Exogenous Brassinosteroid Enhances Plant Defense Against Colletotrichum gloeosporioides by Activating Phenylpropanoid Pathway in Camellia sinensis L.

The phytohormones brassinosteroids (BRs) regulate multiple aspects of plant growth, development, and responses to stress. However, the role of BRs in the defense response of tea (Camellia sinensis L.), one of the most important beverage crops, remains largely unknown. Previously, we reported that BRs improve tea quality both under normal and unfavorable temperature conditions. Here, we showed that 24-epibrassinolide (EBR, a bioactive BR) enhanced defense against Colletotrichum gloeosporioides in tea plants, which was associated with EBR-induced reduction in H2O2 accumulation in tea leaves. C. gloeosporioides-caused necrotic lesions and its actin gene expression increased over the postinoculation period, but exogenous EBR remarkably suppressed C. gloeosporioides spread. Time-course analysis of a key enzyme, phenylalanine ammonia-lyase (PAL), involved in phenylpropanoid biosynthesis, revealed that PAL activity gradually increased from 6 to 24h postinoculation with C. gloeosporioides following an initial decline. Meanwhile, exogenous EBR sharply increased PAL activity of inoculated leaves compared with that of only C. gloeosporioides inoculation. Expression analysis of genes involved in phenylpropanoid pathway showed that both exogenous EBR and C. gloeosporioides inoculation increased transcript levels of CsPAL, CsC4H, and Cs4CL; however, combined treatment with EBR and C. gloeosporioides resulted in a greater increase. Furthermore, CsPR1 and CsLOX1 expression analyses revealed that EBR potentially activates systemic induced tolerance, but not the lipoxygenase pathway to enhance tea plant resistance to C. gloeosporioides. These findings indicate a positive role of BR in strengthening disease resistance and thus may have potential implications in the control of C. gloeosporioides-caused disease in tea plants.