutants. A remarkable decrease in germination percentage, shoot and root elongation, as well as lateral root numbers were observed in 3MB. Furthermore, ROS production increased by 2MB and 3MB, suggesting that pentanol isomers could induce cytotoxicity in rice seedlings. The enhancement of peroxidase (POD) and catalase (CAT) activity provided evidence that pentanol isomers activated the enzymatic antioxidant scavenging systems, with a more significant effect observed in 3MB. Furthermore, 3MB induced higher activity levels of glutathione (GSH), oxidized glutathione (GSSG), and the GSH/GSSG ratio in rice compared to the levels induced by 2MB. Additionally, qRT-PCR analysis showed more up-regulation in the expression of glutaredoxins (GRXs), peroxiredoxins (PRXs), thioredoxins (TRXs), and glutathione S-transferases (GSTUs) genes in 3MB. Taking the impacts of pentanol isomers together, the present study suggests that 3MB exhibits more cytotoxic than 2MB, as such has critical effects on germination and the early seedling stage of rice. Our results provide molecular insights into how isomeric indoor microbial volatile pollutants affect plant growth through airborne signals.

Transcriptomic and physio-biochemical features in rice (Oryza sativa L.) in response to mercury stress

Mercury (Hg) is a toxic and nonessential element for organisms, and its contamination in the environment is a global concern. Previous research has shown that Hg stress may cause severe damage to rice roots; however, the transcriptomic changes in roots and physio-biochemical responses in leaves to different levels of Hg stress are not fully understood. In the present study, rice seedlings were exposed to 20, 80, and 160 μM HgCl₂ for three days in hydroponic experiments. The results showed that the majority of Hg was accumulated in rice roots after Hg exposure, and the 80- and 160-μM Hg stresses significantly increased the root-to-shoot translocation factors relative to 20-μM Hg stress, resulting in elevated Hg concentrations in rice shoots. Only the 160-μM Hg stress significantly inhibited root growth compared with the control, while photosynthesis capacity in leaves was significantly reduced under Hg stress. RNA transcriptome sequencing analyses of the roots showed that common responsive differentially expressed genes were strongly associated with glutathione metabolism, amino acid biosynthesis, and secondary metabolite metabolism, which may play significant roles in Hg accumulation by rice plants. Nine crucial genes identified by protein-protein interaction network analysis may be used as candidate target genes for further investigation of the detoxification mechanism, encoding proteins involved in jasmonic acid synthesis, sugar metabolism, allene oxide synthase, glutathione peroxidase, dismutase, and catalase. Furthermore, physio-biochemical analyses of the leaves indicated that higher production of reactive oxygen species was induced by Hg stress, while glutathione and antioxidant enzymes may play crucial roles in Hg detoxification. Our findings provide transcriptomic and physio-biochemical features of rice roots and shoots, which advance our understanding of the responsive and detoxification mechanisms in rice under different levels of Hg stress.

Auxin-salicylic acid cross-talk ameliorates OsMYB-R1 mediated defense towards heavy metal, drought and fungal stress

The MYB TF family is an immensely large and functionally diverse class of proteins involved in the regulation of cell cycle, cell morphogenesis to stress signaling mechanism. The present study deciphered the hormonal cross-talk of wound inducible and stress-responsive OsMYB-R1 transcription factor in combating abiotic [Cr(VI) and drought/PEG] as well as biotic (Rhizoctonia solani) stress. OsMYB-R1 over-expressing rice transgenics exhibit a significant increase in lateral roots, which may be associated with increased tolerance under Cr(VI) and drought exposure. In contrast, its loss-of-function reduces stress tolerance. Higher auxin accumulation in the OsMYB-R1 over-expressed lines further strengthens the protective role of lateral roots under stress conditions. RNA-seq. data reveals over-representation of salicylic acid signaling molecule calcium-dependent protein kinases, which probably activate the stress-responsive downstream genes (Peroxidases, Glutathione S-transferases, Osmotins, Heat Shock Proteins, Pathogenesis Related-Proteins). Enzymatic studies further confirm OsMYB-R1 mediated robust antioxidant system as catalase, guaiacol peroxidase and superoxide dismutase activities were found to be increased in the over-expressed lines. Our results suggest that OsMYB-R1 is part of a complex network of transcription factors controlling the cross-talk of auxin and salicylic acid signaling and other genes in response to multiple stresses by modifying molecular signaling, internal cellular homeostasis and root morphology.