So in my last blog post I mentioned how grocery stores are now using a method involving the spraying of fruit with ethylene gas to induce ripening of fruit. Ethylene gas increase is necessary in fruit for ripening to occur so this is why ethylene gas can be exploited to induce ripening artificially. Well turns out you don’t need to be a big chain grocery store to take advantage of this fact! Here’s a neat little experiment you can try at home to show how ethylene gas can induce ripening of fruit:
How is it that as Canadians we can walk into a grocery store and have our pick of perfectly ripened, exotic fruit? The fruit has come long distances to be displayed for you, being shipped by plane, train or boat. Despite the long journey it endured to get to a grocery store in Canada, the fruit is at peak consumption ripeness. How is this possible? It is accomplished through a mix of science and common sense. The common sense aspect is that certain fruit will be picked just after reaching maturity. This is to ensure that during transport the fruit will start to ripen, but will be in peak consumption condition upon reaching their destination. These fruit are called climacteric fruit and are capable of ripening once harvested. A few examples of common fruit which demonstrate this method of development are bananas, mangoes, and pineapples. On the other hand, there are also fruit which are not capable of ripening after being harvested. These fruit are called non-climacteric fruit, with examples being strawberries, oranges, and grapes. So how are we able to purchase fully ripened, non-climacteric fruit if they can’t ripen once harvested? These fruit will often be harvested just before ripening and shipped rapidly to their destination. The result is fruit which are of diminished quality when compared to fruit which is at peak ripeness and picked fresh off the stem, but oftentimes the fruit is still excellent. Another method that is used for ripening climacteric fruit is that the fruit will be harvested long before they enter the ripening phase of their life cycle and once they arrive at their destination, the science aspect of fruit ripening is applied. The fruit are sprayed with ethylene gas, as increased ethylene levels within fruit result in the ripening of the fruit.
While this system works well and allows Canadians to access fresh produce during the winter months, there are a few drawbacks. One drawback is that since the fruit is picked pre-ripening, the resultant taste is often inferior to fresh produce. The overall quality is also often diminished, with softer and mushier fruit often being the result of ethylene treatment and the shelf life of these fruit is also decreased. So what is being done to avoid the outcomes of ethylene gas treatment? Well, the aim of this blog post is to introduce you to a method that is currently being developed to produce fruit which will not require post shipment treatment, and the new method will also improve the quality of fruit being imported.
The new method that is being researched takes advantage of abscisic acid’s (ABA) (seen in Figure 1.) role in fruit development. ABA is a plant hormone which is involved not only with fruit development, but is also involved in adapting the plant to different biotic and abiotic factors. It also plays a key role in regulating seed dormancy, a process which ensures that a seed will not germinate when environmental conditions are unfavourable. While the mechanism of action of ABA is still being discovered, there is new research that is looking to discover why exogenously applied ABA results in fruits which have prolonged shelf lives, and are superior in quality to fruits sold by using ethylene treatment.
An experiment was carried out by researchers in Italy where they harvested peaches in varying stages of physiological development, and sprayed them for five days with ABA. The peaches were assessed at the end of the five days, not only for how ripe they were (this was done using a DA-meter, which can be seen in Figure 2) but also for how much water they used over the same period, as the peaches had each been placed in a tube containing water and the level was recorded each day. The DA-meter uses a non-invasive means of measuring fruit ripeness by testing the chlorophyll content in the fruit, a precise means to discover what stage the fruit is in.
So what did the researchers discover with their peaches? They discovered that ABA treatment had to occur before a certain point in the lifespan to have peaches which would be beneficial to ship long distances. Fruit which were further along in their physiological development actually had ripening occur at an increased rate when ABA was sprayed on it. There are a number of reasons it was thought this occurred. One reason is that the researchers discovered ABA actually affected the expression of other hormones and enzymes related to ripening differently, depending on what stage in life the fruit were at. One affected enzyme that is of particular importance is the 9-cis-epoxycarotenoid dehydrogenase (NCED) enzyme. This enzyme is responsible for the synthesis of ABA in the plant naturally, and levels of NCED, and therefore ABA, increase as plants approach maturation. Figure 3 shows the process through which NCED mediates the synthesis of ABA. Plants sprayed with ABA would either have an increase in NCED if they were past a certain point in their development, meaning ripening would happen at an accelerated rate, or they would have decreased concentrations of NCED if sprayed before a certain point. The decrease in NCED results in a decreased concentration of ABA and therefore plant maturation is delayed.
