Management of the Tolerance to Bolting for Spinach ( Spinacia Oleracea L .): Retrospect and Prospects

Management of the Tolerance to Bolting for Spinach ( Spinacia Oleracea L .): Retrospect and Prospects.


Introduction
Spinach is a diploid (2n = 12) that is native to the mid-east probably Iran Ryder [1], and it was first mentioned in China around 600 A.D. as the herb of Persia Nonnicke [2]. It is a member of the family Amaranthaceae, which also contains sugar beet, chard and quinoa as well as some weeds like lamb's-quarters Morelock, et al. [3]. Spinach is an important and nutritious green leafy vegetable, and there were rich carotenoids, folate, vitamin C, calcium and iron in spinach Dicoteau [4]. Besides, spinach is a good source of antioxidants and has one of the highest ORAC (oxygen radical absorbance capacity) values of any vegetable Prior [5]. Normally, spinach was used as a cooked vegetable or a salad or cooked meat and vegetable dishes or as an ingredient in fresh or frozen spinach or canned spinach Morelock, et al. [3]. Spinach were popular all around the world, and the production increased to ~28 million tons in 2017, including ~26 million tons accounting for 92% in China (http://www.fao.org/faostat/zh/?#data/QC). Spinach is a longday plant, which is harvested when its rostte leaves are nearly fully expanded at its vegetative growth stage Hartmann et al. [6].
The onset and development of the reproductive phase involve the distinct and sequential processes of bolting followed DOI: 10.26717/BJSTR.2020. 28.004595 by flowering Mutasa-Göttgens, et al. [7]. After transition to the reproductive stage with flowering and bolting, spinach loses its value as an agricultural product, becoming bitter and hard Abe, et al. [8]. Bolting is an important agronomy trait to consider in relation to developing spinach cultivars for year-round production, because of its sensitivity to photoperiod Chun, et al. [9]. Because some commercially grown spinach is cut multiple times Morelock, et al. [3], overwintered spinach that is susceptible to bolting in the spring reduces the number of harvests that may be taken and therefore reduces overall yield. To prevent this phenomenon, the appropriate cultivars are selected for cropping. This review would summarize the late-bolting research progress and the factors that influencing bolting in spinach, and may lay the foundation for the molecular design late-bolting breeding in spinach.

Factors that Influencing Bolting
As every plant individual with a short commodity value growth period, spinach no longer has a good flavor when it begins to bolt. Therefore, it is important to find a method of inhibiting or delaying bolting Borkowski, et al. [13]. Resistance to bolting among different cultivars has been known for a long time, which has the character of longer growth periods Webb, et al. [14]. It has been shown that the thorn varieties have a strong bolting tolerance. Generally, oriental cultivars that Japanese and other Asian customers prefer bolt easy Chun, et al. [9]. Low bolting rate was found in the cultivars 'Fidalgo', 'Springfield', and 'Springer' Goreta, et al. [10], which will improve the efficiency of breeding and producing in spinach. . Bolting is not enhanced if seed is exposed to low temperatures during development on the motherplant Wiebe [15]. But bolting rate increase if cold exposure of growing plants is followed by higher temperatures and long days (Yamaguchi, 1983). Chun et al. (1999) showed that manipulating the day length and temperature during transplant production can prevent bolting by using artificial light. Spinach is a long-day plant, a long photoperiod causes it to bolt and flower, Kim, et al. [16] reported that spinach plants began to sense the photoperiod during transplant production, and that flower bud development during this period was enhanced under a longer photoperiod. The photoperiod during transplant production has a significant effect on the percentage of bolting plant, and the delay of bolting that occurs when the photoperiod is reduced Chun, et al. [17]. And its flowerstalk length increased with increasing photoperiod (e.g., at 14 DAT, from 15 mm at the shorter photoperiods to 80 mm at 16 hours/ day) Chun, et al. [17]. Before transplanting, short photoperiod treatments can retard bolting while long-day exposure induces bolting Goreta, et al. [10].

