Bke2 Biochemistry Lectures

Lecture 27: Plant biochemistry
Reading material:

Abstract:
Biological cycles
Living things on this planet depend on the cycling of living matter and atmospheric CO₂, a process powered by the solar energy trapped by photosynthetic organisms. This is a short formulation of the carbon cycle. All major biological elements are subject to comparable cyclic processes, and the most important, from both ecological and economic viewpoints, is the nitrogen cycle.

Nitrogen passes through various forms and valence states as a result of its interaction with different living forms. Ammonia, NH₃, is the form in which nitrogen is incorporated into organic materials but is less often available to plants and bacteria than other forms of nitrogen. The most available form of nitrogen is gaseous N₂ which constitutes about 80 % of the atmosphere. Nitrogen in this form is chemically unreactive and thus unavailable to most plants and bacteria. This unavailability is a limiting factor for plant growth. To be incorporated in organic matter nitrogen from the atmosphere must be reduced to NH₃, a process called nitrogen fixation. Because of the inertness of N₂ gas a very specific and sophisticated enzyme, nitrogenase, is required to catalyse the reaction.

Nitrogen fixation is carried out by a few species of bacteria that synthesise the enzyme nitrogenase, for instance in members of the genus Rhizobium that live symbiotically on leguminous plants, in some free-living soil bacteria such as Azobacter, Klebsiella, and Clostridium, and by cyanobacteria in aquatic environments. Most plants require a supply of fixed nitrogen from the environment. Sources of NH₃ available to plants are from excess fixed and oxidised nitrogen excreted by micro-organisms, decayed animal and plant tissue, and fertiliser. Vertebrates ultimately obtain fixed nitrogen by feeding on those organisms that can fix atmospheric nitrogen.

Nitrogenase consists of two protein components including electron transport systems, an 4Fe-4S cluster, an Fe-containing centre and an Fe-Mo containing centre. O₂ strongly inactivates the metal centres and nitrogenase is thus protected from oxygen for instance by the presence of leghemoglobin in legume root nodules. A strong reducing agent and ATP are required for the reduction of N₂ to NH₃ by nitrogenase.

Another way by which plants and micro-organisms can obtain ammonia is by reducing nitrate (NO₃^-) and nitrite (NO₂^-). These reactions are catalysed by nitrate and nitrite reductases. The process in the other direction, oxidation of NH₃ to nitrite and nitrate is called nitrification and is performed by micro-organisms such as Nitrosomas and Nitrobacter. Yet other bacteria can reduce nitrate or nitrite to N₂, a process called denitrification.

Ammonia generated from N₂ or nitrogen oxides is assimilated into metabolites, often via the amino acids glutamate and glutamine.

The Glyoxylate cycle
The glyoxylate cycle is a modification of the citric acid cycle and constitutes a specialised anabolic pathway in certain organisms, especially oily seed plants. In plants, bacteria, and yeast, but not in animals, two carbon molecules such as ethanol or acetate are converted to four carbon molecules and ultimately to glucose by the glyoxalate cycle. Cells that contain the glyoxalate cycle enzymes can synthesise all their required carbohydrates from any substrate that is a precursor of acetyl CoA. In plants the glyoxylate cycle is compartmentalised is special organelles called glyoxysomes.

The glyoxylate cycle can be regarded as a shunt within the citric acid cycle. Some of its reactions are shared with the citric acid cycle, while two reactions provide a bypass around the two CO₂ producing steps of the citric acid cycle. The two reactions unique to the glyoxylate cycle are those responsible for the generation and subsequent utilisation of glyoxylate. Isocitrate is converted to succinate (4C) and glyoxylate (2C) in a reaction catalysed by isocitrate lyase. Glyoxylate then condenses with acetyl CoA in a second reaction catalysed by malate synthase. The succinate arising from the glyoxylate cycle activity is first taken care of by the enzymes of the citric acid cycle in mitochondria and then transported to the cytosol for further anabolic steps (Gluconeogenesis). In the same manner the glyoxylate bypass permits the synthesis of sugars from the degradation of fatty acids.

Plant hormones
Plant hormones (also called growth regulators) coordinate the growth and development of plants. The major hormones discovered to date fall into a number of classes: auxins, cytokinins, gibberellins, abscisic acid, ethylene, oligosaccharides and steroids. All of these are low molecular compounds, ethylene is even a gas. Hormones are active in very small quantities. Each class of compounds elicit many diverse responses (e.g. in different tissues or at different times), and there is considerable interaction among different plant hormones. In addition, the same process is controlled by different hormones in different species. Auxins, cytokinins and gibberellins for instance stimulate growth in different ways whereas abscisic acid works antagonistically, i.e. inhibits growth. Ethylene stimulates transverse growth (as opposed to longitudinal) and ripening and senescence of fruit. It is clear that plant hormones are essential for the development of plants, but the exact way in which this is accomplished and how the processes are regulated is still not fully known.

Key concepts:
Photosyntyhesis
Nitrogen cycle
Nitrogen fixation
Rhizobium
Symbiosis
Nitrification
Denitrification
Nitrate reductase
Glyoxylate
Glyoxysome
Plant hormone
Auxin/cytokinin ratio
Gibberellin
Ethylene
Abscisic acid
Growth regulation and differentiation

Links:

Lecture index

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]