Endogenous cytokinins, auxins and abscisic acid inUlva fasciata(Chlorophyta) andDictyota humifusa(Phaeophyta): towards understanding their biosynthesis and homoeostasis

Abstract

Hormones are present in seaweeds, but little is known about their biosynthesis and regulation. The aim of this study was to investigate variation in concentration and composition of endogenous cytokinins, auxins and abscisic acid in two seaweeds collected over a 1 year period to gain a more complete picture of the types of hormones present during a range of environmental and developmental conditions. Ulva fasciata (Ulvales, Chlorophyta) and Dictyota humifusa (Dictyotales, Phaeophyta) were collected bimonthly from the inter-tidal zone at Rocky Bay, South Africa. Ethanol extracts of the samples containing a mixture of internal standards were purified using a combined DEAE-Sephadex-octadecylsilica column, followed by immuno-affinity chromatography and analysed by high-performance liquid chromatography -mass spectrometry to quantify different cytokinins, auxins and abscisic acid. cis-Zeatin, isopentenyladenine and their ribotide and riboside conjugates were the main cytokinins present in both U. fasciata and D. humifusa with low concentrations of trans-zeatin, dihydrozeatin and aromatic cytokinins. Very low concentrations of O-glucosides and riboside-O-glucosides and no N-glucosides were detected in any of the samples. Based on the cytokinins detected, we propose that as in higher plants, the ribotides are also the first intermediates formed during de novo biosynthesis in seaweeds and are subsequently converted to free-bases and ribosides. The only indole compounds detected in both species were free indole-3-acetic acid and indole-3-acetamide (IAM), suggesting that IAM is the intermediate in auxin biosynthesis. Endogenous abscisic acid was detected in most samples with levels in U. fasciata generally being higher than those measured in D. humifusa.

Euglena gracilis(Euglenophyceae) produces abscisic acid and cytokinins and responds to their exogenous application singly and in combination with other growth regulators

“…These results support the theory that the CK de novo synthesis pathway only evolved within the seed plants, whereas tRNA degradation is the predominant pathway of CK production in lower organisms (reviewed in Spichal, 2012). This is also consistent with earlier findings pertaining to endogenous CK profiles of seaweeds and microalgal Chlorophyta and Cyanophyta strains (Stirk et al, 1999(Stirk et al, , 2003(Stirk et al, , 2009Ordog et al, 2004).…”

“…However, in non-vascular plants such as mosses or algae, the free bases iP and cZ as well as their riboside and nucleotide conjugates often predominate (Auer, 1997;Stirk et al, 2003Stirk et al, , 2009Stirk & Van Staden, 2010). The results of this study identify the main CKs in Euglena gracilis as the cZ derivatives: cZR, cZNT, MeSZ and MeSZR, and to a lesser extent, the free base iP and its derivatives: iPR, iPNT, MeSiP and MeSiPA.…”

“…Recently published reports on the ligand preferences of CK receptors indicate that tZ and its conjugates are the most active type of endogenous CKs in vascular plants, while iP-type CKs are considered to have lower biological activity, especially in aboveground organs (Romanov et al, 2006;Hirose et al, 2008;Stirk et al, 2009). However, in non-vascular plants such as mosses or algae, the free bases iP and cZ as well as their riboside and nucleotide conjugates often predominate (Auer, 1997;Stirk et al, 2003Stirk et al, , 2009Stirk & Van Staden, 2010).…”

“…Glucoside derivatives were not detected in the present study or in Rhizobium. Stirk et al (2003Stirk et al ( , 2009 observed only low levels of O-glucosides in microalgae and seaweeds. O-glucosides are considered to be the hormone storage forms, thought to function in higher plants as a slow-release reserve refilling the active aglycons, thereby ensuring a continuous supply of free CKs over a prolonged period of time (Van Staden & Drewes, 1991;Strnad, 1997).…”

“…Both isoprenoid and aromatic CKs have been identified in green, red and brown macroalgae and in microalgae in the Chlorophyta, Cyanophyta and Chrysophyta. Despite these taxa belonging to diverse evolutionary lineages, their CK profiles are similar, in that derivatives of iP and cZ dominate (Stirk et al, 2003(Stirk et al, , 2009(Stirk et al, , 2011Ordog et al, 2004;Piotrowska & Czerpak, 2009;Gupta et al, 2011;Mekhalfi et al, 2012).…”