Aux/IAA gene Cla004102, is involved in synergistic regulation of various endogenous hormones, regulating flesh firmness in watermelon

Abstract

Fruit flesh firmness is the key trait of watermelon (Citrullus lanatus) that affects fruit texture, commerciality, and shelf life. However, the knowledge about the underlying molecular mechanisms responsible for variations in fruit flesh firmness is still limited. We constructed a near-isogenic line HWF with high fruit flesh firmness by crossing a wild watermelon PI 271769 as the donor parent and an inbred line 203Z as the recurrent parent. An F2 sub-segregation population was developed by crossing 203Z and HWF for bulked segregant analysis (BSA-seq). The results revealed a candidate region of 6.1Mb (Chr6: 11557845–17658672) containing the genes related to watermelon flesh firmness. RNA Sequencing (RNA-Seq) analysis of the candidate region led to the identification of a candidate gene, Cla004102 (Aux/IAA), an auxin-responsive protein. The expression of Aux/IAA decreased with fruit developmental stages and a reduction in fruit flesh firmness was observed. Moreover, correlation analysis showed a high positive correlation of Aux/IAA with Cla004120, an ethylene-responsive factor1 (ERF1). The RT-qPCR analysis revealed a higher expression of Aux/IAA and ERF1 consistent with flesh firmness. Higher contents of abscisic acid (ABA) were observed in soft fleshed fruits than hard fleshed fruits. Further, applying exogenous ethylene to watermelon fruit reduced flesh firmness, and ABA content was increased much faster in ethylene-treated fruits than in the control. Interestingly, Aux/IAA expression was also decreased with the reduction in flesh firmness and inclined ABA contents. These findings add considerably to the existing knowledge about watermelon fruit flesh firmness. A more in-depth study would better understand watermelon's genetic mechanisms underlying flesh firmness.