Kewei Zhang

One important mechanism underlying the promotive action of rhizosphere bacteria on plants is their ability to produce plant hormones, particularly cytokinins. Cytokinin must be delivered from the roots to stimulate plant growth and exert beneficial effects on the shoots. This work aimed to study the transport of bacterial cytokinins in barley plants. We studied the effects of cytokinin-producing bacteria on the growth, cytokinin content, and expression of the HvABCG gene, a homolog of the rice OsABCG18 and Arabidopsis AtABCG14 genes encoding transporters of these hormones. Rhizosphere of 3-days-old barley seedlings were inoculated with Bacillus subtilis IB-22, and cytokinins were immunoassayed on the sixth day after bacterial treatment. On day 11, leaf growth stimulation was detected under the bacterial treatment, indicating that bacterial cytokinins affect shoot performance. This hypothesis was confirmed by a 1.5 increase in zeatin riboside levels registered in the shoots of bacteria-treated plants. Estimating the expression of the HvABCG gene showed its upregulation in the roots of bacteria-treated plants, suggesting the involvement of this transporter in the delivery of cytokinins to the shoots.

Abscisic acid (ABA) is a key phytohormone that regulates stomatal movement, seed germination, root development, and stress responses. Here, we report that Arabidopsis (Arabidopsis thaliana) ATP-binding cassette (ABC) transporter G family member 21 (AtABCG21) functions as an ABA transporter that plays a role in seed germination under salt stress. AtABCG21 exhibits efflux transporter activity toward ABA in tobacco (Nicotiana benthamiana) and yeast (Saccharomyces cerevisiae) cells. β-glucuronidase (GUS) staining showed that AtABCG21, predominantly expressed in embryos and vascular tissues, is induced by ABA and salt stress, respectively. Subcellular localization analysis of GFP-AtABCG21 in N. benthamiana leaves indicated that AtABCG21 localizes to the plasma membrane. The inflorescence intensity of the ABSCISIC ACID RESPONSIVE ELEMENT (ABRE)-based synthetic promoter-driven GFP reporter ABRE::GFP was significantly reduced in the embryos of the atabcg21 mutant under salt stress, suggesting that disruption of AtABCG21 compromised ABA signaling in the embryo. Consistently, quantification of ABA and ABA-GE revealed that the levels of ABA, but not ABA-GE, in the seeds of the atabcg21 mutant were significantly reduced compared to those in wild-type plants under salt stress. Consequently, the seed germination rate of the atabcg21 mutants was increased compared to that of wild-type plants under salt stress. These results demonstrate that AtABCG21 acts as an ABA efflux transporter, facilitating ABA translocation and influencing seed germination under salt stress. Our findings provide new insights into the mechanisms of ABA-mediated stress adaptation in plants.

Salicylic acid (SA) is a pivotal phytohormone for plant responses to biotic and abiotic stresses. Plants have evolved two pathways to produce SA: the isochorismate synthase and phenylalanine ammonia lyase (PAL) pathways¹. Whereas the isochorismate synthase pathway has been fully identified^2–4, the PAL pathway remains incomplete. Here we report the full characterization of the PAL pathway for SA biosynthesis via functional analysis of rice (Oryza sativa) SA-DEFICIENT GENE 1 (OSD1) to OSD4. The cinnamoyl-coenzyme A (CoA) ligase OSD1 catalyses the conversion of trans-cinnamic acid to cinnamoyl-CoA, which is subsequently transformed to benzoyl-CoA via the β-oxidative pathway in peroxisomes. The resulting benzoyl-CoA is further converted to benzyl benzoate by the peroxisomal benzoyltransferase OSD2. Benzyl benzoate is subsequently hydroxylated to benzyl salicylate by the endoplasmic reticulum membrane-resident cytochrome P450 OSD3, which is ultimately hydrolysed to salicylic acid by the cytoplasmic carboxylesterase OSD4. Evolutionary analyses reveal that the PAL pathway was first assembled before the divergence of gymnosperms and has been conserved in most seed plants. Activation of the PAL pathway in rice significantly enhances salicylic acid levels and plant immunity. Completion of the PAL pathway provides critical insights into the primary salicylic acid biosynthetic pathway across plant species and offers a precise target for modulating crop immunity.

