Embracing substrate multispecificity in plant ABC transporters

Mechanisms of auxin action in plant growth and development

The phytohormone auxin is a major signal coordinating growth and development in plants. The variety of its effects arises from its ability to form local auxin maxima and gradients within tissues, generated through directional cell-to-cell transport and elaborate metabolic control. These auxin distribution patterns instruct cells in a context-dependent manner to undergo predefined developmental transitions. In this Review, we discuss advances in auxin action at the level of homeostasis and signalling. We highlight key insights into the structural basis of PIN-mediated intercellular auxin transport and explore two novel non-transcriptional auxin signalling mechanisms: one involving intracellular Ca2+ transients and another involving cell-surface auxin perception that mediates global, ultrafast phosphorylation. Furthermore, we examine emerging evidence indicating the involvement of cyclic adenosine monophosphate as a second messenger in the transcriptional auxin response. Together, these recent developments in auxin research have profoundly deepened our understanding of the complex and diverse activities of auxin in plant growth and development.

Plant steroids on the move: mechanisms of brassinosteroid export

Brassinosteroids (BRs) are essential plant steroidal hormones that regulate growth and development. The recent discoveries of ATP-binding cassette subfamily B (ABCB) members, ABCB19 and ABCB1, as BR transporters highlight the significance of active export to the apoplast in maintaining BR homeostasis and enabling effective signaling. This review focuses on the latest progress in understanding ABCB-mediated BR transport, with particular attention to the structural and functional characterization of arabidopsis ABCB19 and ABCB1. These findings reveal both conserved and distinct features in substrate recognition and transport mechanisms, providing valuable insights into their roles in hormonal regulation. Additionally, the evolutionary conservation of ABC transporters in mediating steroid-based signaling across biological kingdoms underscores their fundamental biological significance.

ABA importers ABCG17 and ABCG18 redundantly regulate seed size in Arabidopsis

The stress hormone abscisic acid (ABA) plays a crucial role in mediating plant responses to the environment and regulating plant development. In this study, we demonstrate that two ABA importers, ABCG17 and ABCG18, control seed size by regulating the ABA levels transported into the embryo. Double knockdown of ABCG17 and ABCG18 resulted in lower ABA accumulation in the embryo, wider siliques, and increased overall seed size. Leaf phloem-specific ABA induction in the aba2-1 background showed that ABA could move from the vasculature to control seed size. ABCG17 and ABCG18 are expressed in leaves, and the reproductive organs septum, and valves but not in the developing seeds, suggesting that ABCG17 and ABCG18 affect seed size maternally. Together, the results shed light on the molecular mechanisms by which ABA is transported to the embryo to determine seed size.

The ABC transporter SmABCG1 mediates tanshinones export from the peridermic cells of Salvia miltiorrhiza root

Plants have mechanisms to transport secondary metabolites from where they are biosynthesized to the sites where they function, or to sites such as the vacuole for detoxification. However, current research has mainly focused on metabolite biosynthesis and regulation, and little is known about their transport. Tanshinone, a class diterpenoid with medicinal properties, is biosynthesized in the periderm of Salvia miltiorrhiza roots. Here, we discovered that tanshinone can be transported out of peridermal cells and secreted into the soil environment and that the ABC transporter SmABCG1 is involved in the efflux of tanshinone ⅡA and tanshinone Ⅰ. The SmABCG1 gene is adjacent to the diterpene biosynthesis gene cluster in the S. miltiorrhiza genome. The temporal-spatial expression pattern of SmABCG1 is consistent with tanshinone accumulation profiles. SmABCG1 is located on the plasma membrane and preferentially accumulates in the peridermal cells of S. miltiorrhiza roots. Heterologous expression in Xenopus laevis oocytes demonstrated that SmABCG1 can export tanshinone ⅡA and tanshinone Ⅰ. CRISPR/Cas9-mediated mutagenesis of SmABCG1 in S. miltiorrhiza hairy roots resulted in a significant decrease in tanshinone contents in both hairy roots and the culture medium, whereas overexpression of this gene resulted in increased tanshinone contents. CYP76AH3 transcript levels increased in hairy roots overexpressing SmABCG1 and decreased in knockout lines, suggesting that SmABCG1 may affect the expression of CYP76AH3, indirectly regulating tanshinone biosynthesis. Finally, tanshinone ⅡA showed cytotoxicity to Arabidopsis roots. These findings offer new perspectives on plant diterpenoid transport and provide a new genetic tool for metabolic engineering and synthetic biology research.