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

Abstract

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.

Keywords: Abscisic acid; Auxin; Ethylene; Fleshy Fruit; Hormonal Signal Production; Ripening.

Fig. 1

The biosynthesis pathway of hormones. The biosynthesis and catabolism pathways of hormones, showing the potential rate-limiting enzymes in each pathway. a ethylene biosynthesis and catabolism; b ABA biosynthesis and catabolism; c IAA biosynthesis and catabolism. The enzymes identified were shown in red. The potential rate-limiting enzymes are shown in solid green circles

Fig. 2

Proposed model for a regulatory pattern of the enzymes in the ABA biosynthesis pathway in fruit development and ripening. Up arrows denote up-regulation; solid green circles denote key enzymes; the size of ‘circled ABA’ denote the content of ABA in fruit cells

Fig. 3

Diagram showing the hormonal function in relation to their communication. a the left, whole fruit with strawberry as a model of NC fruit, showing the structural relationship between achenes/seeds and receptacle/flesh; the middle, amplification of the part in boxed in green in the whole fruit, showing the pattern of hormonal transport in relation to their signal production; the right, showing the changing patterns in the levels of IAA and ABA from fruit set to ripening. b the left, the whole fruit with tomato as a model of CL fruit, showing the structural relationship between achenes/seeds and receptacle/flesh; the middle, amplification of the part in boxed in green in the whole fruit, showing the pattern of hormonal transport in relation to their signal production; the right, showing the changing patterns in the levels of IAA, ABA and ET from fruit set to ripening

Fig. 4

Diagram showing the signaling network upstream of ethylene and ABA production. a the signaling network implicated in the regulation of ethylene biosynthesis; b the signaling network implicated in the regulation of ABA biosynthesis. Different cascades are shown in different colors. Note that the key enzymes can be regulated both transcriptionally and post-transcriptionally, and that the transcriptional regulation is determined by the transcription factors, which are again modulated by the upstream signaling proteins, and the post-transcriptional regulation is directly modulated by the upstream signaling proteins

Fig. 5

Diagram showing the cell biological mechanism of hormonal signal production. Fertilization triggers IAA production in the embryo. Development-associate cell dehydration triggers ABA production in the seed. Changes in CWI and membrane tension likely function as mechanisms to trigger ET production. The questioned words in brackets denote the signals potentially produced. Boxed are amplified areas. CWI, cell wall integrity