The precise timing of flowering is crucial for the transition from vegetative to reproductive phases and is regulated by both endogenous and exogenous signals. Here, we demonstrate that DTX44, a member of the multidrug and toxic compound extrusion (MATE) transporter family, plays a role in facilitating flowering in Arabidopsis thaliana . Overexpressing of DTX44 resulted in early flowering, while a dtx44 mutant exhibited delayed flowering. The early‐flowering phenotype induced by DTX44 overexpression was repressed by the ft ( FLOWERING LOCUS T ) mutant. In contrast, the delayed flowering phenotype of dtx44 mutants was suppressed by the flc ( FLOWERING LOCUS C ) mutant. Subcellular localization analysis of GFP‐DTX44 in the Arabidopsis protoplasts and Nicotiana benthamiana leaves suggests that DTX44 localizes to the endoplasmic reticulum. GUS staining of the DTX44pro:DTX44‐GUS transgenic plants indicated that the DTX44 gene is widely expressed in the shoots and roots. The grafting experiments between the dtx44 mutant or DTX44 overexpression lines and wild type suggested that DTX44 expression in the shoots and roots contribute to flowering time. RNA‐seq analysis of dtx44 mutant and wild‐type seedlings suggests that DTX44 is involved in the biosynthesis or signaling of multiple hormones in response to abiotic stresses. In summary, our study reveals the biological function of the MATE transporter DTX44 in Arabidopsis flowering. The substrate of DTX44 remains to be elucidated in future studies.

Parthenocarpy, the development of fruit without fertilization, is a significant trait in fingered citron (Citrus medica L. var. sarcodactylis Swingle). This study aims to elucidate the regulatory mechanisms underlying parthenocarpy through comparative transcriptome analysis and phytohormone profiling between fingered citron and its non-parthenocarpic counterpart, citron. A total of 66 differentially expressed hormone-related genes were identified, with the auxin pathway emerging as the most prominent in fingered citron. Protein–protein interaction analysis revealed a potential interaction between auxin and abscisic acid (ABA). Phytohormone content analysis indicated that fingered citron exhibited higher levels of indole-3-acetic acid (IAA) and lower levels of ABA compared to citron. A weighted gene co-expression network analysis (WGCNA) suggested that the interplay between auxin and ABA is crucial for triggering parthenocarpy. Virus-induced gene silencing (VIGS) experiments demonstrated that silencing CmsABI5 led to decreased ABA levels, while auxin levels remained unchanged, resulting in disrupted parthenocarpy and increased ethylene levels, indicating a secondary hormonal response. Additionally, gene expression changes associated with failed parthenocarpy showed down-regulation of SAUR50 and up-regulation of PP2C 56, linking these changes to ABA signaling. Our findings highlight the central regulatory role of auxin in parthenocarpy in fingered citron, with ABA acting as a critical modulator. The suppression of ABA alone disrupts parthenocarpy despite stable auxin levels, emphasizing the necessity of balanced hormone interactions. This research underscores the significant role of auxin, rather than gibberellin (GA), in regulating parthenocarpy in fingered citron, contributing valuable insights to the understanding of fruit development mechanisms.

Peptide hormones regulate plant development, growth, and stress responses. Sulfated peptides represent a class of proteins that undergo posttranslational modification by tyrosylprotein sulfotransferase (TPST), followed by specific enzymatic cleavage to generate mature peptides. This process contributes to the formation of various bioactive peptides, including PSKs (PHYTOSULFOKINEs), PSYs (PLANT PEPTIDE CONTAINING SULFATED TYROSINE), CIFs (CASPARIAN STRIP INTEGRITY FACTOR), and RGFs (ROOT MERISTEM GROWTH FACTOR). In the past three decades, significant progress has been made in understanding the molecular mechanisms of sulfated peptides that regulate plant development, growth, and stress responses. In this review, we explore the sequence properties of precursors, posttranslational modifications, peptide receptors, and signal transduction pathways of the sulfated peptides, analyzing their functions in plants. The cross-talk between PSK/RGF peptides and other phytohormones, such as brassinosteroids, auxin, salicylic acid, abscisic acid, gibberellins, ethylene, and jasmonic acid, is also described. The significance of sulfated peptides in crops and their potential application for enhancing crop productivity are discussed, along with future research directions in the study of sulfated peptides.