The Banana MaWRKY18, MaWRKY45, MaWRKY60 and MaWRKY70 Genes Encode Functional Transcription Factors and Display Differential Expression in Response to Defense Phytohormones

Abstract

WRKY transcription factors (TFs) play key roles in plant defense responses through phytohormone signaling pathways. However, their functions in tropical fruit crops, especially in banana, remain largely unknown. Several <i>WRKY</i> genes from the model plants rice (<i>OsWRKY45</i>) and <i>Arabidopsis</i> (<i>AtWRKY18</i>, <i>AtWRKY60</i>, <i>AtWRKY70</i>) have shown to be attractive TFs for engineering disease resistance. In this study, we isolated four banana cDNAs (<i>MaWRKY18</i>, <i>MaWRKY45</i>, <i>MaWRKY60</i>, and <i>MaWRKY70</i>) with homology to these rice and <i>Arabidopsis</i><i>WRKY</i> genes. The <i>Ma</i><i>WRKY</i> cDNAs were isolated from the wild banana <i>Musa acuminata</i> ssp. <i>malaccensis</i>, which is resistant to several diseases of this crop and is a progenitor of most banana cultivars. The deduced amino acid sequences of the four <i>MaWRKY</i> cDNAs revealed the presence of the conserved WRKY domain of ~60 amino acids and a zinc-finger motif at the N-terminus. Based on the number of WRKY repeats and the structure of the zinc-finger motif, <i>MaWRKY18</i> and <i>MaWRKY60</i> belong to group II of WRKY TFs, while <i>MaWRKY45</i> and <i>MaWRKY70</i> are members of group III. Their corresponding proteins were located in the nuclei of onion epidermal cells and were shown to be functional TFs in yeast cells. Moreover, expression analyses revealed that the majority of these <i>MaWRKY</i> genes were upregulated by salicylic acid (SA) or methyl jasmonate (MeJA) phytohormones, although the expression levels were relatively higher with MeJA treatment. The fact that most of these banana <i>WRKY</i> genes were upregulated by SA or MeJA, which are involved in systemic acquired resistance (SAR) or induced systemic resistance (ISR), respectively, make them interesting candidates for bioengineering broad-spectrum resistance in this crop.