Functional investigation of two 1-aminocyclopropane-1-carboxylate (ACC) synthase-like genes in the moss Physcomitrella patens

Two ACC synthase-like (ACL) proteins in the moss Physcomitrella patens have no ACS activity, and PpACL1 functions as an L-cystine/L-cysteine C-S lyase. The ethylene biosynthetic pathway has been well characterized in higher plants, and homologs of a key enzyme in this pathway, ACS, have been reported in several algae and mosses, including Physcomitrella patens. However, the function of the ACS homologs in P. patens has not been investigated. In this research, we cloned two putative ACS genes from the P. patens genome, namely PpACS-Like 1 and 2, and investigated whether their encoded proteins had in vitro and in vivo ACS activity. In vitro biochemical assays using purified PpACL1 and PpACL2 showed that neither protein had ACS activity. Subsequently, we generated transgenic Arabidopsis lines expressing 35S:PpACL1 and 35S:PpACL2, and found that the transgenic etiolated seedlings that overexpressed either of these proteins lacked the constitutive triple response phenotype and did not emit excess levels of ethylene, indicating that neither of the PpACS-Like proteins had in vivo ACS activity. Furthermore, we found that PpACL1 functions as a C-S lyase that uses l-cystine and l-cysteine as substrates, rather than as an aminotransferase. Together, these results indicated that PpACL1 and PpACL2 are not true ACS genes as those found in higher plants.