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Fruit trees bloom earlier and earlier

Global warming is significantly affecting the flowering time of many plant species [1]. This especially concerns plants flowering in spring, as they are observed to flower earlier and earlier each decade [2]. This phenomenon has been very well documented in Japan where the dates of cherry blossoms have been recorded for centuries (Figure 1) [3, 4]. However, changes in flowering patterns are observed all over the world [2].

graph florescence over centuries

Figure 1: Dates of full bloom of cherry trees and March temperatures in Kyoto, Japan. A: Full flowering dates observed since the year 812 AD. B: Mean temperatures recorded in March in the period of 1881-2021 (blue; y-axis reversed) and full flowering dates observed in the same period. Figure adopted from Nikolaos Christidiset al. (2022) [3].

Why is this a problem?

Climate change manifests itself not only in rising temperatures but also in increasingly strong fluctuations in weather patterns [5]. For plants, one of the most dangerous phenomena are frosts occurring right after a period of warm temperatures which promote flower development. When forming very tight structures, buds can survive temperatures as low as -30°C. However, tolerance to frost drops quickly upon the buds’ transformation to flowers, which are highly susceptible even to mild frost [6, 7]. This problem affects many agriculturally important plant species. For instance, sweet cherry (Prunus avium L.) is one of the earliest flowering fruit trees, making it highly vulnerable to frost damage [7, 8]. In the case of stone fruits, such as apricots, plums, peaches and cherries, a single frost event can reduce the yield by 90% [9].

Economic and societal impact

Frost damage to flowers is the main climate-related cause of economic losses in global fruit production [10]. A week of spring frosts that hit the United States in 2007 resulted in losses estimated at over 2 billion US dollars [11]. A decade later, a wave of late-spring frosts that swept through Europe caused losses to the fruit sector of 3.3 billion euros [12]. This accounts for nearly 15% of the total fruit production in the EU that year [13]. This event affected nearly the entire continent, as the strongest effects were felt by growers from Italy, France, Germany, Poland, Spain, and Switzerland. Another series of overnight frosts that occurred in the spring of 2021 also had catastrophic consequences for European fruit growers. The most tragic damage was reported in France where, due to an extremely poor grape harvest, wine production was reduced by a third [14]. Farmers in the Netherlands are also well aware of this problem. For Jochem van Dijke, one of the cherry growers with whom we met, 2015 was especially difficult, as he lost 50% of his harvest. However, as he says, sometimes losses can be as high as 80% of the yield.

And the examples do not end here:

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Figure 2: World map with examples of frost damage.

The effects of frost damage are felt not only by farmers but also by consumers, as low yields cause fruit prices to skyrocket (Figure 2).

cherry prices

Figure 3: Prices of cherries in Poland between 1995 and 2019 in US dollars per ton. Red points represent years in which frost events were recorded [15-19].

What are the protective measures used?

When spring frosts are predicted, fruit farmers use various methods to protect their orchards from getting damaged. One of the most applied solutions is sprinkling the trees with water which turns into a protective ice layer upon freezing. Alternatively, farmers can artificially raise the temperature around the trees by lighting big paraffin candles or using heat cannons. A similar, albeit much smaller, effect can also be achieved by covering the entire orchard with foil. However, none of these solutions are ideal - they are costly, provide only short-term benefits, and the effectiveness of most of them is highly dependent on weather conditions, such as wind [20]. Moreover, most of them have a negative environmental impact, as sprinkling requires large amounts of water, heat cannons are highly energy-intensive, and candles contribute to air pollution [20].

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Nipping the problem in the bud

Plants' internal mechanisms regulating the flowering process are very complex (Figure 4) and consist of numerous interconnecting genetic pathways controlled by different internal and external signals. Ultimately, all of them control the expression of floral pathway integrators (FPIs) which include FT (FLOWERING LOCUS T), TSF (TWIN SISTERS OF FT), and SOC1 (SUPPRESSION OF OVEREXPRESSION OF CO 1) [21]. These, in turn, act on the floral meristem identity (FMI) genes to induce flowering [21].

flowering control pathways

Figure 4: A simplified scheme of the main flowering control pathways in the model plant Arabidopsis thaliana. The figure includes pathways controlled by photoperiod (orange and yellow), ambient temperature (red), age (green), gibberellins (brown), autonomous (sky blue), vernalization (light blue). Grey boxes represent the main floral integrators FT/TSF and SOC1. Squared boxes show genes with a central role in a specific pathway, while rounded boxes represent several genes or complexes. Solid lines indicate direct regulation and dotted lines indicate indirect regulation. Black arrows indicate positive regulation, red T-ends indicate negative regulation. Adopted from Leijtenet al., (2018) [22].

