A tetraspanin gene regulating auxin response and affecting orchid perianth size and various plant developmental processes

Abstract

The competition between L (lip) and SP (sepal/petal) complexes in P-code model determines the identity of complex perianth patterns in orchids. Orchid tetraspanin gene <i>Auxin Activation Factor</i> (<i>AAF</i>) orthologs, whose expression strongly correlated with the expansion and size of the perianth after P code established, were identified. Virus-induced gene silencing (VIGS) of <i>OAGL6-2</i> in L complex resulted in smaller lips and the down-regulation of <i>Oncidium OnAAF</i>. VIGS of <i>PeMADS9</i> in L complex resulted in the enlarged lips and up-regulation of <i>Phalaenopsis PaAAF</i>. Furthermore, the larger size of <i>Phalaenopsis</i> variety flowers was associated with higher <i>PaAAF</i> expression, larger and more cells in the perianth. Thus, a rule is established that whenever bigger perianth organs are made in orchids, higher <i>OnAAF</i>/<i>PaAAF</i> expression is observed after their identities are determined by P-code complexes. Ectopic expression Arabidopsis <i>AtAAF</i> significantly increased the size of flower organs by promoting cell expansion in transgenic <i>Arabidopsis</i> due to the enhancement of the efficiency of the auxin response and the subsequent suppression of the jasmonic acid (JA) biosynthesis genes (<i>DAD1</i>/<i>OPR3)</i> and <i>BIGPETAL</i> gene during late flower development. In addition, auxin-controlled phenotypes, such as indehiscent anthers, enhanced drought tolerance, and increased lateral root formation, were also observed in 35S::<i>AtAAF</i> plants. Furthermore, 35S::<i>AtAAF</i> root tips maintained gravitropism during auxin treatment. In contrast, the opposite phenotype was observed in palmitoylation-deficient AtAAF mutants. Our data demonstrate an interaction between the tetraspanin AAF and auxin/JA that regulates the size of flower organs and impacts various developmental processes.