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

Abstract

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.