Abstract
The transcription factor WRKY is widely distributed in the plant kingdom, playing a significant role in plant growth, development and response to stresses. Walnut is an economically important temperate tree species valued for both its edible nuts and high-quality wood, and its response to various stresses is an important factor that determines the quality of its fruit. However, in walnut trees themselves, information about the WRKY gene family remains scarce. In this paper, we perform a comprehensive study of the WRKY gene family in walnut. In total, we identified 103 WRKY genes in the common walnut that are clustered into 4 groups and distributed on 14 chromosomes. The conserved domains all contained a WRKY domain, and motif 2 was observed in most WRKYs, suggesting a high degree of conservation and similar functions within each subfamily. However, gene structure was significantly differentiated between different subfamilies. Synteny analysis indicates that there were 56 gene pairs in J. regia and A. thaliana, 76 in J. regia and J. mandshurica, 75 in J. regia and J. microcarpa, 76 in J. regia and P. trichocarpa, and 33 in J. regia and Q. robur, indicating that the WRKY gene family may come from a common ancestor. GO and KEGG enrichment analysis showed that the WRKY gene family was involved in resistance traits and the plant-pathogen interaction pathway. In anthracnose-resistant F26 fruits (AR) and anthracnose-susceptible F423 fruits (AS), transcriptome and qPCR analysis results showed that JrWRKY83, JrWRKY73 and JrWRKY74 were expressed significantly more highly in resistant cultivars, indicating that these three genes may be important contributors to stress resistance in walnut trees. Furthermore, we investigate how these three genes potentially target miRNAs and interact with proteins. JrWRKY73 was target by the miR156 family, including 12 miRNAs; this miRNA family targets WRKY genes to enhance plant defense. JrWRKY73 also interacted with the resistance gene AtMPK6, showing that it may play a crucial role in walnut defense.
Highlights
WRKY transcription factors are ubiquitous among higher plants, and they harbor a highly conserved WRKYGQK amino acid sequence that is followed by a zinc-finger motif at the N-terminal domain [1,2]
Based on the similar domain in which walnut WRKY genes were identified, the WRKY genes were classified into four groups, and the fourth group had the largest number of genes, including 51 members
With the transcriptome data of anthracnose-resistant F26 fruits (AR) fruits and anthracnose-susceptible F423 fruits (AS) fruits, we investigated the expression profile between these two fruits; three genes, JrWRKY83, JrWRKY732 and JrWRKY74, were highly expressed in AR fruits, indicating that these three genes increased their expression level when infected by the stress of Colletotrichum gloeosporioides, which predominantly affects walnut anthracnose through C. gloeosporioides can cause leaf scorches or defoliation, as well as fruit gangrene, which is currently the most challenging disease in walnut production [52,69,70]
Summary
WRKY transcription factors are ubiquitous among higher plants, and they harbor a highly conserved WRKYGQK amino acid sequence that is followed by a zinc-finger motif at the N-terminal domain [1,2]. The WRKY gene is an important factor in the regulation of plant growth and development, as well as in a plant’s response to different kinds of stress [27], including drought, dehydration and salt stress [28]; the most important aspect of the WRKY gene is its ability to respond to abiotic stresses. In Brassica rapa, WRKY gene family members act against abiotic and biotic stresses [30,31]
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