Even though scientific knowledge mainly comes from research using Arabidopsis thaliana [23], studies have shown that the three FPIs are highly conserved among different plant species [24]. Especially FT, a gene expressed in leaves and travelling to the shoot apical meristem through the phloem, is generally acknowledged as the main systemic inducer of flowering (also referred to as florigen). Its overexpression in vivo has been shown to promote earlier flower development [21].

The exact mechanisms leading to flowering repression are not yet fully understood. Nevertheless, studies indicate that other proteins, namely antiflorigens, are responsible for this process [21]. There are two possible ways in which they can act [25]. Antiflorigens could directly block the expression of genes involved in flowering induction, such as FT (Figure 5A). Alternatively, they could act as a competitive inhibitor, preventing the florigen from forming an active complex by binding to its activator (Figure 5B). Regardless of the mechanism, it is hypothesised that the flowering time of plants is determined by the ratio of florigens and antiflorigens [25].


Figure 5: Schematic representation of the two possible mechanisms of flowering repression. A – Antiflorigen (anti-FT) binds to the promoter of the FT gene preventing its expression. B – (top) FT binds to FD and forms an active complex that induces the expression of genes involved in the flowering cascade such as AP1. (bottom) Antiflorigen binds to FD, preventing the formation of the active complex. As a result, the flowering cascade is not induced.

Pseudomonas fluorescens: a tiny organism with great capabilities

Pseudomonas fluorescens is a gram-negative bacterium inhabiting various environments including the phyllosphere and rhizosphere of many plant species [26]. Due to its ability to produce biologically active compounds, antibiotics, and phytohormones, P. fluorescens is characterised as a plant growth-promoting rhizobacterium (PGPR) [27]. Bacteria belonging to this group can colonise roots and directly or indirectly improve plant growth. These beneficial properties have made P. fluorescens useful in agriculture as a biocontrol agent and component of biostimulants [28]. P. fluorescens SBW25, the strain used in this project, was isolated for the first time in 1989 from the phyllosphere of sugar beet in Oxfordshire, United Kingdom [26]. The genome sequence of P. fluorescens SBW25 was first published by Silby et al. in 2009 [29] and was updated by Fortmann-Grote et al. at the beginning of 2023 [30]. Given its native root colonizing abilities, plant growth-promoting properties, and genome availability, P. fluorescens SBW25 is an ideal chassis for our project in the agricultural sector.

What has been already done?

The serious consequences of frost damage motivated farmers and scientists to seek for novel methods of preventing crop losses over the last 50 years. In the 1980’s, Dr. Lindow from the University of California developed a genetically engineered strain of Pseudomonas syringae to be applied to fruit trees. The engineered strain prevented frost damage as a consequence of the native strains inducing ice crystal formation [31]. Interestingly, this bacterium was the first GMO to be released into the environment [32].

The problem of frost damage affecting fruit crops has also been previously addressed in the iGEM competition. The 2017 team IONIS-Paris constructed a bacterial system to protect grapevines from damage caused by extremely low and high temperatures. Their system secreted different kinds of proteins depending on the environmental conditions [33]. The 2021 team from UNILausanne developed a method to protect apricot trees from freezing temperatures by spraying them with a solution containing antifreeze proteins, while simultaneously targeting a plant pathogen Pseudomonas syringae [34]. Several other parts of our project are also related to work done previously by other iGEM teams. For instance, Flower Fairy E. coli, the project of the 2012 team from Kyoto, Japan, was aimed at triggering the blooming of plants by delivering florigen to plant cells by a modified E. coli [35].

Our project, however, stands out from the others for several reasons. In contrast to former iGEM teams, we challenge ourselves to engineer Pseudomonas fluorescens, a non-model organism native to the rhizosphere and beneficial for fruit trees. Furthermore, rather than focusing on protecting developed flowers or fruit tissue, we use the inherent frost-protective capacity of flower buds to help plants protecting themselves against frost damage. To discover how PseuPomona can solve one of the biggest agricultural challenges, explore our Overview and Results pages.

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