The Long-Distance Transport of Some Plant Hormones and Possible Involvement of Lipid-Binding and Transfer Proteins in Hormonal Transport

Abstract

Adaptation to changes in the environment depends, in part, on signaling between plant organs to integrate adaptive response at the level of the whole organism. Changes in the delivery of hormones from one organ to another through the vascular system strongly suggest that hormone transport is involved in the transmission of signals over long distances. However, there is evidence that, alternatively, systemic responses may be brought about by other kinds of signals (e.g., hydraulic or electrical) capable of inducing changes in hormone metabolism in distant organs. Long-distance transport of hormones is therefore a matter of debate. This review summarizes arguments for and against the involvement of the long-distance transport of cytokinins in signaling mineral nutrient availability from roots to the shoot. It also assesses the evidence for the role of abscisic acid (ABA) and jasmonates in long-distance signaling of water deficiency and the possibility that Lipid-Binding and Transfer Proteins (LBTPs) facilitate the long-distance transport of hormones. It is assumed that proteins of this type raise the solubility of hydrophobic substances such as ABA and jasmonates in hydrophilic spaces, thereby enabling their movement in solution throughout the plant. This review collates evidence that LBTPs bind to cytokinins, ABA, and jasmonates and that cytokinins, ABA, and LBTPs are present in xylem and phloem sap and co-localize at sites of loading into vascular tissues and at sites of unloading from the phloem. The available evidence indicates a functional interaction between LBTPs and these hormones.

Keywords: abscisic acid; cytokinins; jasmonic acid; lipid-binding and transfer proteins; long-distance transport; phytohormones.

Figure 1

Long-distance transport and signaling of cytokinins, jasmonates, and ABA. Nitrates stimulate the synthesis of cytokinins in roots, while NaCl and deficiency of water stimulate the synthesis of jasmonates and ABA, which are transported from roots to shoots through the xylem, where cytokinins stimulate leaf growth, whereas ABA and jasmonates close the stomata. LTP possibly contributes to JA loading into the xylem and transport to the leaves. Damage to one leaf can result in the transfer of JA through the phloem to another leaf, thereby contributing to increased resistance. Transport of JA and ABA from leaves to roots up-regulates the expression of aquaporin genes, thereby increasing hydraulic conductance and influencing root growth. Nitrogen foliar feeding increases the loading of leaf cytokinins into the phloem, which possibly bind to PR-10 (PDB 2FLH, PR-10 from Vigna radiata forms a complex with zeatin [105]). This complex is transported from shoots to roots. PR-10 of Vigna radiata in complex with zeatin (PDB 2FLH) and LTP of Pisum sativum, capable of binding JA and ABA (PDB 2N81), are shown [72,77].

Figure 2

LTP (a,b) and JA (c,d) immunohistochemical localization in the central cylinder of the basal part of the roots of the control (a,c) and salt-treated (75 mM NaCl) (b,d) pea plants. The intensity of fluorescence of the second antibody against rabbit immunoglobulins is color-coded; a green color corresponds to lower fluorescence, while blue and red colors reflect a gradual increase in fluorescence. x—xylem, ph—phloem. Scale 100 µm. The principle of the experimental design is described in [77]. The novelty of our results lies in the use of antibodies against JA (Agrisera, Vannas, Sweden) to visualize the localization of jasmonates by an Olympus FV3000 Fluoview (FV31-HSD) confocal laser scanning microscope (Olympus, Tokyo, Japan) with an excitation laser wavelength of 561 nm. The results have not been published anywhere before.