Cation Measurements and Gene Expression Analysis Suggest Tomato Leaf Marginal Necrosis is Caused by a Jasmonate Signal Induced by K+ Starvation in the Tip Region of Leaflets

Several Japanese tomato cultivars develop a physiological disorder called leaf marginal necrosis, which occurs in relatively young compound leaves. Although the positions of the observed symptoms differ from those caused by inadequate potassium (K+) supplementation, previous studies have reported a relationship between the reduction of K+ content and the occurrence of this disorder. However, the mechanism of the relationship between K+ deficiency and leaf marginal necrosis remains unstudied. In the present study, the relationship between K+ deficiency in leaflets and leaf marginal necrosis was investigated by cation measurement and gene expression analysis to understand the possible mechanism responsible for the induction of leaf marginal necrosis. First, cation measurement of the two cultivars differing in their symptom intensities showed a trend of K+ reduction in the 'CF Momotaro J' cultivar developing leaf marginal necrosis at the tip leaflets positioned under the flowering fruit truss. Next, a comparison between the basal and tip region of the leaflet from four cultivars differing in their symptom intensities revealed that the K+ concentration in tip leaflets was significantly lower in the tip regions compared to the basal region, especially in the two cultivars 'CF Momotaro J' and 'Momotaro grande', leading to leaf marginal necrosis. The gene expression analysis of the basal and tip regions identified that the expression patterns of jasmonate-related genes were upregulated in the tomato leaflets with low concentration. The gene expression of a leaf senescence marker gene, a homologue of the SAG12 gene of Arabidopsis thaliana, was detected only in the leaf tip region samples with the lowest K+ concentration. Furthermore, 'CF Momotaro J' plants cultivated with K+-supplemented medium showed an increase in the K+ concentration, a decrease in the occurrence of leaf marginal necrosis, and down-regulation of the expression of jasmonate-related genes in tip leaflets. These results indicate that tomato leaf marginal necrosis occurs because of K+ starvation in the tip region of leaflets, leading to the activation of jasmonate-induced signal for necrosis.