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.

As a second messenger, calcium is involved in auxin signaling, and previous studies demonstrated that Arabidopsis CAX1 (Ca 2+ /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 Ca 2+ /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 Ca 2+ -dependent growth and in the Ca 2+ /auxin-regulated expression of the CAX genes was investigated in etiolated maize seedlings. Exogenous Ca 2+ 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 Ca 2+ was observed when seedlings were exposed to exogenous Ca 2+ . In the mesocotyl of abp4 and double mutants, the expression of ZCAX3 was significantly reduced in the absence of exogenous Ca 2+ , whereas exogenous Ca 2+ significantly up-regulated its expression in both mutants. This effect of Ca 2+ 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, Ca 2+ , and ZCAX3 during growth of etiolated maize mesocotyl.Résumé : En tant que messager secondaire, le calcium joue un rôle dans la voie de signalisation de l'auxine. Des études ont démontré que CAX1 (échangeur Ca 2+ /H + ) d'Arabidopsis est impliqué dans cette voie de signalisation. Cette étude a été menée de façon à déterminer le rôle des protéines liant l'auxine, ABP1 et ABP4, dans la croissance du maïs (Zea mays L.) régulée par Ca 2+ /auxine. Nous avons identifié et cloné chez le maïs deux homologues de AtCAX1, ZCAX2 et ZCAX3. En utilisant les mutants de maïs perte-de-fonction abp1 et abp4, nous avons étudié le rôle des ABPs d'une part dans la croissance dépendante du Ca 2+ et d'autre part dans l'expression des gènes CAX régulée à la fois par le Ca 2+ et l'auxine. Le Ca 2+ exogène stimule la croissance du mésocotyle mais pas du coléoptile chez le WT, les mutants abp1, abp4 et abp1/abp4, même si l'effet optimum est observé pour le mutant abp4. De la même façon, chez le mutant abp4, l'accumulation maximale de Ca 2+ est observée lorsque les plantules sont exposées au Ca 2+ exogène. Dans le mésocotyle de abp4 et du double mutant, l'expression de ZCAX3 est significativement réduite en l'absence de Ca 2+ exogène ; à l'opposé, l'application de Ca 2+ induit significativement l'expression de ZCAX3 chez ces deux mutants. Cet effet du Ca 2+ n'est pas observé dans le coléoptile. En l'absence de ...