AUXIN BINDING PROTEIN 4 is involved in the Ca2+/auxin-regulated expression of ZCAX3 gene in maize (Zea mays)

As a second messenger, calcium is involved in auxin signaling, and previous studies demonstrated that Arabidopsis CAX1 (Ca2+/H+ exchanger) is involved in the auxin transduction pathway. This study was performed to investigate the possible role of auxin-binding proteins ABP1 and ABP4 in Ca2+/auxin-regulated growth in maize (Zea mays L.). We identified and cloned two AtCAX1 homologs in maize, ZCAX2 and ZCAX3. Using maize loss-of-function abp1 and abp4 mutants, the role of ABPs in Ca2+-dependent growth and in the Ca2+/auxin-regulated expression of the CAX genes was investigated in etiolated maize seedlings. Exogenous Ca2+ enhanced mesocotyl but not coleoptile growth in WT, abp1, abp4, and abp1/abp4 mutants, but the maximum stimulation was in abp4. As well, in the abp4 mutant, maximum accumulation of Ca2+ was observed when seedlings were exposed to exogenous Ca2+. In the mesocotyl of abp4 and double mutants, the expression of ZCAX3 was significantly reduced in the absence of exogenous Ca2+, whereas exogenous Ca2+ significantly up-regulated its expression in both mutants. This effect of Ca2+ was not observed in the coleoptile. In the absence of NAA, knockout of ABP4 led to significant drop of ZCAX3 expression in the mesocotyl, and exogenous auxin significantly inhibited expression of the ZCAX3 in WT, but not in abp mutants. This effect of auxin was not observed in the coleoptile. Our results indicate that in the absence of NAA, functional ABP4 is required for ZCAX3 expression and that ABP4 mediates the inhibitory effect of NAA on ZCAX3 expression. We provided evidence for a cross talk between ABP4, exogenous auxin, Ca2+, and ZCAX3 during growth of etiolated maize mesocotyl.

Maize AUXIN-BINDING PROTEIN 1 and AUXIN-BINDING PROTEIN 4 impact on leaf growth, elongation, and seedling responsiveness to auxin and light

In maize, at least five auxin-binding proteins (ABPs) have been identified, yet their functions remain unclear. The present study reports the use of maize abp1, abp4, and abp1abp4 mutants to investigate the role of ABPs during maize growth and development. Single and double abp mutant plants grown in a greenhouse differ from the wild type (WT) in their leaf declination and leaf blade growth. The effect of the dark (D), blue light (BL), red light (RL), and exogenous auxin on the development of mutant seedlings was also studied. Relative to WT, etiolated mutant seedlings were shorter and showed a reduced responsiveness to exogenous auxin. In BL or RL, the responsiveness of maize seedlings to auxin was distinctly less than in D. The reducing effect of light on seedling responsiveness to auxin is mediated at least by phytochromes. The suppression of ABP1 and (or) ABP4 led to a distinct accumulation of free indole-3-acetic acid (IAA) in etiolated and light-grown seedling organs. We concluded that ABP1 and ABP4 participate in the growth of maize seedlings, mediate seedling responses to auxin, and interact with light signaling pathway(s). We also deduce a functional interaction between ABP1 and ABP4, which is that the relationship between them is light-, organ- and response-dependent.

Oxidation of indole-3-acetic acid by dioxygen catalysed by plant peroxidases: specificity for the enzyme structure

Indole-3-acetic acid (IAA) can be oxidized via two mechanisms: a conventional hydrogen-peroxide-dependent pathway, and one that is hydrogen-peroxide-independent and requires oxygen. It has been shown here for the first time that only plant peroxidases are able to catalyse the reaction of IAA oxidation with molecular oxygen. Cytochrome c peroxidase (CcP), fungal peroxidases (manganese-dependent peroxidase, lignin peroxidase and Arthromyces ramosus peroxidase) and microperoxidase were essentially inactive towards IAA in the absence of added H2O2. An analysis of amino acid sequences allowed five structurally similar fragments to be identified in auxin-binding proteins and plant peroxidases. The corresponding fragments in CcP and fungal peroxidases showed no similarity with auxin-binding proteins. Five structurally similar fragments form a subdomain including the catalytic centre and two residues highly conserved among ‘classical’ plant peroxidases only, namely His-40 and Trp-117. The subdomain identified above with the two residues might be responsible for the oxidation of the physiological substrate of classical plant peroxidases, IAA.