Decreased cellulose-degrading enzyme activity causes pod hardening of okra (Abelmoschus esculentus L. Moench)

Abstract

Okra (Abelmoschus esculentus L. Moench) is an important tropical and subtropical crop species, but okra pods age rapidly after they meet harvest standards. The underlying mechanisms by which okra pods harden are unclear. In this study, we determined the cellulose and lignin contents of ‘Chaowuxing’ okra pods from 4 to 14 days postanthesis (DPA). Based on the histochemical staining of okra fruit during the active period of cellulose accumulation, we found that the hardening of okra fruit is due to the rapid accumulation of cellulose in the cell walls of vascular cells in the pulp. We used RNA-sequencing (RNA-Seq) analyses to investigate the genes that regulate okra fruit aging. Transcriptome sequencing data showed that after 7 DPA, expression of the cellulose synthase gene (CesA) decreased with time. In addition, expression of the gene encoding the first functional enzyme involved in cellulose hydrolysis (endoglucanase) also decreased, but its rate of decrease was much faster than that of the CesA. The quantitative real-time PCR (qRT-PCR) results were consistent with the RNA-Seq data. Accordingly, we speculate that the accumulation of cellulose during okra pod hardening occurs via a reduction in cellulose hydrolysis activity. The above results suggest that thickening of the cell wall caused by a significant increase in cellulose content in the vascular bundles causes okra hardening. The accumulation of cellulose is not directly achieved via increased expression of CesA but rather indirectly via decreased hydrolysis of cellulose.