Genetic Analysis of the Late-Bolting Trait
All the studies on the inheritance all over the world showed that the bolting trait is regulated by multigene, and early-bolting gene is incomplete dominant to late-bolting gene Yu, et al. [18]. In order to develop such spinach cultivars, it is necessary to understand flowering and bolting mechanisms of spinach at the genetic level.
Genetic and molecular studies have revealed that many genes are involved and can be assigned to distinct regulatory pathways including photoperiod, vernalization, gibberellin, autonomous, ambient temperature, and age pathways Fornara, et al. [24]). These Fornara, et al. [24,25].
In Arabidopsis, most key flowering genes have been identified and functionally characterized. SOC1 encodes a MADS-box protein that integrates multiple flowering signals derived from photoperiod, temperature, hormone and age-related pathways (Lee, 2010).
Interestingly, it has been discovered that many homologous genes For assessing genetic diversity within accessions, Hu, et al. [29] analyzed genetic variability among 38 germplasm accessions and 10 commercial hybrids of spinach by using target region amplification polymorphism (TRAP) markers. In Kuwahara's study, the simple sequence repeat (SSR) data were used to study the population structure and genetic diversity of the 48 spinach accessions Kuwahara, et al. [30]. A total of 288 spinach accessions from United States Department of Agriculture (USDA) were used as the association panel to discovered the single-nucleotide polymorphisms (SNPs) through genotyping by sequencing (GBS), and three SNP markers, AYZV02001321_398, AYZV02041012_1060, and AYZV02118171_95 were identified to be associated with bolting Chitwood, et al. [11]. The draft genome sequence of spinach was reported, and a number of known markers and quantitative trait loci (QTLs) associated with bolting were obtained in the region from 44.7 to 50.5 Mb of Chromosome 2 Xu, et al. [12]. In addition, spinach and sugar beet were both belong to the Amaranthaceae family. In sugar beet, 40 genes associated with bolting were respectively obtained by amplified fragment length polymorphism (AFLP) and iTRAQ technology Liang, et al. [31]. The studies about molecular markers of bolting on other Amaranthaceae family specials may be laid the foundation for the further study in spinach.

Breeding of the Late-Bolting Varieties
Understanding the molecular genetic mechanism of the bolting could provide evidence for their application in breeding. There were the gaps between the bolting research and the spinach crop, and much work should been done to develop and analyze the bolting genes in spinach. The bolting genes could be applied in breeding in many ways, such as transformation of the bolting genes, molecular marker assisted selection (MAS), late-bolting genes pyramiding, and so on. It is our goal to take full advantage of the bolting genetic information for breeding.

Prospects of the Bolting Trait in Spinach
Spinach cultivation has a long history, wide geographical distribution and rich germplasm resources, which provide the basis of plant breeding and genetic research. There were a total of 1,938 germplasm resources of spinach that have been counted in the world Kik [32]). The germplasm resources of spinach are widely There is no uniform international standard for the bolting trait. At present, the identification of morphological index of bolting is the common evaluation criteria for the bolting traits, but this is highly subjective. In Chinese cabbage, the varieties with early flower bud differentiation and early bolting were sensitive to the signal stimulation Yu, et al. [18]. The content of CTK and three polyamines (spermidine putamine spermidine) of late bolting varieties were lower than that of early bolting varieties Cheng, et al. [33].
These physiological changes during the bolting could be used as a reference for the selection of late-bolting varieties.
The combination of physiological indexes and morphological index would be helpful to the study of the bolting trait. Genetic variation among spinach and resistance to bolting among different cultivars for bolting has been documented for many years Goreta, et al. [10], and therefore late-bolting cultivars could be developed through breeding efforts Brandenberger, et al. [34]. Areas with mild climate conditions are suitable for growing winter spinach, and successful production depends on choosing late bolting cultivars. Change the seasonal timing of flowering could allow the production of novel varieties adapted to local climatic and environmental conditions Jung, et al. [35]. Breeding targets for the control of bolting in cultivating must meet the dual, but opposing, needs for high yielding crops and seed production Mutasa-Göttgens, et al. [7].
Late-bolting spinach is preferred to increase market value, while early-bolting spinach is beneficial for seed production. As a longday plant, the photoperiod has a great limitation on the year-round production of spinach. Early-bolting or late-bolting is both good for grown spinach if it lacks the sensitivity to photoperiod. So further analysis need to understand both the early-bolting and late-bolting about the photoperiod in spinach Chun, et al. [9]. Arabidopsis thaliana is a model facultative long-day (LD) plant Jung, et al. [35].

CONSTANS (CO) plays a central role in the photoperiod pathway in
Arabidopsis, and the CO transcription activates transcription of FT and TSF transcription only during long days Fornara, et al. [24]).
CO-mutant was insensitive to the day length (Re'dei, 1962).
In genotypes where CO transcription shifts, CO activation is no longer restricted to long days but also occurs during short days, leading to earlier FT transcription and flowering under short-day conditions Fornara, et al. [24]. In the next study, CO-mutant or other gene involved in the photoperiod pathway can be used for creating new germplasm to achieve anniversary production in spinach. The flowering genes in Arabidopsis have been well studied, making Arabidopsis a viable reference species for comparative genomic studies. To date, knowledge of the spinach genome is limited and few reports have been published on the use of molecular markers in spinach Khattak, et al. [36]. Molecular assisted selection (MAS) could be further used to select specific genes alleles in spinach breeding. Transgenic approaches are also an obvious and convenient option for crop improvement, and are made easier by the hybrid nature. The use of inducible transgenic targets enabled bolting and flowering to be activated only in the specified lines Mutasa-Göttgens, et al. [7]. To detect the genetic mechanism and regulatory genes of bolting traits by combining biotechnology with conventional breeding will be an important means for future research on bolting breeding of spinach [37][38][39][40][41][42].