Physiological manifestations of environmental stress in plants often implicate cell membranes as the primary site of injury. At low temperatures, a decrease in membrane fluidity leads to a decrease in the activity of membrane-bound enzymes and loss of semi permeable membrane properties. This is due to the transition of membrane lipids from a fluid liquid-crystalline phase to a viscous gel crystalline phase. Membrane lipids determine membrane fluidity. The lipids are composed of fatty acids, either straight chain or branched chain families. A small number of microorganisms use branched chain fatty acids as their membrane constituents to maintain membrane fluidity, instead of unsaturated fatty acids. In the genus Bacillus, the majority of fatty acids contributing to membrane fluidity are branched chain fatty acids. Studies of cold tolerant microorganisms suggest that the fatty acid composition of their cell membranes differs from that of their counterparts adapted to warmer ecological niches. Membrane fatty acid analysis of Antarctic bacteria showed that branched chain fatty acids were predominant in the membrane fatty acids in the psychrophiles. Deep-sea barotolerant bacteria contain branched chain fatty acids as a major component of membrane lipids. Legionella shakespearei sp. isolated from cooling tower water contained branched chain fatty acids as the major membrane lipid component. These findings suggest that membrane fluidity may be enhanced at low temperatures by increasing the ratio of branched chain fatty acids in cell membranes.

The biosynthetic pathway of branched chain fatty acids in bacteria is better understood. These organisms use branched chain alpha-oxo acids related to valine, isoleucine and leucine as precursors. These alpha-oxo acid substrates are decarboxylated to yield the precursor compounds for fatty acid synthesis reactions. The decarboxylase, named as branched chain alpha-oxo acid decarboxylase, has been found to be essential in fatty acid synthesis from alpha-oxo acids. The enzyme is a heterodimer consisting of two alpha subunits and two beta subunits, with a molecular weight of 144.4 KDa. Previously, three genes encoding branched chain alpha-oxo acid dehydrogenase complex (EC1.2.1.25) were cloned and sequenced from B. subtilis 168s. The first two genes encode alpha subunit and beta subunits of the branched chain alpha-oxo acid decarboxylase; the third gene encodes the dihydrolipoyl acyltransferase and is considered to be a part of the dehydrogenase complex.

Branched chain fatty acids are utilized by many organisms for supporting their growth at low temperatures. This is accomplished by providing the required membrane fluidity at suboptimal temperatures, so that biological functions may take place. Studies over the past three decades showed that it is very difficult to increase the composition of unsaturated fatty acids in membranes of higher plants. Since engineering of higher plants for increased cold tolerance requires a chemical modification of membrane fluidity in both organelles and cytoplasm of plant cells, cloning of genes essential for branched chain fatty acid biosynthesis provides a new approach in this research area. In this report, we presented the transfer and expression of the first two genes that encode branched chain alpha-oxo decarboxylase in the tomato plant, L. esculentum, and the potential for enhanced biosynthesis of branched chain fatty acids in plants to increase cold tolerance capability. In this paper we described the introduction of genes encoding branched chain alpha-oxo acid decarboxylase into tomato plants. Molecular analysis showed that both the A and B genes were active at transcriptional and translational levels. Comparative analysis of a small number of transgenic tomato plants grown at 4ºC showed that transformed plants had enhanced tolerance to low temperature (4ºC) compared to non-transformed plants. In conclusion, tomato plants transformed with branched chain fatty acid synthesis genes may provide a new model for engineering cold tolerance in plants.

In addition to cold tolerance, branched chain fatty acids may contribute to plant development processes such as fruit ripening, senescence, and disease resistance. Delay of fruit ripening in tomato plants has a positive economical impact. The role of oxidative breakdown of membrane lipids in plant senescence has attracted new research for many years. It was reported that unsaturated fatty acids are used to synthesize compounds that inhibit the fruit ripening of tomato plants. If the incorporation of branched chain fatty acids can replace unsaturated fatty acids at normal plant growth conditions, the free unsaturated fatty acid pool will be enlarged, resulting in the potential delay of tomato ripening.