Promoter profiling of Arabidopsis amino acid transporters: clues for improving crops

CRISPR/Cas9-induced knockout of an amino acid permease gene (AAP6) reduced Arabidopsis thaliana susceptibility to Meloidogyne incognita

BACKGROUND

Plant-parasitic root-knot nematode (Meloidogyne incognita) causes global yield loss in agri- and horticultural crops. Nematode management options rely on chemical method. However, only a handful of nematicides are commercially available. Resistance breeding efforts are not sustainable because R gene sources are limited and nematodes have developed resistance-breaking populations against the commercially available Mi-1.2 gene-expressing tomatoes. RNAi crops that manage nematode infection are yet to be commercialized because of the regulatory hurdles associated with transgenic crops. The deployment of the CRISPR/Cas9 system to improve nematode tolerance (by knocking out the susceptibility factors) in plants has emerged as a feasible alternative lately.

RESULTS

In the present study, a M. incognita-responsive susceptibility (S) gene, amino acid permease (AAP6), was characterized from the model plant Arabidodpsis thaliana by generating the AtAAP6 overexpression line, followed by performing the GUS reporter assay by fusing the promoter of AtAAP6 with the β-glucuronidase (GUS) gene. Upon challenge inoculation with M. incognita, overexpression lines supported greater nematode multiplication, and AtAAP6 expression was inducible to the early stage of nematode infection. Next, using CRISPR/Cas9, AtAAP6 was selectively knocked out without incurring any growth penalty in the host plant. The 'Cas9-free' homozygous T3 line was challenge inoculated with M. incognita, and CRISPR-edited A. thaliana plants exhibited considerably reduced susceptibility to nematode infection compared to the non-edited plants. Additionally, host defense response genes were unaltered between edited and non-edited plants, implicating the direct role of AtAAP6 towards nematode susceptibility.

CONCLUSION

The present findings enrich the existing literature on CRISPR/Cas9 research in plant-nematode interactions, which is quite limited currently while compared with the other plant-pathogen interaction systems.

Genome-wide analysis of the MADS-box gene family involved in salt and waterlogging tolerance in barley (Hordeum vulgare L.)

MADS-box transcription factors are crucial members of regulatory networks underlying multiple developmental pathways and abiotic stress regulatory networks in plants. Studies on stress resistance-related functions of MADS-box genes are very limited in barley. To gain insight into this gene family and elucidate their roles in salt and waterlogging stress resistance, we performed genome-wide identification, characterization and expression analysis of MADS-box genes in barley. A whole-genome survey of barley revealed 83 MADS-box genes, which were categorized into type I (Mα, Mβ and Mγ) and type II (AP1, SEP1, AGL12, STK, AGL16, SVP and MIKC*) lineages based on phylogeny, protein motif structure. Twenty conserved motifs were determined and each HvMADS contained one to six motifs. We also found tandem repeat duplication was the driven force for HvMADS gene family expansion. Additionally, the co-expression regulatory network of 10 and 14 HvMADS genes was predicted in response to salt and waterlogging stress, and we proposed HvMADS11,13 and 35 as candidate genes for further exploration of the functions in abiotic stress. The extensive annotations and transcriptome profiling reported in this study ultimately provides the basis for MADS functional characterization in genetic engineering of barley and other gramineous crops.

Amino acid transporter gene TaATLa1 from Triticum aestivum L. improves growth under nitrogen sufficiency and is down regulated under nitrogen deficiency

TaATLa1 was identified to respond to nitrogen deprivation through transcriptome analysis of wheat seedlings. TaATLa1 specifically transports Gln, Glu, and Asp, and affects the biomass of Arabidopsis and wheat. Nitrogen is an essential macronutrient and plays a crucial role in wheat production. Amino acids, the major form of organic nitrogen, are remobilized by amino acid transporters (AATs) in plants. AATs are commonly described as central components of essential developmental processes and yield formation via taking up and transporting amino acids in plants. However, few studies have reported the detailed biochemical properties and biological functions of these AATs in wheat. In this study, key genes encoding AATs were screened from transcriptome analysis of wheat seedlings treated with normal nitrogen (NN) and nitrogen deprivation (ND). Among them, 21 AATs were down-regulated and eight AATs were up-regulated under ND treatment. Among the homoeologs, TaATLa1.1-3A, TaATLa1.1-3B, and TaATLa1.1-3D (TaATLa1.1-3A, -3B, and -3D), belonging to amino acid transporter-like a (ATLa) subfamily, were significantly down-regulated in response to ND in wheat, and accordingly were selected for functional analyses. The results demonstrated that TaATLa1.1-3A, -3B, and -3D effectively transported glutamine (Gln), glutamate (Glu), and aspartate (Asp) in yeast. Overexpression of TaAILa1.1-3A, -3B, and -3D in Arabidopsis thaliana L. significantly increased amino acid content in leaves, storage protein content in seeds and the plant biomass under NN. Knockdown of TaATLa1.1-3A, -3B, and -3D in wheat seedlings resulted in a significant block of amino acid remobilization and growth inhibition. Taken together, TaATLa1.1-3A, -3B, and -3D contribute substantially to Arabidopsis and wheat growth. We propose that TaATLa1.1-3A, -3B, and -3D may participate in the source-sink translocation of amino acid, and they may have profound implications for wheat yield improvement.

