Temperature-Sensitive Plant Cells with Shunted Indole-3-Acetic Acid Conjugation

Abstract

~ ~Cells of henbane (Hyoscyamus muticus L.) grow indefinitely i n culture without exogenous auxin. Cells of its temperature-sensitive variant XIlB2 grow like the wild type at 26OC but die rapidly at 33°C unless auxin is added to the medium. Despite this temperaturesensitive auxin auxotrophy, XllB2 produces wild-type amounts of indole-3-acetic acid (IAA). IAA i s the predominant auxin and is important for plant growth and development. Since the IAA production of the variant is functional, we investigated whether the synthesis or degradation of IAA metabolites, possibly active auxins themselves, is altered. The IAA metabolites were IAA-aspartate (IAAsp) and IAA-glucose. The wild type converted IAA mainly to IAAsp, whereas the variant produced mainly IAA-glucose. Exogenous auxin corrected the shunted IAA metaholism of the variant.The half-life of labeled IAAsp i n the variant was reduced 21-fold, hut in the presence of exogenous auxin it was not different from the wild type. The temperature sensitivity of XllB2 was also corrected by supplying IAAsp. Pulse-chase experiments revealed that henhane rapidly metabolizes IAAsp to compounds not identical to IAA. The data show that the variant XIlB2 is a useful tool to study the function of IAA conjugates to challenge the popular hypothesis that IAA conjugates are merely slow-release storage forms of IAA.

The Conjugated Auxin Indole-3-Acetic Acid–Aspartic Acid Promotes Plant Disease Development

“…To date, no biological activity has been ascribed to IAA-Asp, although two studies have hinted toward a role in temperature sensitivity and grape ripening (Oetiker and Aeschbacher, 1997;Bö ttcher et al, 2010). Our results now show that IAA-Asp is biologically active and promotes disease by two pathogens and in at least three plant species.…”

“…Since free IAA is established as being the biologically active form of auxin, only certain hydrolysable conjugates have been shown to be active in auxin bioassays (Woodward and Bartel, 2005;Ludwig-Mü ller, 2011). Although a role for IAA-Asp in abiotic stress and ripening has been suggested in henbane (Hyoscyamus niger) and grape (Vitis vinifera) (Oetiker and Aeschbacher, 1997;Bö ttcher et al, 2010), a direct biological function for IAA-Asp and IAA-Glu has not been clearly demonstrated, and these conjugates are rather considered as the starting point of auxin catabolism (Woodward and Bartel, 2005;Ludwig-Mü ller, 2011). Auxin conjugation to amino acids is carried out by the family of GH3 proteins.…”

Crystal Structure of an Indole-3-Acetic Acid Amido Synthetase from Grapevine Involved in Auxin Homeostasis

“…The amino acid conjugated can determine the fate of the bound auxin as some conjugates are oxidized (Tsurumi and Wada, 1986;Plüss et al, 1989;Östin et al, 1998), whereas others can release free IAA through hydrolysis (reviewed in Ludwig-Müller, 2011). There is also evidence that some of the conjugates themselves have biological activity in plant development and disease response (Cohen and Baldi, 1983;Magnus et al, 1992;Oetiker and Aeschbacher, 1997;Staswick, 2009;González-Lamothe et al, 2012). It should be kept in mind that the metabolism of IAA conjugates as well as their biological activity may differ from one plant species to another.…”

A gene encoding a protein modified by the phytohormone indoleacetic acid

“…In addition, some longer-term responses, such as resistance to higher temperatures (2), are mediated by the conjugated form of the phytohormone. IAA in plants occurs in both conjugated and free forms, and there is increasing evidence that the ratio of free to conjugated IAA is controlled by tissue-specific and developmentally regulated processes (3).…”