Group VII ethylene response factors forming distinct regulatory loops mediate submergence responses.

Group VII ethylene response factors (ERFVIIs), whose stability is oxygen concentration-dependent, play key roles in regulating hypoxia response genes in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) during submergence. To understand the evolution of flooding tolerance in cereal crops, we evaluated whether Brachypodium distachyon ERFVII genes (BdERFVIIs) are related to submergence tolerance. We found that three BdERFVIIs, BdERF108, BdERF018, and BdERF961, form a feedback regulatory loop to mediate downstream responses. BdERF108 and BdERF018 activated the expression of BdERF961 and PHYTOGLOBIN 1 (PGB1), which promoted nitric oxide turnover and preserved ERFVII protein stability. The activation of PGB1 was subsequently counteracted by increased BdERF961 accumulation through negative feedback regulation. Interestingly, we found that OsERF67, the orthologue of BdERF961 in rice, activated PHYTOGLOBIN (OsHB2) expression and formed distinct regulatory loops during submergence. Overall, the divergent regulatory mechanisms exhibited by orthologs collectively offer perspectives for the development of submergence-tolerant crops.

The Arabidopsis defensin gene, AtPDF1.1, mediates defence against Pectobacterium carotovorum subsp. carotovorum via an iron-withholding defence system.

Plant defensins (PDFs) are cysteine-rich peptides that have a range of biological functions, including defence against fungal pathogens. However, little is known about their role in defence against bacteria. In this study, we showed that the protein encoded by ARABIDOPSIS THALIANA PLANT DEFENSIN TYPE 1.1 (AtPDF1.1) is a secreted protein that can chelate apoplastic iron. Transcripts of AtPDF1.1 were induced in both systemic non-infected leaves of Arabidopsis thaliana plants and those infected with the necrotrophic bacterium Pectobacterium carotovorum subsp. carotovorum (Pcc). The expression levels of AtPDF1.1 with correct subcellular localization in transgenic A. thaliana plants were positively correlated with tolerance to Pcc, suggesting its involvement in the defence against this bacterium. Expression analysis of genes associated with iron homeostasis/deficiency and hormone signalling indicated that the increased sequestration of iron by apoplastic AtPDF1.1 overexpression perturbs iron homeostasis in leaves and consequently activates an iron-deficiency-mediated response in roots via the ethylene signalling pathway. This in turn triggers ethylene-mediated signalling in systemic leaves, which is involved in suppressing the infection of necrotrophic pathogens. These findings provide new insight into the key functions of plant defensins in limiting the infection by the necrotrophic bacterium Pcc via an iron-deficiency-mediated defence response.

Overexpression of Arabidopsis thaliana tryptophan synthase beta 1 (AtTSB1) in Arabidopsis and tomato confers tolerance to cadmium stress.

Tryptophan (Trp) is an essential amino acid in humans, and in plants, it plays a major role in the regulation of plant development and defence responses. However, little is known about Trp-mediated cadmium (Cd) tolerance. Gene expression analysis showed that Arabidopsis thaliana tryptophan synthase beta 1 (AtTSB1) is up-regulated in plants treated with Cd; hence, we investigated whether this gene is involved in Cd tolerance. Exogenous application of Trp to wild-type Arabidopsis enhances Cd tolerance. Cd tolerance in the Trp-overproducing mutant trp5-1 was associated with high chlorophyll levels and low lipid peroxidation, as indicated by malondialdehyde 4-hydroxyalkenal level, whereas the wild-type developed symptoms of severe chlorosis. Moreover, the Trp-auxotroph mutant trp2-1 was sensitive to Cd. CaMV 35S promoter-driven AtTSB1 enhanced Trp accumulation and improved Cd tolerance in transgenic Arabidopsis and tomato plants without increasing the level of Cd. Moreover, reverse transcription-polymerase chain reaction confirmed that enhanced level of Trp in AtTSB1 transgenic Arabidopsis plants affected the expression of AtZIP4 and AtZIP9 metal transporters, which interfered with Cd ion trafficking, a mechanism of transcriptional regulation that does not exist in wild-type plants. Overexpression of AtTSB1 in transgenic tomato also produced higher Trp synthase-beta enzyme activity than that in wild-type plants. These results implicate that Trp could be involved in Cd defence.