**2. Hormones involved in strawberry ripening**

Ethylene controls almost all ripening processes in climacteric fruits; however, many hormones are actively involved in the ripening process of non-climacteric fruits, which makes these fruits attractive for research. In this context, strawberry (*Fragaria x ananassa*) fruit has become a model of non-climacteric fruit. A complete hormonal profile of woodland strawberry *Fragaria vesca* fruit was reported that auxin is produced mainly in achenes (seeds), whilst abscisic acid (ABA), ethylene, gibberellins, and bioactive free base cytokinins are chiefly produced in receptacles [9]. The report also indicated that ABA promotes ripening while auxin delays it. Moreover, endogenous auxin GA levels are greatly reduced in the late stages of strawberry ripening when the abscisic acid (ABA) level increases dramatically [10]. Indole-3 acetic acid (IAA) has a significant role in cell expansion, determination of fruit size, and ripening of strawberry fruits. A recent RNA-seq study describes the expression profile of auxin biosynthesis and signaling during the development and maturation of F × ananassa [11]. Based on this study, the auxin content drops by 50% in the receptacle but remains constant during ripening, supporting the idea that auxins may involve in strawberry fruit ripening in later stages.

Auxin has been shown to delay ripening by altering the expression of many genes associated with ripening [5]. Expression of FaPL and FaEGase, which are the most important enzymes responsible for softening, increased at the beginning of strawberry ripening and decreased with exogenous auxin application [12]. However, in another study, the expression of two genes encoding Xyloglucan endotransglycosylase/hydrolases (XTH), FaXTH1, and FaXTH2, significantly up-regulated by auxins treatment [13]. In the same study, gibberellins and abscisic acid up-regulated both gene expressions. Increases of FaAux/IAA1 and FaAux/IAA2 transcripts increased by the influence of naphthalene acetic acid (NAA) at the stage of large green and white fruit [14]. During strawberry fruit ripening, the ABA content increasingly grows from the green stage to the red stage (and the commencement of this rise overlaps with declines in IAA levels [15]. In other words, the ripening of strawberries is controlled by ABA in an ethylene-independent manner [16]. Exogenous application of ABA to strawberry fruits has fluctuating results during fruit ripening. IAA has been shown to play an important role in inducing cell division and expansion, which is related to the early stages of strawberry fruit development. In the later stages of ripening, ABA and sucrose are the main molecules that play a role in controlling gene expression. Expression of ABA and sucrose signaling genes and ripening-related genes such as endo-β-(1,4)-glucanases 2 (CEL2), 9-cis-epoxycarotenoid dioxygenase 2 (NCED2),

#### *Pests, Diseases, Nematodes, and Weeds Management on Strawberries DOI: http://dx.doi.org/10.5772/intechopen.103925*

MYB5, Sucrase synthase (SuSy), as endo-β-(1,4)-glucanases 1 (CEL1), Sucrose nonfermenting 1 (SNF1)—related protein kinase 2(SnRK2.2), and9-cis-epoxycarotenoid dioxygenase 1 (NCED1) were all considerably up-regulated by ABA or sucrose treatment alone, and especially with ABA + sucrose treatment [17]. However, postharvest ripening of strawberry fruits varied from the fruits attached to the plant, proposing ripening is related to the signal activated by ABA, as the application of ABA caused the modified amassing of numerous compounds including sugars, ABA, anthocyanins, and ABA-GE [18]. Treatment of ABA positively regulated the expression of FaRGL [19]. ABA is a crucial signal molecule in the advancement of strawberry ripening which was proved by the study [20]. Downregulation of FaNCED1 (9-cisepoxycarotenoi dioxygenase, a key gene in ABA biosynthesis) inhibits ripening. A recent study showed that ABA biosynthesis is firmly connected by response and forward loops to limit ABA contents for fruit growing and to rapidly raise ABA contents for the commencement of fruit ripening [21]. To summarize the role of ABA in strawberry ripening, the expression of ABA-related genes significantly increases, and the hormone regulates many ripening-related metabolic pathways.

Gibberellic acid (GA) plays in the regulation of the growth of non-climacteric fruits, especially strawberries [6]. The expansion of receptacle cells during fruit development is coordinated by endogenous GAs. Among the bioactive GAS (GA1, GA3, and GA4), GA1 and GA4 are the most abundant in the early stages of strawberry fruit development and drop to lower levels as the fruit ripens. Exogenous treatment of GA3 retarded red color development and the loss of fruit firmness throughout the ripening period was significantly reduced in strawberry cultivars [22]. In a recent study, the application of GA3 affected the fruits quality of strawberries by changing organic acid and individual phenolic compound composition [23].

Although strawberry is known as climacteric fruit, ethylene could play a role at early stages of fruit ripening in strawberries [24]. Initially, an increase in FaPG1 gene expression was found in response to ethylene [25], and in later studies, several other genes, such as FaPG1, FaGal1, and FaGal2, were involved in cell wall modification were found to be modified by ethylene application [26]. Exogenous ethephon treatment increased the expression of biosynthesis and signaling genes, FaERF2 and FaACO1, and influenced the phytochemical profile of phenolic compounds, vitamin C contents, anthocyanins, and sugars [27]. Studies have shown that ethylene elicitors or inhibitors affect some significant feature qualities in strawberry fruits, including firmness [28]. Different hormonal treatments differentially affected hemicellulose metabolism during strawberry fruit ripening and under postharvest conditions. For example, postharvest 1-methylcyclopropene treatment up-regulated FaXynA and FaXynC expressions [29]. The physiological consequences of ethylene on strawberry fruit have been shown to depend on the developing phase of the fruit [30]. Up-regulation of ethylene-responsive transcription factor, ERF105-like gene is significantly induced under cold stress, showing that ethylene could also play an important role in abiotic stress resistance in strawberries. Although ethylene appears to be involved in a secondary role compared to abscisic acid in non-climacteric strawberry ripening, this does not disregard that ethylene may adjust some certain occurrences linked to the ripening progression.

Polyamines (PAs), ubiquitous aliphatic amines and biogenic regulators present in all living organisms, are involved in many developmental and physiological processes involving plant aging, stress, and plant growth [31]. The content of spermine (Spm) rises strongly following the commencement of fruit coloring to red, and Spm is the dominant component of the ripe strawberry fruits. The predominance of spm in

ripe fruit over other PAs is due to abundant expression of the strawberry S-adenosyl-L-Met decarboxylase gene (FaSAMDC), which encodes an enzyme that produces a residue required for PA biosynthesis [32]. Polyamine oxidase 5, FaPAO5, negatively adjusts strawberry fruit ripening, as down-regulation of FaPAO5 stimulated Spd, Spm, and ABA amassing, which ultimately enhanced ripening. The opposite results were shown in FaPAO5-overexpressing in the same study [33]. The results showed that FaPAO5 plays a role in the terminal catabolism of Spd and Spm. Application of putrescine (PUS) reduced the adverse effects of osmotic stress of the nutrient solution and increased plant resistance against salt stress, showing that PAs are important regulators against abiotic stress conditions [34].
