FASCICLIN-LIKE 18 Is a New Player Regulating Root Elongation in Arabidopsis thaliana

The functional conservation of SSR1 homologs in plants

AtSSR1 (Arabidopsis thaliana short and swollen root 1) is a plant-specific gene that encodes a mitochondrial protein containing TPR (tetratricopeptide repeat) domains, and was shown to be required for maintaining mitochondrial function. However, the evolution of its function in the plant lineage is not known. In this paper, SSR1 homologs were cloned from seven representative plant species ranging from lower to higher plants. Their structural and functional conservation were characterized in detail. The results demonstrated that most of the SSR1 homologs are predicted to have mitochondrial localization except for the one from Physcomitrella patens and all of them possess various numbers of TPR domains. Upon introduction into the Arabidopsis ssr1-2 knock-out mutant, all SSR1 homologs were capable of fully rescuing the short-root and stress hypersensitive phenotype of the mutant. In addition, in vitro pull-down analysis showed that similar to AtSSR1, the selected SSR1 homologs were also able to interact with AtHSCA2 and AtISU1, two components of mitochondrial iron-sulfur cluster assembly pathway, suggesting that SSR1 homologs from various plant species are functionally conserved. Despite the conserved function, different SSR1 homologs shared relatively low sequence identity with AtSSR1. Instead, their 3D structures display a common feature of a globular shape, mainly dominated by α-helices and two β-sheets embedded in the center. Taken together, our results suggest that SSR1, as a plant-specific mitochondrial protein, might have complete functionality in plant development and stress response already in the early stage of plant evolution.

The banana MaFLA27 confers cold tolerance partially through modulating cell wall remodeling

Fasciclin-like arabinogalactan proteins (FLAs) have been shown to improve plant tolerance to salt stress. However, their role in cold tolerance (CT) remains unclear. Here, we report that banana MaFLA27 positively regulates CT in Arabidopsis. MaFLA27-overexpression (OE) caused the upregulation of differentially expressed arabinogalactan proteins (AGPs) and genes involved in the biosynthesis of cellulose, lignin, and xylan, as well as the degradation of pectin and xyloglucan. Correspondingly, MaFLA27-OE plants exhibited increased cell wall thickness, enhanced cellulose lignin and starch granule content, elevated levels of partially homogalacturonans recognized by JIM5 and JIM7 antibodies, xyloglucan components recognized by CCRC-M39/104 and LM15 antibodies, LM14 antibody binding AGPs. In contrast, transgenic plants showed a decreased degree of pectin methyl-esterification and accumulated less reactive oxygen species after cold acclimation when compared to wild-type plants. A higher number of pectin methylesterases and cellulose and xylan biosynthesis genes were elevated after cold acclimation. Additionally, both Arabidopsis mutant cesa8 and cellulose inhibitor-treated plants displayed decreased freezing tolerance. Our data suggested that MaFLA27-OE in Arabidopsis may perceive and transmit low-temperature stress signals to the cellulose synthase complexes, activating cellulose synthesis and enhancing cold tolerance. These findings reveal a previously unreported cold-tolerance function of FLAs and highlight associated cell wall-mediated tolerance mechanisms.

Insights into the ameliorative effect of ZnONPs on arsenic toxicity in soybean mediated by hormonal regulation, transporter modulation, and stress responsive genes

Arsenic (As) contamination of agricultural soils poses a serious threat to crop productivity and food safety. Zinc oxide nanoparticles (ZnONPs) have emerged as a potential amendment for mitigating the adverse effects of As stress in plants. Soybean crop is mostly grown on marginalized land and is known for high accumulation of As in roots than others tissue. Therefore, this study aimed to elucidate the underlying mechanisms of ZnONPs in ameliorating arsenic toxicity in soybean. Our results demonstrated that ZnOB significantly improved the growth performance of soybean plants exposed to arsenic. This improvement was accompanied by a decrease (55%) in As accumulation and an increase in photosynthetic efficiency. ZnOB also modulated hormonal balance, with a significant increase in auxin (149%), abscisic acid (118%), gibberellin (160%) and jasmonic acid content (92%) under As(V) stress assuring that ZnONPs may enhance root growth and development by regulating hormonal signaling. We then conducted a transcriptomic analysis to understand further the molecular mechanisms underlying the NPs-induced As(V) tolerance. This analysis identified genes differentially expressed in response to ZnONPs supplementation, including those involved in auxin, abscisic acid, gibberellin, and jasmonic acid biosynthesis and signaling pathways. Weighted gene co-expression network analysis identified 37 potential hub genes encoding stress responders, transporters, and signal transducers across six modules potentially facilitated the efflux of arsenic from cells, reducing its toxicity. Our study provides valuable insights into the molecular mechanisms associated with metalloid tolerance in soybean and offers new avenues for improving As tolerance in contaminated soils.

