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Bacterial fruit blotch, caused by the seedborne gram-negative bacterium Acidovorax citrulli , is one of the most destructive bacterial diseases of cucurbits (gourds) worldwide. Despite its prevalence, effective and reliable means to control bacterial fruit blotch remain limited. Transcriptomic analyses of tissue culture-based regeneration processes have revealed that organogenesis-associated cellular reprogramming is often associated with upregulation of stress- and defense-responsive genes. Yet, there is limited evidence supporting the notion that the reprogrammed cellular metabolism of the regenerated tissued confers bona fide antimicrobial activity. Here, we explored the anti-bacterial activity of protocorm-like-bodies (PLBs) of Phalaenopsis aphrodite . Encouragingly, we found that the PLB extract was potent in slowing growth of A. citrulli , reducing the number of bacteria attached to watermelon seeds, and alleviating disease symptoms of watermelon seedlings caused by A. citrulli . Because the anti-bacterial activity can be fractionated chemically, we predict that reprogrammed cellular activity during the PLB regeneration process produces metabolites with antibacterial activity. In conclusion, our data demonstrated the antibacterial activity in developing PLBs and revealed the potential of using orchid PLBs to discover chemicals to control bacterial fruit blotch disease.

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli (Aac), is a devastating bacterial disease to the melon industry. The scarcity of resistant germplasms has hindered in-depth research into its resistance mechanisms. In this study, we combined comparative transcriptomics, physiological assays, and hormonal profiling to explore the defense mechanisms of resistant (ZT145) and susceptible (ZT146) melon germplasms. The results indicated that resistant plants rapidly initiated a coordinated defense mechanism within 12 h after inoculation. This was characterized by an induced increase in salicylic acid (SA), activation of core immune pathways (plant–pathogen interaction, MAPK signaling pathway, etc.), and upregulation of phenylpropanoids and other biosynthetic processes. In contrast, susceptible plants exhibited a delayed and less coordinated response, characterized by SA inhibition, a surge in jasmonic acid (JA), and the broad but non-coordinated activation of multiple pathways, ultimately leading to physiological dysregulation. Through comparative analysis, we identified nine key genes that were early responders to pathogen challenge, as well as 21 genes that might be responsible for maintaining resistance. Our findings suggest that the resistance of melon to BFB is determined not by the abundance of defense-related genes but rather by the plant’s ability to rapidly activate a coordinated, SA-dominated defense network during early infection. This study provides an integrative theoretical framework for deciphering the molecular and physiological mechanisms against bacterial diseases in melon.

Bacterial fruit blotch of cucurbits is a destructive disease caused by Acidovorax avenae subsp. citrulli, which is a typical seedborne pathogen. In seed health testing for this disease, we have detected many strains of Acidovorax with some differences from A. avenae subsp. citrulli. Their 16S rRNA sequences were divided into six types. The most common sequence was completely consistent with that of A. avenae subsp. avenae originally isolated from rice. The other sequences were over 99% similar but not identical to those of A. avenae subsp. avenae and A. avenae subsp. citrulli. Some commercialized antibodies against A. avenae subsp. citrulli reacted with several of these strains. Some of these strains incited yellow spots or brownish water-soaked lesions mainly on young true leaves of cucumber and squash after spray inoculation. Histological observations showed that these strains entered the leaf tissues of cucurbit plants through stomata and multiplied in the intercellular spaces of parenchymatous tissues as well as in the vascular tissues. The amount of bacterial multiplication and spread in the tissues differed among the strains, presumably reflecting their ability to induce symptoms. These isolated strains are therefore different from A. avenae subsp. citrulli, and their potential threat to the cultivation of cucurbits is lower than that of A. avenae subsp. citrulli.

Background Bacterial fruit blotch (BFB), a disease caused by Acidovorax citrulli, results in significant economic losses in melon. The causal QTLs and genes for resistance to this disease have yet to be identified. Resistance (R)-genes play vital roles in resistance to plant diseases. Since the complete genome sequence of melon is available and genome-wide identification of R-genes has been performed for this important crop, comprehensive expression profiling may lead to the identification of putative candidate genes that function in the response to BFB. Results We identified melon accessions that are resistant and susceptible to BFB through repeated bioassays and characterized all 70 R-genes in melon, including their gene structures, chromosomal locations, domain organizations, motif distributions, and syntenic relationships. Several disease resistance-related domains were identified, including NBS, TIR, LRR, CC, RLK, and DUF domains, and the genes were categorized based on the domains of their encoded proteins. In addition, we profiled the expression patterns of the genes in melon accessions with contrasting levels of BFB resistance at 12 h, 1 d, 3 d, and 6 d after inoculation with A. citrulli. Six R-genes exhibited consistent expression patterns (MELO3C023441, MELO3C016529, MELO3C022157, MELO3C022146, MELO3C025518, and MELO3C004303), with higher expression levels in the resistant vs. susceptible accession. Conclusion We identified six putative candidate R-genes against BFB in melon. Upon functional validation, these genes could be targeted for manipulation via breeding and biotechnological approaches to improve BFB resistance in melon in the future.