An ACC Oxidase Gene Essential for Cucumber Carpel Development

Sex determination in plants gives rise to unisexual flowers that facilitate outcrossing and enhance genetic diversity. In cucumber and melon, ethylene promotes carpel development and arrests sta- men development. Five sex-determination genes have been identified, including four encoding 1-aminocyclopropane-l-carboxylate （ACC） synthase that catalyzes the rate-limiting step in ethylene biosynthesis, and a transcription factor gene CmWIP1 that corresponds to the Mendelian locus gynoecious in melon and is a negative regulator of femaleness. ACC oxidase （ACO） converts ACC into ethylene; how- ever, it remains elusive which ACO gene in the cucumber genome is critical for sex determination and how CmWIP1 represses development of female flowers. In this study, we discovered that mutation in an ACO gene, CsAC02, confers androecy in cucumber that bears only male flowers. The mutation disrupts the enzymatic activity of CsAC02, resulting in 50% less ethylene emission from shoot tips. CsAC02 was ex- pressed in the carpel primordia and its expression overlapped with that of CsACS11 in female flowers at key stages for sex determination, presumably providing sufficient ethylene required for proper CsACS2 expression. CmAC03, the ortholog of CsACO2, showed a similar expression pattern in the carpel region, suggesting a conserved function of CsACO2/CmACO3. We demonstrated that CsWlP1, the ortholog of CmWlP1, could directly bind the promoter of CsAC02 and repress its expression. Taken together, we propose a presumably conserved regulatory module consisting of WlP1 transcription factor and ACO controls unisexual flower development in cucumber and melon.

Structural analysis of receptor-like kinase SOBIR1 reveals mechanisms that regulate its phosphorylation-dependent activation

Plant leucine-rich repeat(LRR)receptor-like kinases(RLKs)and LRR receptor-like proteins(RLPs)comprise a large family of cell surface receptors that play critical roles in signal perception and transduction.Both LRR-RLKs and LRR-RLPs rely on regulatory LRR-RLKs to initiate downstream signaling pathways.BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3(BAK1/SERK3)and SUPPRESSOR OF BIR1-1(SOBIR1)are important and extensively studied regulatory LRR-RLKs with distinct functions.Although the regulatory mechanism of BAK1 activation has been studied in detail,the activation mechanism of SOBIR1 remains poorly understood.Here,the crystal structures of the catalytically inactive kinase domain of SOBIR1(SOBIR1-KD)from Arabidopsis thaliana were determined in complexes with AMP-PNP and Mg^(2+).The results show that SOBIR1-KD contains a uniquely long β3-αC loop and adopts an Src-like inactive conformation with an unusual architecture at the activation segment,which comprises three helices.Biochemical studies revealed that SOBIR1 is transphosphorylated by BAK1 following its autophosphorylation via an intermolecular mechanism,and the phosphorylation of Thr529 in the activation segment and the β3-αC loop are critical for SOBIR1 phosphorylation.Further functional analysis confirmed the importance of Thr529 and the β3-αC loop for the SOBIR1-induced cell death response in Nicotiana benthamiana.Taken together,these findings provide a structural basis for the regulatory mechanism of SOBIR1 and reveal the important elements and phosphorylation events in the special stepwise activation of SOBIR1-KD,the first such processes found in regulatory LRR-RLKs.