The mechanisms by which plants regulate levels of the phytohormone indole-3-acetic acid (IAA) are complex and not fully understood. One level of regulation appears to be the synthesis and hydrolysis of IAA conjugates, which function in both the permanent inactivation and temporary storage of auxin. Similar to free IAA, certain IAA-amino acid conjugates inhibit root elongation. We have tested the ability of 19 IAA-L-amino acid conjugates to inhibit Arabidopsis seedling root growth. We have also determined the ability of purified glutathione S-transferase (GST) fusions of four Arabidopsis IAAamino acid hydrolases (ILR1, IAR3, ILL1, and ILL2) to release free IAA by cleaving these conjugates. Each hydrolase cleaves a subset of IAA-amino acid conjugates in vitro, and GST-ILR1, GST-IAR3, and GST-ILL2 have K m values that suggest physiological relevance. In vivo inhibition of root elongation correlates with in vitro hydrolysis rates for each conjugate, suggesting that the identified hydrolases generate the bioactivity of the conjugates.

Cation levels within the cytosol are coordinated by a network of transporters. Here, we examine the functional roles of calcium exchanger 1 (CAX1), a vacuolar H 1 /Ca 21 transporter, and the closely related transporter CAX3. We demonstrate that like CAX1, CAX3 is also localized to the tonoplast. We show that CAX1 is predominately expressed in leaves, while CAX3 is highly expressed in roots. Previously, using a yeast assay, we demonstrated that an N-terminal truncation of CAX1 functions as an H At the whole-plant level, it has been well documented that there is a complex interplay among various ions (Marschner, 1995). For example, supplemental Ca 21 is known to mitigate the adverse effects of salinity on plant growth (Epstein, 1972). Recently, it has become possible to measure the sum total of the plant's mineral nutrient and trace element composition, termed the ionome (Lahner et al., 2003). The ionome phenotypes now allow investigators to assess how alterations in specific transporters affect these ionic relationships.Ca 21 and other cations can accumulate to millimolar levels in the vacuole, whereas the concentrations of these cations are maintained in the micromolar range in the cytosol (Taiz et al., 1990; Marty, 1999 (Sze et al., 2000). The driving force for cation antiport activity is the pH gradient generated by two electrogenic proton pumps located on the membrane, an ATPase and a pyrophosphatase (PPase; Sze et al., 1999). In principle, the proton pumps and the H 1 / cation exchangers can both dramatically alter the cation content of the vacuoles.Plant H Article, publication date, and citation information can be found at www.plantphysiol.org/cgi

Plants can perceive a wide range of biotic attackers and respond with targeted induced defenses. Specificity in plant non-selfrecognition occurs either directly by perception of pest-derived elicitors or indirectly through resistance protein recognition of host targets that are inappropriately proteolyzed. Indirect plant perception can occur during interactions with pathogens, yet evidence for analogous events mediating the detection of insect herbivores remains elusive. Here we report indirect perception of herbivory in cowpea (Vigna unguiculata) plants attacked by fall armyworm (Spodoptera frugiperda) larvae. We isolated and identified a disulfide-bridged peptide ( ؉ ICDINGVCVDA ؊ ), termed inceptin, from S. frugiperda larval oral secretions that promotes cowpea ethylene production at 1 fmol leaf ؊1 and triggers increases in the defenserelated phytohormones salicylic acid and jasmonic acid. Inceptins are proteolytic fragments of chloroplastic ATP synthase ␥-subunit regulatory regions that mediate plant perception of herbivory through the induction of volatile, phenylpropanoid, and protease inhibitor defenses. Only S. frugiperda larvae that previously ingested chloroplastic ATP synthase ␥-subunit proteins and produced inceptins significantly induced cowpea defenses after herbivory. Digestive fragments of an ancient and essential plant enzyme, inceptin functions as a potent indirect signal initiating specific plant responses to insect attack.elicitor ͉ guard hypothesis ͉ indirect perception ͉ insect herbivory ͉ plant defense A mechanistic understanding and targeted improvement of plant resistance traits are recognized as essential in combating yield losses from crop pests. Plants can perceive and defensively respond to attack either directly by impeding pest growth or indirectly by promoting advantageous interactions with beneficial organisms (1-7). Great progress has been made in the identification of plant receptor-like kinase families mediating perception of biotic attack and the subsequent activation of signal transduction cascades spanning interactions of GTP binding proteins, mitogen-activated protein kinases, phytohormones, transcription factors, and ultimately induced biochemical defenses (2,8). Despite these advances, relatively few candidate elicitors and ligands responsible for the initiation and specificity of induced plant defenses to pest attack have been identified (1, 2). This void is especially acute in the case of insect herbivore perception and is surprising given both the significance of plant-insect interactions in arthropod and angiosperm evolution and the role of insects in facilitating plant pathogen entry (9, 10).Induced plant defenses are initiated in part by the direct perception of elicitors derived from offending organisms. For example, maize (Zea mays) and tobacco (Nicotiana attenuata) perceive insect attack through the direct detection of fatty acid amino acid conjugate (FAC) elicitors present in insect oral secretions (OS). Plants respond with indirect defenses in the form of indu...

