Mechanism of Naphthaleneacetic Acid Conjugation

Abstract

Formation of naphthaleneacetic acid-glucose (NAGlu) in detached leaves, floating on a-naphthaleneacetic acid-1-V4C (NAA, 0.05 microcurie per milliliter, 3.1 ,uM)-buffer solution (phosphate-citrate, pH 4.2) began immediately while there was a 2-to 4-hour lag before NAA-asparatate (NAAsp) could be detected. Subsequent increase in the NAAsp conjugate reflected a decrease in free NAA to 1 to 2 % of the total radioactivity taken up. Pretreatment with 31 gM 'C-NAA for 18 hours doubled NAAsp formation after transfer for 4 hours to '4C-NAA. Pretreatment with ethylene, as ethephon (up to 400 milligrams per liter) or ethylene gas (10 microliters per liter), did not induce NAAsp formation. In the presence of NAA, ethylene had no effect on NAA conjugation. Similarly, CO2 (5%) did not modify the formation of the conjugates. Rhizobitoxine (1.87 uM) inhibited NAA-induced ethylene production but did not prevent NAA-induced formation of NAAsp. We concluded that the conjugation of NAA with aspartic acid is not mediated by ethylene.

Mechanism of Indole-3-acetic Acid Conjugation

“…Rhizobitoxine (1.87 ,uM) inhibited indole-3-acetic acid-induced ethylene production but did not prevent the formation of indole-3-acetylaspartic acid. In view of the similarity of these results and those previouslv obtained with a-naphthaleneacetic acid, it is concluded that ethylene has no role in the auxin-induced indole-3-acetylaspartic acid formation in cowpea leaves.Although conjugation of exogenous IAA and NAA3 with aspartic acid in plant tissues is well documented (2,5,8,(10)(11)(12), see also literature review 5), the mechanism of induction of the still unisolated (L)-aspartic acid-N-acylase, considered responsible for this conjugation (11), remains unclear. The formation of aspartic acid conjugates with both IAA and NAA usually occurs after a lag period of about 2 hr (2,5,8,11).…”

Abscission of Citrus Leaf Explants

“…The ability of plant tissues to conjugate IAA and NAA to glucose and aspartic acid is well established (3,9,10). We have found (11) an in situ increase in the level of auxin in AZs sections of isolated Citrus leaf during the first hours of abscission, and a marked decrease in endogenous auxin at later stages.…”

“…Our results, however, do not exclude the possibility that auxin conjugation in the leaf blade may have a role in the regulation of abscission. We were unable to show any inductive effect of ethylene on either NAA or IAA conjugation rate (9,10). Beyer and Morgan (5) reported that ethylene-induced inhibition ofauxin transport is not due to its increased conjugation rate to aspartic acid.…”

“…Dipping type II explants ( Fig. 2) in 1.5 pM NAA for 15 min did not affect abscission, but was sufficient for conjugation studies (7,9), and was therefore adopted as one of the two treatments required. Increasing concentrations of NAA gradually delayed abscission, as expected (2); we chose to treat explants in the following experiment with 150 Am.…”

“…Extraction was done by 80%1o ethanol as previously described (9). Extracts to find the concentration of NAA which does not delay abscission when applied to the whole explant at excision time, and to compare its metabolism to that of a higher concentration which does delay it.…”