Abstract
Ethylene is an important plant hormone that regulates developmental processes in plants. The ethylene biosynthesis pathway is a highly regulated process at both the transcriptional and post-translational level. The transcriptional regulation of these ethylene biosynthesis genes is well known. However, post-translational modifications of the key ethylene biosynthesis enzyme 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) are little understood. In vitro kinase assays were conducted on the type III ACS, AtACS7, fusion protein and peptides to determine whether the AtACS7 protein can be phosphorylated by calcium-dependent protein kinase (CDPK). AtACS7 was phosphorylated at Ser216, Thr296, and Ser299 by AtCDPK16 in vitro. To investigate further the function of the ACS7 gene in Arabidopsis, an acs7-1 loss-of-function mutant was isolated. The acs7-1 mutant exhibited less sensitivity to the inhibition of root gravitropism by treatment with the calcium chelator ethylene glycol tetraacetic acid (EGTA). Seedlings were treated with gradient concentrations of ACC. The results showed that a certain concentration of ethylene enhanced the gravity response. Moreover, the acs7-1 mutant was less sensitive to inhibition of the gravity response by treatment with the auxin polar transport inhibitor 1-naphthylphthalamic acid, but exogenous ACC application recovered root gravitropism. Altogether, the results indicate that AtACS7 is involved in root gravitropism in a calcium-dependent manner in Arabidopsis.
Highlights
Gravity is one of the most important environmental cues that control growth direction (Morita, 2010)
These results indicate that a pathway similar to the stress-induced MAPK (SIPK) pathway in tobacco operates in Arabidopsis and that mitogen-activated protein kinase 6 (MPK6) phosphorylates aminocyclopropane-1-carboxylase synthase (ACS) proteins, thereby decreasing their turnover and increasing ethylene biosynthesis after pathogen stress
The type III ACC synthase ACS7 has a shorter C-terminal domain and does not have a predicted calcium-dependent protein kinase (CDPK) phosphorylation site, ACS7 may still be a substrate of CDPK
Summary
Gravity is one of the most important environmental cues that control growth direction (Morita, 2010). Shoots generally grow upward (i.e. negative gravitropism) and roots grow downward (i.e. positive gravitropism). It is unknown how plants receive and respond to the gravity signal. One widely accepted theory is that starch-accumulating amyloplast movement along the gravity vector within gravity-sensing cells (statocytes) is a likely trigger of subsequent intracellular signalling (Morita, 2010). Several studies have demonstrated that starch is important but not essential for gravity sensing (Strohm et al, 2011; Wolverton et al, 2011). The receptor that responds to gravity has not yet been identified
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