Auxins are hormones important for numerous processes throughout plant growth and development. Plants use several mechanisms to regulate levels of the auxin indole-3-acetic acid (IAA), including the formation and hydrolysis of amide-linked conjugates that act as storage or inactivation forms of the hormone. Certain members of an Arabidopsis amidohydrolase family hydrolyze these conjugates to free IAA in vitro. We examined amidohydrolase gene expression using northern and promoterb-glucuronidase analyses and found overlapping but distinct patterns of expression. To examine the in vivo importance of auxin-conjugate hydrolysis, we generated a triple hydrolase mutant, ilr1 iar3 ill2, which is deficient in three of these hydrolases. We compared root and hypocotyl growth of the single, double, and triple hydrolase mutants on IAA-Ala, IAA-Leu, and IAAPhe. The hydrolase mutant phenotypic profiles on different conjugates reveal the in vivo activities and relative importance of ILR1, IAR3, and ILL2 in IAA-conjugate hydrolysis. In addition to defective responses to exogenous conjugates, ilr1 iar3 ill2 roots are slightly less responsive to exogenous IAA. The triple mutant also has a shorter hypocotyl and fewer lateral roots than wild type on unsupplemented medium. As suggested by the mutant phenotypes, ilr1 iar3 ill2 imbibed seeds and seedlings have lower IAA levels than wild type and accumulate IAA-Ala and IAA-Leu, conjugates that are substrates of the absent hydrolases. These results indicate that amidohydrolases contribute free IAA to the auxin pool during germination in Arabidopsis.Although auxins are essential regulators of many aspects of plant growth and development, our understanding of how levels of this hormone are controlled remains incomplete. One component of auxin homeostasis is conjugation of the auxin indole-3-acetic acid (IAA) to different moieties, including esterification to sugars and amide linkage to amino acids and peptides. IAA-Leu, IAA-Ala, IAA-Asp, IAA-Glu, and IAA-Glc have been identified in Arabidopsis seedlings (Tam et al., 2000;Kowalczyk and Sandberg, 2001). Several IAA-peptide conjugates have been identified in bean seeds (Bialek and Cohen, 1986;Walz et al., 2002) and Arabidopsis (Walz et al., 2002); in fact, IAApeptide conjugates are the major IAA conjugates in Arabidopsis seeds Park and Cohen, 2003). Different IAA conjugates apparently have specific functions in plants, such as storage, transport, or inactivation of IAA (Cohen and Bandurski, 1982;Bartel et al., 2001). In general, endogenous IAA conjugates that are biologically active and hydrolyzed in plants may function as auxin storage forms, whereas conjugates inactive in bioassays may have roles in IAA degradation Ljung et al., 2002).Several Arabidopsis screens have uncovered IAAamino acid conjugate-resistant mutants that help to delineate conjugate functions (Bartel and Fink, 1995;Campanella et al., 1996;Davies et al., 1999;Lasswell et al., 2000;Magidin et al., 2003;LeClere et al., 2004). Two of these mutants, ilr1 and iar3, are defective...