Gibberellins treatment usually leads to an increase in the grape rachis hardening. Under the treatment of gibberellic acid, study the morphological and transcriptomic differences, screen gibberellic acid related functional genes, would help develop grape rachis softening measures in production.

The Honey grape was treated with gibberellic acid at the flowering stage, and the water treatment was used as the control. The middle part of the rachis was collected for morphological observation and transcriptomic analysis on the 15th day after treatment. The differentially expressed genes (DEGs) related to lignin synthesis were screened between the control and gibberellic acid treatment, and real-time fluorescent quantitative RT-qPCR was used to verify their expression levels. In order to identify the key genes involved in rachis hardening, a correlation analysis was conducted between phenotypic traits, key differentially expressed genes, related enzyme activities, and lignin content.

After gibberellic acid treatment, no visible "intercellular space" was observed between the xylem and the cambium. Cellulose structures were observed in the epidermis, phloem, and pith. A total of 4459 DEGs were identified, among which 26 were involved in the phenylpropanoid biosynthesis pathway-a key route for lignin formation. Four key differentially expressed genes (F6HNF5 (Vitvi13g00622), D7SYR4 (Vitvi01g00658), F6H218 (Vitvi19g00186), and F6GS10 (Vitvi17g00898)) were screened from the pathway. Their qRT-PCR expression patterns were consistent with the trends in the transcriptomic analysis. The activities of phenylalanine ammonia-lyase (PAL) and cinnamaldehyde dehydrogenase (CAD) which are regulated by these genes increased significantly and lignin content increased substantially. These results indicate that the four lignin synthesis genes screened are pivotal in the process of rachis hardening.

Gibberellic acid treatment caused significant changes in the morphological structure of grape rachis. F6HNF5, D7SYR4, F6H218 and F6GS10 may be the key genes affecting rachis hardening of 'Honey' grape.