Bethany Stone

Bethany is a graduate student in Dale's lab working on her PhD in Biological Sciences at the University of Missouri - Columbia. She completed her Bachelor of Science from Southwest Missouri State University where she majored in biology and minored in chemistry. She entered into Mizzou's graduate program in fall of 1996 and is just getting into the meat of her research. Currently she is studying plant's requirements for boron. Click here to see some new maize data!

In 1923 Warington showed that boron is an essential element for broad bean (Vicia faba) and other legumes. She did this by demonstrating that without boron these plants die before reproducing. Initially the plants produced stunted, dark green shoots and short, thickened roots. Death occurred at all of the growing points. These symptoms could only be rescued by applications of boric acid. Since then, similar boron-deficiency symptoms have been characterized in many plants, including most of the agriculturally important species. Diseases such as "black spot" in tomato (Bennett, 1993), "heart rot" in sugar beet (Mengel and Kirkby, 1982), and "hollow heart" in peanut (Bennett, 1993) have been attributed to a lack of boron. Brittleness of the leaves or stem is also a symptom of boron deficiency (Gauch and Dugger, 1954). This is agriculturally important because one of Pioneer's more popular maize lines are brittle (Blevins, personal communication). Boron deficiency is the most widespread micronutrient deficiency in agriculture (Gupta, 1993). Boron toxicity is also a problem in agriculture where it can cause necrotic leaves, delayed flowering and decreased yield (Francois, 1992; Nable et al., 1997). Because of the dramatic effects of boron deficiency and toxicity on agricultural productivity, it is important to understand boron's action in plants.

The question of boron's function in plants is difficult to approach for several reasons. First, for most plant species the concentration range of boron required for optimal growth is very narrow (Loomis and Durst, 1992). An additional few milligrams of boron per liter can change deficiency into toxicity (Mengel and Kirkby, 1982; Eaton, 1944). Secondly, different species have different boron requirements. Plants are broken into four categories depending on their requirement for boron: lactifers, cole crops, most dicots and monocots, and graminaceous monocots. Lactifers, such as the rubber tree, appear to have very high requirements for boron. Cole crops, include the brassicas and have high requirement as well. The next group is the largest and is made up of the remaining dicots and many of the monocots. The fourth group includes the graminaceous plants. These plants have a very low requirement for boron, especially during vegetative growth. However, their need for boron during reproductive growth is undeniable (Shkolnik, 1984; Shelp, 1993; Blevins, unpublished). In addition to the differences between these groups, requirements can vary greatly between similar species and even between cultivars within a species (Nable, 1988; Garnett et al., 1993). Another difficulty in studying boron arises because boron is a micronutrient, so the plant requires very little of it to function normally (Warington, 1923). Additionally, it has been shown that light influences the severity of boron deficiency (Cakmak et al., 1995; Marschner, 1995). The influence of light on boron deficiency is not known to be due to a general effect on growth of plants in higher intensity light or if it is a specific, boron-related response. Finally, the most critical factor that makes studying boron's role in plants difficult is the extreme number of physiological disorders that result from removing boron. All of these disorders happen rapidly, making it difficult to determine what is the primary response and which are secondary.

Although much research has been done on the role of boron in plants, the above complications make it difficult to draw conclusions or find a unifying theme. The boron literature is full of contradictions. So, even though the requirement for boron has been known for over 70 years, much remains a mystery. Most researchers agree that boron is involved in cell wall structure and synthesis (Loomis and Durst, 1992; Hu et al., 1996; Matoh, 1997). It is known that the cell wall pectic polysaccharide, rhamnogalacturonan-II (RG-II), forms a dimer that is crosslinked by a borate ester (O'Neill, et al., 1996; Kobayashi et. al, 1996). Many agree that boron is involved in plingham et al. (1970), who used barley roots and found a passive uptake that was dependant only on pH. However, the results of all of these experiments can be questioned by a critical examination of the methods. In all of these experiments an extremely high concentration of boron was used, usually millimolar levels. It is possible that plants have a boron deficiency inducible active uptake system, but this system is not "active" when there is sufficient boron present. For example, potassium and iron both utilize high-affinity transporters that are expressed only when the ion is limiting (Schachtman and Schroeder, 1994; Eide et al., 1996).

Once boron is in the plant, it is immobile in most species. This is commonly demonstrated by the symptoms of boron deficiency. Much like calcium's deficiency symptoms (another non-mobile ion), boron deficiency symptoms show up primarily in the younger tissues. However, it has been shown that boron is mobile in species where polyols such as sorbitol, mannitol or dulcitol are the primary photosynthetic products. In these species, boron is transported in a complex with polyols (Brown and Hu, 1996; Brown and Shelp, 1997). It has been shown that environmental conditions affect the boron mobility by influencing which sugar is the primary photosynthetic product. When polyols are not being produced in high quantities in these species, the amount of boron transport goes down significantly (Delgado et al., 1994).

Bethany hopes to use molecular biology techniques to address the question of boron's role in plants. Hopefully it will not be another 70 years before this mystery is solved.

Reference List

 

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E-mail: budell@mail.orion.org

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