Combining Genetic and Transcriptomic Approaches to Identify Transporter-Coding Genes as Likely Responsible for a Repeatable Salt Tolerance QTL in Citrus

The excessive accumulation of chloride (Cl-) in leaves due to salinity is frequently related to decreased yield in citrus. Two salt tolerance experiments to detect quantitative trait loci (QTLs) for leaf concentrations of Cl-, Na+, and other traits using the same reference progeny derived from the salt-tolerant Cleopatra mandarin (Citrus reshni) and the disease-resistant donor Poncirus trifoliata were performed with the aim to identify repeatable QTLs that regulate leaf Cl- (and/or Na+) exclusion across independent experiments in citrus, as well as potential candidate genes involved. A repeatable QTL controlling leaf Cl- was detected in chromosome 6 (LCl-6), where 23 potential candidate genes coding for transporters were identified using the C. clementina genome as reference. Transcriptomic analysis revealed two important candidate genes coding for a member of the nitrate transporter 1/peptide transporter family (NPF5.9) and a major facilitator superfamily (MFS) protein. Cell wall biosynthesis- and secondary metabolism-related processes appeared to play a significant role in differential gene expression in LCl-6. Six likely gene candidates were mapped in LCl-6, showing conserved synteny in C. reshni. In conclusion, markers to select beneficial Cleopatra mandarin alleles of likely candidate genes in LCl-6 to improve salt tolerance in citrus rootstock breeding programs are provided.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Integrated multi-omics analysis provides insights into genome evolution and phosphorus deficiency adaptation in pigeonpea (Cajanus cajan)

Pigeonpea (Cajanus cajan) is an important legume food crop and plays a crucial role in a secure food supply in many developing countries. Several previous studies have suggested that pigeonpea has great potential for phosphorus (P) deficiency tolerance, but little is known about the underlying mechanism. In this study, the physiological and molecular responses of pigeonpea roots to phosphate (Pi) starvation were investigated through integrating phenotypic, genomic, transcriptomic, metabolomic, and lipidomic analyses. The results showed that low-Pi treatment increased total root length, root surface area, and root acid phosphatase activity, and promoted the secretion of organic acids (e.g. citric acids, piscidic acids, and protocatechuic acids) and the degradation of phospholipids and other P-containing metabolites in the roots of pigeonpea. Consistent with the morphological, physiological, and biochemical changes, a large number of genes involved in these Pi-starvation responses were significantly upregulated in Pi-deficient pigeonpea roots. Among these Pi-starvation response genes upregulated by low-Pi treatment, four gene families were expanded through recent tandem duplication in the pigeonpea genome, namely phosphate transporter 1 (PHT1), phosphoethanolamine/phosphocholine phosphatase (PECP), fasciclin-like arabinogalactan protein (FLA), and glutamate decarboxylase (GAD). These gene families may be associated with Pi uptake from the soil, phospholipid recycling, root morphological remodeling, and regulation of organic acid exudation. Taken together, our results suggest that pigeonpea employs complex Pi-starvation responses to strengthen P acquisition and utilization during low-Pi stress. This study provides new insights into the genome evolution and P deficiency adaptation mechanism of pigeonpea.

Comprehensive Analysis of Arabinogalactan Protein-Encoding Genes Reveals the Involvement of Three BrFLA Genes in Pollen Germination in Brassica rapa

Arabinogalactan proteins (AGPs) are a superfamily of hydroxyproline-rich glycoproteins that are massively glycosylated, widely implicated in plant growth and development. No comprehensive analysis of the AGP gene family has been performed in Chinese cabbage (Brassica rapa ssp. chinensis). Here, we identified a total of 293 putative AGP-encoding genes in B. rapa, including 25 classical AGPs, three lysine-rich AGPs, 30 AG-peptides, 36 fasciclin-like AGPs (FLAs), 59 phytocyanin-like AGPs, 33 xylogen-like AGPs, 102 other chimeric AGPs, two non-classical AGPs and three AGP/extensin hybrids. Their protein structures, phylogenetic relationships, chromosomal location and gene duplication status were comprehensively analyzed. Based on RNA sequencing data, we found that 73 AGP genes were differentially expressed in the floral buds of the sterile and fertile plants at least at one developmental stage in B. rapa, suggesting a potential role of AGPs in male reproductive development. We further characterized BrFLA2, BrFLA28 and BrFLA32, three FLA members especially expressed in anthers, pollen grains and pollen tubes. BrFLA2, BrFLA28 and BrFLA32 are indispensable for the proper timing of pollen germination under high relative humidity. Our study greatly extends the repertoire of AGPs in B. rapa and reveals a role for three members of the FLA subfamily in pollen germination.