Mobile elements in plants

Field: Biochemistry, Genetics and Molecular Biology (Life Sciences)

Before beginning this review of transposable elements in plants, a few comments on general concepts are appropriate. The collective feature that encompasses this review is variegation in plants (Figure 1 A, B, C, and D). This variegation has been tied to genetic elements (sequences of small pieces of DNA) intimately integrated into genes, whose mobility (transposition) in and out of genes yields the variegated phenotype. Any particular variegation that one sees, e.g., the colored spots in Figure 1A or in Figure 10, is the outward expression (phenotype) of the mobility (excision) of these transposing elements, and for any one variegated phenotype it is one particular element in one gene. Without this association of the element with a specific gene that monitors the element, variegation would not be obvious. Any gene could have one of these elements integrated and any one of many different kinds (systems) of elements could be integrated into any one gene. Thus far, no gene is known to be immune to “visitation” or “invasion”. Variegation (e.g., white going to red in the petunia flower as in Figure 1B) generally comes from the integration of this mobile element into a gene component that disturbs or interrupts the normal functioning of the gene and therefore a white phenotype results. Only by the movement of the element out of the gene or by some change so that gene function is not impaired will gene expression return, thus, red appears as spots or stripes (Figure 1A, B, C, and D). In maize variegation, there are generally two component parts that interact to yield this spotting or striping phenotype. One part, called a receptor element, is integrated in the gene, thereby impairing gene function, often simply by interrupting normal transcription. As in our example (Figure 1B), white will be expressed. Elsewhere in the genome is another element, called a regulator element, that recognizes the receptor and excises it with the aid of a specific enzyme(s), thereby mobilizing it and freeing the gene of this impairment. This receptor‐regulator combination represents a system and it is quite specific. Only the product of a specific regulatory element recognizes a specific receptor element (and very likely the terminal ends to be later described as the TIR) and together these two components comprise a system. There are 11 or 12 (very likely, more) such systems in maize. In cases studied thus far, receptor elements are defective derivatives of regulatory elements that are deficient in the capacity to self‐transpose. Generally, the variegation cycle is as follows. There is a wild‐type functional gene that is the site of a transposon inserting and this is followed by the impairment of gene functioning, resulting, for example, in a white phenotype. During the development of the flower, transposition of that element occurs out of that gene, returning the gene to normal (red) or in some instances slightly modified gene functioning, e.g., pale red (Figure 1B). Both elements, the regulator and the receptor, have compositional features which lead to changes that control the timing and frequency of these transposition events; this leads to identifiable patterns (Figures 1 and 10) that are distinguished by the type and degree of spotting or striping. The component of the element that controls the timing and frequency of mutation events itself mutates thus yielding changes in phenotypic expression. Recent findings on the composition of the elements to be discussed later provide a hypothesis to explain the timing and frequency events. This mobility of elements expressed via phenotypic variegation has been found in most organisms studied and is considered a major force in genomic restructuring and in nucleotide addition and deletion as forces in evolution. Many features of these mobile elements are discussed.