Identification and functional analysis of the mitochondrial cysteine synthase TtCsa2 from Tetrahymena thermophila

Cysteine is a crucial component for all organisms and plays a critical role in the structure, stability, and catalytic functions of many proteins. Tetrahymena has reverse transsulfuration and de novo pathways for cysteine biosynthesis. Cysteine synthase is involved in the de novo cysteine biosynthesis and catalyzes the production of cysteine from O-acetylserine. The novel cysteine synthase TtCSA2 was identified from Tetrahymena thermophila. The TtCSA2 showed high expression levels at the log-phase and the sexual development stage. The TtCsa2 was localized on the outer mitochondrial membrane throughout different developmental stages. However, the truncated N-terminal signal peptide mutant TtCsa2-ΔN23 was localized into the mitochondria. His-TtCsa2 was expressed in Escherichia coli and purified using affinity chromatography. The His-TtCsa2 showed O-acetylserine sulfhydrylase and serine sulfhydrylase activities. Cysteine and glutathione contents decreased in the csa2KD mutant. Furthermore, mutant cells were sensitive to cadmium and copper stresses. This study indicated that the TtCSA2 was involved in the cysteine synthesis in mitochondria and related to heavy metal stresses resistance in Tetrahymena.

Identification and Validation of Reference Genes for RT-qPCR Analysis in Switchgrass under Heavy Metal Stresses

Switchgrass (Panicum Virgatum L.) has been recognized as the new energy plant, which makes it ideal for the development of phytoremediation on heavy metal contamination in soils with great potential. This study aimed to screen the best internal reference genes for the real-time quantitative PCR (RT-qPCR) in leaves and roots of switchgrass for investigating its response to various heavy metals, such as cadmium (Cd), lead (Pb), mercury (Hg), chromium (Cr), and arsenic (As). The stability of fourteen candidate reference genes was evaluated by BestKeeper, GeNorm, NormFinder, and RefFinder software. Our results identified U2AF as the best reference gene in Cd, Hg, Cr, and As treated leaves as well as in Hg, Pb, As, and Cr stressed root tissues. In Pb treated leaf tissues, 18S rRNA was demonstrated to be the best reference gene. CYP5 was determined to be the optimal reference gene in Cd treated root tissues. The least stable reference gene was identified to be CYP2 in all tested samples except for root tissues stressed by Pb. To further validate the initial screening results, we used the different sets of combinatory internal reference genes to analyze the expression of two metal transport associated genes (PvZIP4 and PvPDB8) in young leaves and roots of switchgrass. Our results demonstrated that the relative expression of the target genes consistently changed during the treatment when CYP5/UBQ1, U2AF/ACT12, eEF1a/U2AF, or 18S rRNA/ACT12 were combined as the internal reference genes. However, the time-dependent change pattern of the target genes was significantly altered when CYP2 was used as the internal reference gene. Therefore, the selection of the internal reference genes appropriate for specific experimental conditions is critical to ensure the accuracy and reliability of RT-qPCR. Our findings established a solid foundation to further study the gene regulatory network of switchgrass in response to heavy metal stress.

The auxin influx carrier, OsAUX3, regulates rice root development and responses to aluminium stress

In rice, there are five members of the auxin carrier AUXIN1/LIKE AUX1 family; however, the biological functions of the other four members besides OsAUX1 remain unknown. Here, by using CRISPR/Cas9, we constructed two independent OsAUX3 knock-down lines, osaux3-1 and osaux3-2, in wild-type rice, Hwayoung (WT/HY) and Dongjin (WT/DJ). osaux3-1 and osaux3-2 have shorter primary roots (PRs), decreased lateral root (LR) density, and longer root hairs (RHs) compared with their WT. OsAUX3 expression in PRs, LRs, and RHs further supports that OsAUX3 plays a critical role in the regulation of root development. OsAUX3 locates at the plasma membrane and functions as an auxin influx carrier affecting acropetal auxin transport. OsAUX3 is up-regulated in the root apex under aluminium (Al) stress, and osaux3-2 is insensitive to Al treatments. Furthermore, 1-naphthylacetic acid accented the sensitivity of WT/DJ and osaux3-2 to respond to Al stress. Auxin concentrations, Al contents, and Al-induced reactive oxygen species-mediated damage in osaux3-2 under Al stress are lower than in WT, indicating that OsAUX3 is involved in Al-induced inhibition of root growth. This study uncovers a novel pathway alleviating Al-induced oxidative damage by inhibition of acropetal auxin transport and provides a new option for engineering Al-tolerant rice species.