Comparative transcriptome profiling reveals that brassinosteroid-mediated lignification plays an important role in garlic adaption to salt stress

Garlic (Allium sativum L.) is an economically important vegetable crop which is used worldwide for culinary and medicinal purposes. Soil salinity constrains the yield components of garlic. Understanding the responsive mechanism of garlic to salinity is crucial to improve its tolerance. To address this problem, two garlic cultivars differing in salt tolerance were used to investigate the long-term adaptive responses to salt stress at phenotype and transcriptome levels. Phenotypic analysis showed four-week salt stress significantly decreased the yield components of salt-sensitive cultivar. Transcriptomes of garlics were de novo assembled and mined for transcriptional activities regulated by salt stress. The results showed that photosynthesis, energy allocation, and secondary metabolism were commonly enriched in both sensitive and tolerant genotypes. Moreover, distinct responsive patterns were also observed between the two genotypes. Compared with the salt-tolerant genotype, most transcripts encoding enzymes in the phenylpmpanoid biosynthesis pathway were coordinately down regulated in the salt-sensitive genotype, resulting in alternation of the content and composition of lignin. Meanwhile, transcripts encoding the enzymes in the brassinosteroid (BR) biosynthesis pathway were also systematically down regulated in the salt-sensitive genotypes. Taken together, these results suggested that BR-mediated lignin accumulation possibly plays an important role in garlic adaption to salt stress. These findings expand the understanding of responsive mechanism of garlic to salt stress.