The triggering mechanism for predominant hormonal signal production in fleshy fruit ripening.

Fleshy Fruit (FF) ripening is regulated by multiple hormones, which can be categorized into two groups, i.e., the positive signals acting to promote FF ripening and the negative signals acting to suppress FF ripening. Ethylene (ET) and abscisic acid (ABA) are two predominant positive signals respectively controlling climacteric (CL) and non-climacteric (NC) FF ripening, whereas auxin (IAA) is the predominant negative signal controlling both FF growth and ripening. Functioning of these hormones is initiated by an alteration of the hormonal levels, which is referred to as the process of Hormonal Signal Production (HSP) in FF development and ripening. While the hormonal regulation of FF ripening has been extensively studied and reviewed, knowledge of HSP has never been summarized and discussed. The purpose of this review is to summarize and discuss the triggering mechanism of HSP. We first summarize the physiological, biochemical and molecular bases of HSP for three crucial hormones, ET, ABA, and IAA, including hormonal metabolism, transport and reciprocal regulation of HSP among different hormones, we then summarize and discuss the recent discoveries on the mechanism of cellular signal transduction of HSP. Finally, we propose several viewpoints to facilitate comprehension of the future research endeavors.