Functional characterisation of Artemisia annua jasmonic acid carboxyl methyltransferase: a key enzyme enhancing artemisinin biosynthesis

MAIN CONCLUSION

Overexpression of Artemisia annua jasmonic acid carboxyl methyltransferase (AaJMT) leads to enhanced artemisinin content in Artemisia annua. Artemisinin-based combination therapies remain the sole deterrent against deadly disease malaria and Artemisia annua remains the only natural producer of artemisinin. In this study, the 1101 bp gene S-adenosyl-L-methionine (SAM): Artemisia annua jasmonic acid carboxyl methyltransferase (AaJMT), was characterised from A. annua, which converts jasmonic acid (JA) to methyl jasmonate (MeJA). From phylogenetic analysis, we confirmed that AaJMT shares a common ancestor with Arabidopsis thaliana, Eutrema japonica and has a close homology with JMT of Camellia sinensis. Further, the Clustal Omega depicted that the conserved motif I, motif III and motif SSSS (serine) required to bind SAM and JA, respectively, are present in AaJMT. The relative expression of AaJMT was induced by wounding, MeJA and salicylic acid (SA) treatments. Additionally, we found that the recombinant AaJMT protein catalyses the synthesis of MeJA from JA with a Km value of 37.16 µM. Moreover, site-directed mutagenesis of serine-151 in motif SSSS to tyrosine, asparagine-10 to threonine and glutamine-25 to histidine abolished the enzyme activity of AaJMT, thus indicating their determining role in JA substrate binding. The GC-MS analysis validated that mutant proteins of AaJMT were unable to convert JA into MeJA. Finally, the artemisinin biosynthetic and trichome developmental genes were upregulated in AaJMT overexpression transgenic lines, which in turn increased the artemisinin content.

Rice tonoplast intrinsic protein member OsTIP1;2 confers tolerance to arsenite stress

The International Agency for Research on Cancer categorizes arsenic (As) as a group I carcinogen. Arsenic exposure significantly reduces growth, development, metabolism, and crop yield. Tonoplast intrinsic proteins (TIPs) belong to the major intrinsic protein (MIP) superfamily and transport various substrates, including metals/metalloids. Our study aimed to characterize rice OsTIP1;2 in As[III] stress response. The gene expression analysis showed that the OsTIP1;2 expression was enhanced in roots on exposure to As[III] treatment. The heterologous expression of OsTIP1;2 in S. cerevisiae mutant lacking YCF1 (ycf1∆) complemented the As[III] transport function of the YCF1 transporter but not for boron (B) and arsenate As[V], indicating its substrate selective nature. The ycf1∆ mutant expressing OsTIP1;2 accumulated more As than the wild type (W303-1A) and ycf1∆ mutant strain carrying the pYES2.1 vector. OsTIP1;2 activity was partially inhibited in the presence of the aquaporin (AQP) inhibitors. The subcellular localization studies confirmed that OsTIP1;2 is localized to the tonoplast. The transient overexpression of OsTIP1;2 in Nicotiana benthamiana leaves resulted in increased activities of enzymatic and non-enzymatic antioxidants, suggesting a potential role in mitigating oxidative stress induced by As[III]. The transgenic N. tabacum overexpressing OsTIP1;2 displayed an As[III]-tolerant phenotype, with increased fresh weight and root length than the wild-type (WT) and empty vector (EV line). The As translocation factor (TF) for WT and EV was around 0.8, while that of OE lines was around 0.4. Moreover, the OE line bioconcentration factor (BCF) was more than 1. Notably, the reduced TF and increased BCF in the OE line imply the potential of OsTIP1;2 for phytostabilization.

Comparative Transcriptome Analysis Reveals the Effect of miR156a Overexpression on Mineral Nutrient Homeostasis in Nicotiana tabacum

Mineral nutrition plays an important role in crop growth, yield and quality. MiR156 is a regulatory hub for growth and development. To date, the understanding of miR156-mediated mineral homeostasis is limited. In this study, we overexpressed Nta-miR156a in the tobacco cultivar TN90 and analyzed the effects of miR156 on mineral element homeostasis in tobacco by comparative transcriptome analysis. The results showed that the overexpression of miR156a caused significant morphological changes in transgenic tobacco. Chlorophyll and three anti-resistance markers, proline, total phenolics, and total flavonoids, were altered due to increased miR156 expression levels. Interestingly, the distribution of Cu, Mn, Zn, and Fe in different tissues of transgenic tobacco was disordered compared with that of the wild type. Comparative transcriptome analysis showed that the overexpression of miR156 resulted in 2656 significantly differentially expressed genes. The expression levels of several metal-transport-related genes, such as NtABC, NtZIP, NtHMA, and NtCAX, were significantly increased or decreased in transgenic tobacco. These results suggest that miR156 plays an essential role in regulating mineral homeostasis. Our study provides a new perspective for the further study of mineral nutrient homeostasis in plants.

The Chlamydomonas reinhardtii chloroplast envelope protein LCIA transports bicarbonate in planta

LCIA is a chloroplast envelope protein associated with the CO2 concentrating mechanism of the green alga Chlamydomonas reinhardtii. LCIA is postulated to be a HCO3- channel, but previous studies were unable to show that LCIA was actively transporting bicarbonate in planta. Therefore, LCIA activity was investigated more directly in two heterologous systems: an E. coli mutant (DCAKO) lacking both native carbonic anhydrases and an Arabidopsis mutant (βca5) missing the plastid carbonic anhydrase βCA5. Both DCAKO and βca5 cannot grow in ambient CO2 conditions, as they lack carbonic anhydrase-catalyzed production of the necessary HCO3- concentration for lipid and nucleic acid biosynthesis. Expression of LCIA restored growth in both systems in ambient CO2 conditions, which strongly suggests that LCIA is facilitating HCO3- uptake in each system. To our knowledge, this is the first direct evidence that LCIA moves HCO3- across membranes in bacteria and plants. Furthermore, the βca5 plant bioassay used in this study is the first system for testing HCO3- transport activity in planta, an experimental breakthrough that will be valuable for future studies aimed at improving the photosynthetic efficiency of crop plants using components from algal CO2 concentrating mechanisms.