AUTHOR=Pan Jian , Wang Gang , Wen Haifan , Du Hui , Lian Hongli , He Huanle , Pan Junsong , Cai Run 

TITLE=Differential Gene Expression Caused by the F and M Loci Provides Insight Into Ethylene-Mediated Female Flower Differentiation in Cucumber

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01091

DOI=10.3389/fpls.2018.01091

ISSN=1664-462X

ABSTRACT=<p>In cucumber (<italic>Cucumis sativus</italic> L.), the differentiation and development of female flowers are important processes that directly affect the fruit yield and quality. Sex differentiation is mainly controlled by three ethylene synthase genes, <italic>F</italic> (<italic>CsACS1G</italic>), <italic>M</italic> (<italic>CsACS2</italic>), and <italic>A</italic> (<italic>CsACS11</italic>). Thus, ethylene plays a key role in the sex differentiation in cucumber. The “one-hormone hypothesis” posits that <italic>F</italic> and <italic>M</italic> regulate the ethylene levels and initiate female flower development in cucumber. Nonetheless, the precise molecular mechanism of this process remains elusive. To investigate the mechanism by which <italic>F</italic> and <italic>M</italic> regulate the sex phenotype, three cucumber near-isogenic lines, namely H34 (<italic>FFmmAA</italic>, hermaphroditic), G12 (<italic>FFMMAA</italic>, gynoecious), and M12 (<italic>ffMMAA</italic>, monoecious), with different <italic>F</italic> and <italic>M</italic> loci were generated. The transcriptomic analysis of the apical shoots revealed that the expression of the B-class floral homeotic genes, <italic>CsPI</italic> (<italic>Csa4G358770</italic>) and <italic>CsAP3</italic> (<italic>Csa3G865440</italic>), was immensely suppressed in G12 (100% female flowers) but highly expressed in M12 (∼90% male flowers). In contrast, <italic>CAG2</italic> (<italic>Csa1G467100</italic>), which is an AG-like C-class floral homeotic gene, was specifically highly expressed in G12. Thus, the initiation of female flowers is likely to be caused by the downregulation of B-class and upregulation of C-class genes by ethylene production in the floral primordium. Additionally, <italic>CsERF31</italic>, which was highly expressed in G12, showed temporal and spatial expression patterns similar to those of <italic>M</italic> and responded to the ethylene-related chemical treatments. The biochemical experiments further demonstrated that <italic>CsERF31</italic> could directly bind the promoter of <italic>M</italic> and promote its expression. Thus, <italic>CsERF31</italic> responded to the ethylene signal derived from <italic>F</italic> and mediated the positive feedback regulation of ethylene by activating <italic>M</italic> expression, which offers an extended “one-hormone hypothesis” of sex differentiation in cucumber.</p>