Over-Expression of FT1 in Plum (Prunus domestica) Results in Phenotypes Compatible with Spaceflight: A Potential New Candidate Crop for Bioregenerative Life Support Systems

The most obvious obstacle for growing trees in a spaceflight environment is their large size. Typical mature plum orchard trees can range in height from 3-4 m (Day et al., 2013), making them impractical for use in any foreseeable spaceflight or planetary exploration plant growth system. In order to be considered as a candidate crop, the entire architecture of the tree needs to be reduced to the point that the tree could be grown in the same systems used for such candidate crops as tomato and pepper. The over-expression of PtFT1 in P. domestica disrupts apical dominance, allowing axillary or secondary buds to develop into branches resulting in phenotypes ranging from bushy to creeping or planar growth habits (Figure 1 A-C), architectural phenotypes that appear to be compatible with spaceflight plant growth systems (Figure 1 B).

Figure 1.

Perhaps less obvious than the architectural barriers, but equally limiting in terms of spaceflight applications, is the prolonged period of exclusively vegetative growth that occurs in the juvenile phase leading up to the first flowering and fruit set. For P. domestica, this juvenile phase lasts between three to seven years (Srinivasan et al., 2012), making it impossible to produce a crop under any reasonable spaceflight scenario. In PtFT1 transformed plums this juvenility phase is significantly reduced – in many cases to less than 12 months – such that comparatively small plants can develop numerous flowers and set large numbers of fruit (Figure 2).

Figure 2.

Although the FT-plum development phase is still a somewhat protracted timeframe for experiments on the International Space Station (ISS), it should be noted the majority of this development time could elapse on the ground as a lead up to a spaceflight experiment. The already accelerated flower development associated with the PtFT1 expression can be reduced even further using clonal propagation from cuttage (Figure 2). Mature tissue can be excised from the parent plant and rooted to generate a large number of clonal plants that will establish roots and flower in as little as eight weeks from the time of cutting (Figure 2). Further, early propagation and spaceflight storage scenario results suggest both rooted and non-rooted cuttings can be stored (4°C; low or no light) for long periods of time (weeks to months) and remain viable (T. Graham, unpublished results), making the FT-plums further amenable to spaceflight experiments.

The use of bioregenerative systems as a crew food supply – in whole or part – during extended duration missions would require a constant production of foodstuffs (Wheeler, 2000). This can be accomplished with staggered plantings, or by using indeterminate crop species capable of continual food production. Although fruit trees are perennial and capable of multiple crops, most are not indeterminate for the purposes of bioregenerative life support. Most tree fruit production is phenologically regulated – particularly those species that evolved in temperate climates (Childers et al., 1995) – implying that sometime during a mission the trees would need to enter a cold dormancy phase, during which time they would neither produce food nor contribute to other life support functions (i.e., air revitalization and water purification). Under the influence of PtFT1 over-expression, P. domestica has no obligate requirement for dormancy. New floral buds can be initiated, develop, and mature in the absence of a chilling phase (Srinivasan et al., 2012) allowing for the continuous production of fruit; essentially the plant has become indeterminate (Figure 3). This said, the degree of floral or vegetative bud development is modulated, to some degree, through changes in ambient temperature. Higher temperatures (e.g., 29°C) promote or favor the development of vegetative buds, while lower temperatures (e.g., 21°C) promote floral buds (Srinivasan et al., 2012; Srinivasan et al., 2014).

Figure 3.

Harvest index is a plant productivity metric used to describe the relative distribution of biomass between the edible and inedible components of a crop (Hay, 1995). It was originally developed primarily for cereal crops but has since been used for a wide range of crops. Tree crops – such as plum – are generally considered to have a low harvest index, at least in the short term, as the plant directs its resources to the development of vegetative and structural elements (i.e., wood). In the long term (i.e., 20-40 years), it can be argued the harvest index is actually quite high given the multiple harvests, and field studies with ultra-dwarf fruit trees have shown biomass partitioning to fruit can be quite high once the trees develop beyond their juvenility phase (Palmer, 1988). The overexpression of PtFT1 in P. domestica circumvents this barrier to spaceflight. The resultant early flowering, fruit production, and vine-like or bushy growth habits combine to increase the edible (fruit) biomass per unit total biomass (Figure 4), even in the first few years of growth.

Figure 4.

Bone loss and its impact on the health of crewmembers has been identified by NASA scientists as one of the greatest challenges to interplanetary space exploration and long-duration stays on the ISS. For crew members of the Russian MIR and ISS, the decrease in bone mineral density can range from of 1.0-1.6% per month in the hip and lumbar spine (LeBlanc et al., 2000). In the space environment, microgravity, radiation exposure, and immunological changes can all contribute to bone loss, but the most pronounced effects result from abnormal loading of the skeleton.

A variety of interventions, including exercise or loading regimens (Baldwin et al., 1996; Yang et al., 2009), drug therapies (Bikle et al., 1994; Turner et al., 1998), and dietary modifications (Globus et al., 2009; Smith et al., 2005; Zwart et al., 2004), have been considered as countermeasures. The appeal of dietary interventions is they could provide a practical and safe component to an osteoprotective regimen through the incorporation of foods rich in specific nutrients or non-nutrient bioactive components (e.g., polyphenolic compounds) or dietary supplements. The ideal dietary intervention would have the capacity to suppress the catabolic activity of the osteoclast cells (i.e., resorption), while maintaining or up-regulating the anabolic activity of osteoblast cells (i.e., formation). Additionally, in vivo and in vitro findings indicate free radicals and pro-inflammatory cytokines generated in the space environment can have detrimental effects on bone (Garrett et al., 1990; Kondo et al., 2010). Thus, a dietary intervention with antioxidant activity to protect against oxidative damage could inhibit bone resorption and stimulate bone formation.

Accumulated scientific evidence has demonstrated the beneficial effects of dried plums (P. domestica L. ‘Improved French’) on bone health. Several studies have shown dried plum supplementation prevents and even reverses bone loss in animal models (Figure 5) (Deyhim et al., 2005; Franklin et al., 2006; Halloran et al., 2010; Rendina et al., 2013; Smith et al., 2014a; Smith et al., 2014b). Importantly, it has also been shown that plum’s capacity to restore bone was similar to that of intermittent parathyroid hormone (PTH), the only FDA-approved bone anabolic therapy (Bu et al., 2007). These and other studies provide a significant body of evidence that suggests supplementation with plum is unique in its osteoprotective effects on bone.

Figure 5.

In terms of their bioactive components, plums are considered a nutrient dense fruit serving as a good dietary source of potassium and vitamin K, as well as a rich source of phenolic compounds (Donovan et al., 1998; Kayano et al., 2004; Stacewicz-Sapuntzakis et al., 2001). Dried plum is of particular interest because it has received the highest oxygen radical absorbance capacity (ORAC) ranking among the most commonly consumed fruits and vegetables (McBride, 1999). In one of our recent studies, we showed an extract of plum phenolic compounds accounted for > 90% of the effects of plum on bone in an aging osteopenic animal model (B.J. Smith, unpublished results). While we recognize other components in plum (e.g., oligosaccharide) likely contribute to the benefits of plum on bone due to their ability to promote calcium uptake by cells (Weaver, 2005; Weaver et al., 2011), our findings indicate the specific phenolic compounds – seemingly unique in plum – are in large part responsible for the beneficial effects on bone.