ABA also affected the expression of other hormones related to maturation. Once again, depending on the life cycle stage of the fruit, these hormones were either up- or down-regulated. One key hormone it affected was ethylene! So this links back to the start of this blog where it was mentioned that ethylene gas is sprayed on fruit to induce maturation. Spraying with ABA either reduced or increased the levels of ethylene in fruit, with delayed maturation occurring in plants which were early in their physiological development. Auxin, another plant hormone involved in the maturation process, was also either up- or down-regulated in response to ABA treatment.
So how did fruit produced through ABA treatment differ from those which are produced naturally? The fruit produced by ABA spraying differed in two distinct ways. One difference was that peaches treated with ABA actually showed decreased water usage. The researchers thought this may have to do with ABA playing a part in the fruits response to abiotic and biotic stimuli, as decreased water usage increases the fruit’s resistance to drought. Another difference was in the hardness of the fruit. Peaches sprayed with ABA were much firmer than those in the control group. Increased hardness is beneficial if the fruit are going to be shipped long distances, as it decreases the rate at which the fruit will rot and will prolong the shelf life of the fruit.
To sum it up, research on ABA and other compounds which prolong fruit life and their ability to be shipped, will benefit not only consumers but also the farmers producing the fruit. Less fruit will be lost and profits will increase. Applying ABA exogenously has not yet shown to produce any detrimental effects to the fruit or to the consumer and may soon be implemented into how fruit is produced. The fruit will be superior in quality to fruit produced through the current means of using ethylene gas mediated maturation, and it will be interesting to see what other applications ABA will have.
That’s all for now,
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The year was 1946 and throughout Australia a troubling phenomenon was occurring. The fertility rates of grazing sheep were dropping when the sheep were put out to pasture. No one knew the cause, as farmers were following the same routine they had been for years; yet, there was quite a noticeable difference, in some cases it was reported that there was a 20-40% drop in lambing rates (1). The cause of the infertility turned out to be what the sheep were eating, specifically, the plant known as red clover, which can be seen in figure 1. It was discovered that the red clover the sheep were eating contained high levels of phytoestrogens, and this was the root cause of the fertility issues.
So what are phytoestrogens? Phytoestrogens are the plant derivative of the steroid hormone estrogen, and like estrogen, there are different forms of phytoestrogens. A few different forms present in plants can be seen in figure 2. In plants, phytoestrogens play a different role than estrogen plays in animals. Phytoestrogens purpose in plants is to help establish a symbiotic relationship with rhizobial bacteria, as well as to aid in plant defense response. The amount of phytoestrogen in the plant can therefore vary and it is seen to be highest pre-flowering, as it is important to be actively interacting with rhizobial bacteria at this stage of the plants’ life (2).
So why were high levels of phytoestrogens causing infertility in sheep? It turns out that a particular phytoestrogen, formononetin (shown in figure 2) is converted into a compound called equol (shown in figure 3) within the rumen of the sheep. Certain bacteria, it is still not known which, convert formononetin into daidzein via a decarboxylation and then convert daidzein into equol via a reduction, depicted in figure 3. Equol, once formed, is capable of being absorbed through the walls of the rumen and can enter the bloodstream of the sheep easily (2). Entrance of equol into the bloodstream is when problems in fertility start to arise, as equol shares a similar hydroxyl group to a form of estrogen known as estradiol, shown in figure 4. The similarity results in equol having the capacity to bind to estrogen receptors in the sheep, with equol being able to bind to the receptor when concentrations are at 1-10nm in the blood (3). Estradiol has a higher affinity for the receptor as it can bind when it`s concentration is between 0.5-1.0nm, but increasing concentrations of equol will result in more and more equol binding to estrogen receptors.