Elucidation of the Flavor Aspects and Flavor-Associated Genomic Regions in Bottle Gourd (Lagenaria siceraria) by Metabolomic Analysis and QTL-seq

Bottle gourd (Lagenaria siceraria) is a commercially important cucurbitaceous vegetable with health-promoting properties whose collections and cultivars differ considerably in their flavor aspects. However, the metabolomic profile related to flavor has not yet been elucidated. In the present study, a comprehensive metabolite analysis revealed the metabolite profile of the strong-flavor collection “J120” and weak-flavor collection “G32”. The major differentially expressed metabolites included carboxylic acids, their derivatives, and organooxygen compounds, which are involved in amino acid biosynthesis and metabolism. QTL-seq was used to identify candidate genomic regions controlling flavor in a MAGIC population comprising 377 elite lines. Three significant genomic regions were identified, and candidate genes likely associated with flavor were screened. Our study provides useful information for understanding the metabolic causes of flavor variation among bottle gourd collections and cultivars. Furthermore, the identified candidate genomic regions may facilitate rational breeding programs to improve bottle gourd quality.

News about amino acid metabolism in plant-microbe interactions

Plants constantly come into contact with a diverse mix of pathogenic and beneficial microbes. The ability to distinguish between them and to respond appropriately is essential for plant health. Here we review recent progress in understanding the role of amino acid sensing, signaling, transport, and metabolism during plant-microbe interactions. Biochemical pathways converting individual amino acids into active compounds have recently been elucidated, and comprehensive large-scale approaches have brought amino acid sensors and transporters into focus. These findings show that plant central amino acid metabolism is closely interwoven with stress signaling and defense responses at various levels. The individual biochemical mechanisms and the interconnections between the different processes are just beginning to emerge and might serve as a foundation for new plant protection strategies.

Amino acid transporter (AAT) gene family in Tartary buckwheat (Fagopyrum tataricum L. Gaertn.): Characterization, expression analysis and functional prediction

Tartary buckwheat (Fagopyrum tataricum L. Gaertn., TB) is an ancient minor crop and an important food source for humans to supplement nutrients such as flavonoids and essential amino acids. Amino acid transporters (AATs) play critical roles in plant growth and development through the transport of amino acids. In this study, 104 AATs were identified in TB genome and divided into 11 subfamilies by phylogenetic relationships. Tandem and segmental duplications promoted the expansion of FtAAT gene family, and the variations of gene sequence, protein structure and expression pattern were the main reasons for the functional differentiation of FtAATs. Based on RNA-seq and qRT-PCR, the expression patterns of FtAATs in different tissues and under different abiotic stresses were analyzed, and several candidate FtAATs that might affect grain development and response to abiotic stresses were identified, such as FtAAP12 and FtCAT7. Finally, combined with the previous studies, the expression patterns and phylogenetic relationships of AATs in multiple species, the functions of multiple high-confidence FtAAT genes were predicted, and the schematic diagram of FtAATs in TB was initially drawn. Overall, this work provided a framework for further functional analysis of FtAAT genes and important clues for the improvement of TB quality and stress resistance.

Effect of ACGT motif in spatiotemporal regulation of AtAVT6D, which improves tolerance to osmotic stress and nitrogen-starvation

Plasma membrane-localized AtAVT6D importing aspartic acid can be targeted to develop plants with enhanced osmotic and nitrogen-starvation tolerance. AtAVT6D promoter can be exploited as a stress-inducible promoter for genetic improvements to raise stress-resilient crops. The AtAVT6 family of amino acid transporters in Arabidopsis thaliana has been predicted to export amino acids like aspartate and glutamate. However, the functional characterization of these amino acid transporters in plants remains unexplored. The present study investigates the expression patterns of AtAVT6 genes in different tissues and under various abiotic stress conditions using quantitative Real-time PCR. The expression analysis demonstrated that the member AtAVT6D was significantly induced in response to phytohormone ABA and stresses like osmotic and drought. The tissue-specific expression analysis showed that AtAVT6D was strongly expressed in the siliques. Taking together these results, we can speculate that AtAVT6D might play a vital role in silique development and abiotic stress tolerance. Further, subcellular localization study showed AtAVT6D was localized to the plasma membrane. The heterologous expression of AtAVT6D in yeast cells conferred significant tolerance to nitrogen-deficient and osmotic stress conditions. The Xenopus oocyte studies revealed that AtAVT6D is involved in the uptake of Aspartic acid. While overexpression of AtAVT6D resulted in smaller siliques in Arabidopsis thaliana. Additionally, transient expression studies were performed with the full-length AtAVT6D promoter and its deletion constructs to study the effect of ACGT-N₂₄-ACGT motifs on the reporter gene expression in response to abiotic stresses and ABA treatment. The fluorometric GUS analyses revealed that the promoter deletion construct-2 (Pro.C2) possessing a single copy of ACGT-N₂₄-ACGT motif directed the strongest GUS expression under all the abiotic conditions tested. These results suggest that Pro.C2 can be used as a stress-inducible promoter to drive a significant transgene expression.