Evidence for 1-(Malonylamino)cyclopropane-1-carboxylic acid being the major conjugate of aminocyclopropane-1-carboxylic acid in tomato fruit

Abstract

Tomato (Lycopersicon esculentum Miller) fruit discs fed with [2, 3-14C]1-aminocyclopropane-1-carboxylic acid (ACC) formed 1-malonyl-ACC (MACC) as the major conjugate of ACC in fruit throughout all ripening stages, from immature-green through the red-ripe stage. Another conjugate of ACC, gamma-glutamyl-ACC (GACC), was formed only in mature-green fruit in an amount about 10% of that of MACC; conjugation of ACC into GACC was not detected in fruits at other ripening stages. No GACC formation was observed from etiolated mung bean (Vigna radiata [L.] Wilczek) hypocotyls, etiolated common vetch (Vicia sativum L.) epicotyls, or pea (Pisum sativum L.) root tips, etiolated epicotyls, and green stem tissue, where active conversion of ACC into MACC was observed. GACC was, however, formed in vitro in extracts from fruit of all ripening stages. GACC formation in an extract from red fruit at pH 7.15 was only about 3% of that at pH 8.0, the pH at which most assays were run. Our present in vivo data support the previous contention that MACC is the major conjugate of ACC in plant tissues, whereas GACC is a minor, if any, conjugate of ACC. Thus, our data do not support the proposal that GACC formation could be more important than MACC formation in tomato fruit.

Figure 1

Metabolites of [2,3-¹⁴C]ACC fed to tomato discs. Results are expressed as percentages of the total radioactivity recovered from the discs. Results are the averages ± sd of two to three fruits for each developmental stage, and two discs from each fruit were fed separately. The fruit stages are: IMG, immature green; MG, mature green; Br, breaker; Tr, turning; LtRd, light red; and Rd, red (an explanation of fruit stages is given in Methods).

Figure 2

Ethylene production rate from discs of tomato fruit at different developmental stages. Discs were incubated without (open bars) or with (hatched bars) ACC (24 μL of 0.63 mm ACC) and after 8 h of ethylene accumulation in the reaction tubes was determined as described in Methods. Results are the averages ± sd for two discs from two fruit. Explanation of fruit-stage abbreviations are given in Figure 1.

Figure 3

Radioactivity scans of electrophoretograms run at pH 7.0 showing radiolabeled products from mature-green (top) and breaker (bottom) tomato fruit fed with [2,3-¹⁴C]ACC. Mobility is relative to N-2,4-dinitrophenyl-l-Ala that moves as an anion with a charge of −1 at this pH.

Figure 4

Formation of GACC in vitro using a homogenate of tomato pericarp tissue and [2,3¹⁴C]ACC as the substrate. The homogenate was from the same fruit that discs were taken from for the in vivo feeding in Figure 1. Results are the averages from two to three fruits, and assays were performed in duplicate. Vertical bars indicate the sd. Assay conditions are given in Methods. Explanation of fruit-stage abbreviations are given in Figure 1. fresh wt, Fresh weight.