Dynamics and control of phloem loading of indole-3-acetic acid in seedling cotyledons ofRicinus communis

Imre A Tamas,Peter J Davies

doi:10.1093/jxb/erw255

Abstract

During seed germination, sugars and auxin are produced from stored precursors or conjugates respectively, and transported to the seedling axis. To elucidate the mode of travel of indole-3-acetic acid (IAA) into the phloem, a solution of [(3)H]IAA, together with [(14)C]sucrose, was injected into the endosperm cavity harboring the cotyledons of germinating seedlings of Ricinus communis Phloem exudate from the cut hypocotyl was collected and the radioactivity recorded. Sucrose loading into the phloem was inhibited at higher IAA levels, and the rate of filling of the transient pool(s) was reduced by IAA. IAA was detected within 10min, with the concentration increasing over 30min and reaching a steady-state by 60min. The kinetics indicated that phloem loading of IAA involving both an active, carrier-based, and a passive, diffusion-based component, with IAA traveling along a pathway containing an intermediary pool, possibly the protoplasts of mesophyll cells. Phloem loading of IAA was altered by sucrose, K(+), and a range of non-specific and IAA-specific analogs and inhibitors in a manner that showed that IAA moves into the phloem from the extra cotyledonary solution by multiple pathways, with a carrier-mediated pathway playing a principal role.

Highlights

Seeds contain food reserves in the form of oils or starch and hormones on a precursor or conjugate form (Normanly et al, 2010)
After a buffered medium containing both [3H]indoleacetic acid (IAA) and [14C]sucrose was injected in the endosperm cavity harboring the cotyledons, both substances were recovered in the phloem exudate from the severed end of the hypocotyl stump about 1 cm below the cotyledons and endosperm (Fig. 1)
The fact that the relative steady state levels for different IAA applied concentrations were in close proximity indicates that IAA loading in the phloem has stabilized at very different IAA levels - stretching over four orders of magnitude - roughly in proportion to the applied concentration, except it was somewhat less than that at the highest concentration (20 mM)

Summary

Introduction

Seeds contain food reserves in the form of oils or starch and hormones on a precursor or conjugate form (Normanly et al, 2010). In the case of the auxin (indole-3-acetic acid [IAA]). These can be on the form of indoleacetyl aspartate, IAA inositol or glycoside conjugates, or complexes thereof, or peptide conjugates. Germinating seeds of castor bean (Ricinus communis) form an ideal model system to address this question because the reserves are held in the endosperm and during germination the solubilized reserves are taken up by leaf-like cotyledons located in the center of the endosperm tissues. These cotyledons function initially as absorptive organs, and only later emerge to function as the first leaves of the seedling. When the hypocotyl is severed the seedlings exude phloem sap from the cut hypocotyl (ibid.)

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