Fig. 1

Sorbitol enhances the resistance of M. micromalus  to pathogenic fungus R1. A-C leaves that have not been treated with sorbitol are more susceptible to Alternaria R1 infection. Young leaves were placed in 50 mM sorbitol solution and pure water for 6 h, and conidiospores of R1 were drop-inoculated onto each leaf and kept in a high humidity environment for 7 days. The representative phenotypes A, plant disease severity B, and cell membrane permeability (C) of R1-inoculated leaves are shown. The disease severity rating in (B) is represented as healthy (< 1%), mild symptoms (< 10%), moderate symptoms (10 -50%), and severe symptoms (> 50%). 40–50 leaves are used for the statistics. Here, 1%,10% and 50% represent the percentage of lesion area to leaf area. Different colors represent different rating. Ordinate represents percentage of different levels. D Sorbitol can induce early Ca²⁺ signaling. Detached leaves were drop-inoculated with conidiospores of R1 for 4 days. NMT technology was used to detect Ca²⁺ flux. The instantaneous Ca²⁺ flux after adding sorbitol to the test solution reflects the effect of sorbitol. The steady-state fluxes were continuously recorded for 5 min. The data are the mean ± SD. *p < 0.05, ***p < 0.001, ****p < 0.0001, two-sided Student’s t-test. Scale bars, 4.5 cm