AUTHOR=Mitoma Mai , Kanno Akira 

TITLE=The Greenish Flower Phenotype of Habenaria radiata (Orchidaceae) Is Caused by a Mutation in the SEPALLATA-Like MADS-Box Gene HrSEP-1

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.00831

DOI=10.3389/fpls.2018.00831

ISSN=1664-462X

ABSTRACT=<p>In <italic>Arabidopsis thaliana</italic>, the E-class <italic>SEPALLATA</italic> (<italic>SEP</italic>) genes are generally expressed across all floral whorls. These genes play fundamental roles in floral organ fate determination during development by interacting with other MADS-box gene products, such as those from A-, B-, and C-class genes. However, the function of <italic>SEP</italic> genes in orchid remains obscure. Here, we analyzed a mutant orchid cultivar with greenish flowers in <italic>Habenaria radiata</italic> and found that this phenotype is caused by the absence of SEP function. Wild type <italic>H. radiata</italic> flowers contain a column and two perianth whorls consisting of three greenish sepals, two white petals, and a lip (labellum). By contrast, the flowers of <italic>H. radiata</italic> cultivar ‘Ryokusei’ appear greenish, with three normal sepals in whorl 1, two greenish petals and a lip in whorl 2, and several sepaloid organs and a ventral column in whorls 3 and 4. We isolated two <italic>SEP</italic>-like genes (<italic>HrSEP-1</italic> and <italic>HrSEP-2</italic>) and two <italic>AGAMOUS</italic>-like genes (<italic>HrAG-1</italic> and <italic>HrAG-2</italic>) from wild type <italic>H. radiata</italic> and compared their expression in the wild type vs. the mutant cultivar. <italic>HrAG-1</italic> and <italic>HrAG-2</italic> were expressed in the column in the wild type, whereas these genes were expressed in the ventral column and in sepaloid organs that had been converted from a column in ‘Ryokusei.’ <italic>HrSEP-1</italic> and <italic>HrSEP-2</italic> were expressed in all floral organs in the wild type. However, in the mutant cultivar, <italic>HrSEP-2</italic> was expressed in all floral organs, while <italic>HrSEP-1</italic> expression was not detected. Thus, we analyzed the genomic structures of <italic>HrSEP-1</italic> in the wild type and ‘Ryokusei’ and identified a retrotransposon-like element in its first exon in ‘Ryokusei.’ Yeast two-hybrid assays demonstrated that HrSEP-1 interacts with HrDEF, HrAG-1, and HrAG-2. These results indicate that the mutant phenotype of ‘Ryokusei’ flowers is caused by the loss of function of <italic>HrSEP-1</italic>. Therefore, this gene plays an important role in column, lip, and petal development in <italic>H. radiata</italic> flowers.</p>