The binding of equol to estrogen receptors is of concern when it occurs in the estrogen receptors which are located in the hypothalamus and pituitary gland, seen in figure 5 (4). The estrogen receptors in these locations play a key role in the feedback mechanism which controls the level of estrogen in the body of the sheep. Upon equol binding, it is observed that there is an increase in sex hormone binding globulins, which results in abnormal metabolism and biological effects of estrogen (4). Some of the health consequences of the abnormality are that the female sheep will have inflamed vulva, as well as having difficulty transporting the spermatozoa through the cervix. These changes in the sheep are the cause of the decreased fertility that was observed.
So why was there a sudden onset of sheep infertility problems in 1946? The hypothesis is that there were a few factors that came into play. One factor is that the clover being consumed by the sheep had high phytoestrogen levels. As explained previously, levels in clover are highest pre-flowering. Therefore, it is suggested that the sheep were allowed to graze on the clover prematurely, as flowered red clover has significantly less phytoestrogen content (5). Another factor was that the sheep were grazing on new pasture. If you’ve ever driven by a newly seeded pasture, or the next time you do, notice how much clover there is. I grew up on a farm and can say first hand that clover will cover the entire pasture in the first year but there will be a significant reduction in clover content every year after that. A large amount of clover in the field would have therefore also increased phytoestrogen intake. These two factors combined are the suggested reason why clover disease occurred in the sheep (5).
So what has been done since 1946 to prevent clover disease? Once it was discovered that clover disease was a problem, the various forms of clover were studied. It was discovered it was only red clover which had high levels of phytoestrogens and only a few unique strains of red clover actually had high phytoestrogen content, with one example being Trifolium subterranium. Farmers therefore avoided planting these strains or made sure to waitt until flowering to graze the sheep on pasture containing red clover. Red clover disease never became an issue in other parts of the world as the strains of clover used elsewhere differed in phytoestrogen content.
Another question that was posed during the process of discovering phytoestrogens in clover is, do other plants contain phytoestrogens? There are in fact many other plants which do contain phytoestrogens, some of which are a major part of the human diet, such as soy. So why have we not observed any negative effects of phytoestrogens on the human body? This can be explained in two parts.
The first part is that not all humans are capable of converting phytoestrogens into equol (6). The cause of this is the fact that phytoestrogens must be converted into equol by bacteria. It was discovered that about two-thirds of people do not in fact contain the necessary microflora to allow the conversion to occur (6). The other reason there has been no negative effects on the human body is that equol has a much lower affinity for the estrogen receptors in humans as compared to sheep (4). So even when equol is formed, it is safely excreted from the body without causing any problematic effects.
So where has all of the research done on equol led? Today, equol is being researched as an inhibitor of specific types of cancer, namely breast cancer. Why is this? As discussed previously equol is quite similar in structure to estrogen, meaning it is capable of binding to estrogen receptors. While the potency of equol is currently very low in humans, the potency can be increased by modifying certain functional groups of the molecule. Breast cancer is often associated with unregulated estrogen circulating in the body, so having a molecule which could competitively bind to estrogen receptors would decrease the effect unregulated estrogen would have (7). This research is in the very early stages, and as such it is not yet known if equol with increased potency would have the same adverse effects on humans as it would sheep. If equol did in fact have adverse, further modification of the equol molecule would be required, but for now research continues in vivo and those obstacles will be overcome if and when they arise.
All this new research stemmed from initially identifying the cause of infertility in sheep. Research on equol is continuing and its effect on humans is slowly starting to be understood. Such research may provide useful insights into how other aspects of our diet affect our daily lives and what can be done to maximize the benefits of how we live.
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I would like to introduce you to my blog, which will be discussing how hormones people mostly attributed to being in animals can be found in everyday foods. An example of this is cabbage and peas containing estrogen. I will be discussing what effects eating natural hormones have on our body, as well as the bodies of the animals that consume them, and will discuss new research into how to best take advantage of the natural hormone sources.