Transport mechanisms of plant hormones

Abstract

Plant growth, development, and response to the environment are mediated by a group of small signaling molecules named hormones. Plants regulate hormone response pathways at multiple levels, including biosynthesis, metabolism, perception, and signaling. In addition, plants exhibit the unique ability to spatially control hormone distribution. In recent years, multiple transporters have been identified for most of the plant hormones. Here we present an updated snapshot of the known transporters for the hormones abscisic acid, auxin, brassinosteroid, cytokinin, ethylene, gibberellin, jasmonic acid, salicylic acid, and strigolactone. We also describe new findings regarding hormone movement and elaborate on hormone substrate specificity and possible genetic redundancy in hormone transport and distribution. Finally, we discuss subcellular, cell-to-cell, and long-distance hormone movement and local hormone sinks that trigger or prevent hormone-mediated responses.

Keywords: ABA; ABC transporters; Abscisic acid; Auxin; Brassinosteroid; Cytokinin; Ethylene; Gibberellin; Hormone transport; IAA; Jasmonic acid; NPF transporters; Phytohormones; Plant hormones; Salicylic acid; Strigolactone; Transport mechanisms.

Fig. 1. Overview of plant hormones transporters.

Known transporters for each hormone are shown. Blue arrows represent importers; orange arrows represent exporters. The inset boxes are magnifications of the indicated organelles. The proteins transport the indicated bioactive hormone unless stated otherwise (ABA-GE stands for ABA Glucosyl Ester; IBA for Indole-3-Butyric Acid; ACC for L-aminocyclopropane-1-Carboxylic Acid; JA-Ile for Jasmonoyl-Isoleucine; OPDA for Cis-12-oxophytodienoic Acid). All hormone transporters were characterized in Arabidopsis thaliana unless stated otherwise (Sl, Solanum lycopersicum; Os, Oryza sativa; Mt, Medicago truncatula; Pa, Petunia axillaris). IC stands for isochorismate.

Fig. 2. Overview of hormone transport mechanisms in plants.

Diverse mechanisms of hormone transport are shown. A. Subcellular transport of hormones in or out of organelles (i.e., ABCD1 for peroxisomes and TOB1 for vacuole) leads to activity, storage, or degradation of the bioactive hormone, intermediates, or conjugated hormone forms. B. Negative sinks reduce hormone action. Several scenarios may lead to this phenomenon. First, insufficient hormone activity may result from import of the hormone into the cytosol, when the hormone receptors operate outside the plasma membrane (i.e., PUP14); second, the transporters may import the hormone into the cytosol where conjugation results in inactivity (i.e., ABCG17 and ABCG18); third, hormone export activity may lead to a decrease in the hormone activity in the cell (i.e., possibly ABCBs). C. Positive sinks enhances the activity of hormones by importing hormones or derivatives into the cell (i.e., NPF3 and NPF7.3). D. Cell-to-cell transport of hormones can occur via an exporter or an importer, allowing hormone gradients (i.e., PINs and AUX\LUXs). E. Long-distance transport of the hormone from root to shoot or vice versa occurs through the vasculature. This type of transport includes xylem and phloem loading and unloading (i.e., ABCG14 and DG1).