We all are familiar with the need for vitamins in the human diet. What about plants, do they require vitamins too? As it turns out, plants do require vitamins, for some of the same reasons that animals do, as well as for others.
Vitamins are needed for the activity of certain enzymes required for cellular metabolism. These enzyme modifiers, called cofactors, bind to proteins to activate them and create fully functioning enzymes. Vitamins also protect molecules from being oxidized thus preserving their function and preventing damage to the structure and function of cells.
Consider vitamin C, also known as ascorbic acid. One function of vitamin C in animals is to donate electrons as part of a reaction pathway that synthesizes collagen, an important protein needed for wound healing (scar tissue), blood vessel development, and other functions. Some of the enzymes used to generate collagen will not work correctly without the presence of vitamin C. It so happens that vitamin C is also essential for plants but as an antioxidant needed to alleviate cellular stress caused by exposure to ultraviolet radiation, ozone, or drought conditions. And, unlike people, who cannot make vitamin C (we have to eat it), plants can manufacture their own.
Recent research has shown that plants possess an enzyme called GDP-L-galactose phosphorylase, which is required for the synthesis of vitamin C and if the enzyme is “knocked out” plants don’t develop past the seedling stage after germination. This makes it pretty clear that vitamin C is needed for normal plant growth and development.
So what other vitamins are important for plants? Unlike hormones, which also effect plant growth and development in small quantities, vitamins in plants have been much less studied. When vitamins in plants have been studied, it has most often been with respect to their presence and activity as required by animals when they eat the plants to obtain vitamins (animals are generally not very good at producing their own vitamins). Despite this lack of plant-directed research, evidence has clearly emerged regarding the importance of vitamins for plant health.
One thing we do know for sure, is that plants possess the required biochemical pathways needed to synthesize vitamins. Why would that be the case if they had no internal need for them? As far as enzyme cofactors go, less is known about their function in plants compared with animals, but the B vitamins in particular are known to be precursors of some cofactors required for plant enzymatic activity.
The B vitamins are a large class of organic compounds and some are known to be converted into enzyme cofactors in plants. Examples of B vitamins are: Thiamine (B1), riboflavin (B2), niacin (B3), and biotin (B8) as well as a few others (see B6 below). Deficiencies in these vitamins, studied by using mutant plants deficient in the production of these compounds, indicate negative effects when sufficient amounts of B vitamins are not available.
In plants, the B vitamins play a role in reducing oxidative stress brought on by excessive temperature (either hot or cold), high salt concentrations, exposure to ultraviolet radiation, or drought conditions. This activity is still far from perfectly understood, but seems to be because certain enzymes involved in the repair of and protection against such damage require vitamin B-derived cofactors. The B vitamins (and likely others) are important enough in plants that they possess repair pathways to correct defects that arise in the structure of B vitamins and related molecules.
Another interesting compound is vitamin B6, which has a crucial role in plant nitrogen metabolism. According to researchers at the University of Geneva in Switzerland, this vitamin is used by plants to monitor the amount of ammonium nitrogen present. Other roles for the B vitamins in plants are likely to be elucidated through further research.
Vitamin C and the B vitamins are water soluble but there is another class of vitamins that are lipid soluble in animals. Lipids are fatty or oily substances. The lipid soluble vitamins are A, E, and K. These vitamins or their analogs and precursors exist in plants as well.
In plants, vitamin A does not exist, but closely related precursor forms of vitamin A are present, sometimes in great abundance, in the form of carotenoids. The carotenoids are colorful compounds (red, yellow, and orange) needed for absorption of light energy during photosyntheses as well as for protection against photooxidation under high light conditions.
Vitamin E exists in plants but performs a role distinct from that which occurs in animals. In animals, vitamin E protects fatty molecules in cell membranes from stress-induced damage. In plants, vitamin E is important for nutrient transport, but only under conditions of cold stress. Experiments done with the model plant Arabadopsis thalliana at Michigan State University and in Canada at the University of Toronto have shown that plants deficient in Vitamin E suffer an abnormal abundance of a carbohydrate called callose. This buildup of callose interferes with the transport of nutrients from the leaves to the rest of the plant.
Vitamin K has been known to exist in plants since the 1930’s. The plant form is vitamin K1, also known as phylloquinone. The prefix “phyllo” means leaf which gives a clue as to where it can be found. Phylloquinone is abundant in green leafy vegetables, which are an important source of the vitamin in human nutrition. In plants, K1 is an electron carrier supporting photosynthesis in tissues that contain a lot of chlorophyll.
It should come as no surprise really that vitamins are as important to plants as they are to us. We can be thankful that plants provide such a rich source of vitamins and vitamin precursors for us to eat. Perhaps it is time that we consider returning the favor and assist plants in getting their vitamins too.