| Summary: | Following photosynthesis, sucrose is translocated to sink organs, where it provides the primary source of carbon and energy to sustain plant growth and development. Sugar transporters from the SWEET (sugar will eventually be exported transporter) family are rate-limiting factors that mediate sucrose transport across concentration gradients, sustain yields, and participate in reproductive development, plant senescence, stress responses, as well as support plant−pathogen interaction, the focus of this study. We identified 25 <i>SWEET</i> genes in the walnut genome and distinguished each by its individual gene structure and pattern of expression in different walnut tissues. Their chromosomal locations, <i>cis</i>-acting motifs within their 5′ regulatory elements, and phylogenetic relationship patterns provided the first comprehensive analysis of the <i>SWEET</i> gene family of sugar transporters in walnut. This family is divided into four clades, the analysis of which suggests duplication and expansion of the SWEET gene family in <i>Juglans regia</i>. In addition, tissue-specific gene expression signatures suggest diverse possible functions for <i>JrSWEET</i> genes. Although these are commonly used by pathogens to harness sugar products from their plant hosts, little was known about their role during <i>Xanthomonas</i> <i>arboricola</i> pv. <i>juglandis</i> (<i>Xaj</i>) infection. We monitored the expression profiles of the <i>JrSWEET</i> genes in different tissues of “Chandler” walnuts when challenged with pathogen <i>Xaj</i>417 and concluded that SWEET-mediated sugar translocation from the host is not a trigger for walnut blight disease development. This may be directly related to the absence of type III secretion system-dependent transcription activator-like effectors (TALEs) in <i>Xaj</i>417, which suggests different strategies are employed by this pathogen to promote susceptibility to this major aboveground disease of walnuts.
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