Mechanical Stress Induces Resistance against Wilt Disease Caused by Fusarium oxysporum in Acacia Koa

Abstract

Acacia koa, an economically important timber tree in the Hawaiian Islands, is suffering from a devastating fungal wilt and dieback disease caused by Fusarium oxysporum f. sp. koae. Previous studies have shown that some environmental stresses, such as drought and wind, can enhance disease resistance in plants. In this research, the effects of a non-wounding mechanical stress on disease resistance were studied in A. koa. The specific objectives were (i) to identify actively transcribed genes through transcriptome analysis, (ii) to identify morphological, biochemical, and transcriptional changes induced by a non-wounding mechanical stress, and (iii) to determine the level of resistance in mechanically stressed A. koa. To identify genes related to disease resistance, 85,533 unigenes were assembled through Illumina sequencing and Trinity de novo software, and 47,038 unigenes were annotated based on search for sequence similarity with known proteins. From these sequences, 4,000 genes related to plant defense and growth were selected for a microarray analysis to determine if they were mechanically induced. For a non-wounding mechanical stress, A. koa seedlings were gently bent once in each of the four cardinal directions. The gene expression analyses showed upregulation of over 50 genes (> 2-fold) within 10-60 min following the treatment. They included genes for calcium, hormone, and MAPK signaling, and disease resistance. Genes for pathogenesis-related proteins and secondary metabolite biosynthesis were also upregulated within 6 h. For morphological and biochemical changes, plants were mechanically treated daily for 2-6 mo; the stressed plants had significantly reduced stem length, whereas they had significantly increased stem diameter, and anthocyanin and lignin contents. The disk diffusion assays and qRT-PCR analyses were used to determine the level of induced resistance. The results showed that the stressed plants had stronger antifungal activities and were ‘primed’ for defense with enhanced expression levels of defense-related genes following fungal inoculation. Consistently, the stressed plants had a significantly higher survival rate following a 100-day F. oxysporum inoculation trial. These results indicate that disease resistance can be induced and studied simply by providing the non-wounding mechanical stress; it may contribute towards identifying useful biomarkers for A. koa seedling selection of disease resistance.