Diversity and strategies of environmental stress-responsive transposons in plants

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In this special issue, we survey the latest findings on the relationship between transposons and environmental stresses in plants. Transposons are mobile genes in the genomes of all organisms, not just plants. From their discovery in Zea mays (corn) to the present day, the biological importance of transposons has become increasingly clear. Transposon insertions not only cause mutations in genes essential to the organism but also affect various biological functions by altering gene expression patterns and transcript levels. This special issue focuses on the roles played by transposons in environmental stress responses in plants.

The relationship between transposons and their host plants varies across plant species. Moreover, even within the same species, different types of transposons have distinct relationships with their hosts. Therefore, to understand the relationship between transposons and their hosts, it is necessary to understand the regulatory mechanisms of transposons in various species. In this issue of Genes & Genetic Systems, Yuji Kishima and colleagues, Tokuko Ujino-Ihara and I each review recently discovered environmental stress-responsive transposons and how they adopt diverse strategies. Transposon transposition is a driving force in genome evolution for host plants. Nevertheless, it is also a powerful source of mutation that is likely to be detrimental to survival. For this reason, transposons are usually silenced in the plant genome. However, it has been reported in many plants that transposons can be activated when environmental stimuli are applied.

Kishima, Shasha Wang and Yohei Koide summarize the relationship between the transposition activity of Tam3, a DNA-type transposon of snapdragon, and low-temperature stress. Tam3 has a unique set of properties that distinguish it from other transposons. Generally, transposons are synonymous with selfish factors. That is, transposons increase their copy number and move copies of themselves independently of the survival of the host organism. Therefore, the host epigenetically regulates the transcriptional activity of transposons dispersed throughout the genome. Analysis of the structure and behavior of Tam3 demonstrates that it possesses features that distinguish it from many other transposons and does not behave in a selfish manner that is detrimental to host survival.

Recent improvements in sequencing technology have yielded complete genomic information in various species. In plants, whole-genome sequences have been determined for many species, not only model plants. With the availability of genome sequences, information on transposons that exist in various plant species has also increased. Ujino-Ihara summarizes transposons and high-temperature stress responses in conifers. Conifers play an essential role in many forest ecosystems and require considerable plasticity to adapt to environmental stresses. Conifers have large genomes, a high percentage of which is composed of repetitive elements. Retrotransposons are the most frequent repeat elements in conifers for which whole-genome sequences have been studied. The large number and diversity of retrotransposons in conifer genomes suggest that they play a role in environmental adaptation.

I summarize recently reported stress-responsive transposons and the types of stress that activate them. There are two types of activation of transposons. One is transcriptional activation. Many transposons have been reported to be transcriptionally activated in response to environmental stress. The other type of activation is transpositional activation. Transposition occurs in somatic cells, but there are few reports of transposition because it is technically challenging to detect. The new insertion is fixed in the next generation when the transposition is transmitted to germ cells. Such cases are called transgenerational transposition, and the copy number of the transposon is changed in the offspring. However, there are also few reports of transgenerational inheritance. This is because transgenerational transposition can be detrimental to the survival of offspring, and the host has acquired an epigenetic mechanism to tightly suppress transposition in the germline cells. Understanding the diverse interactions between transposons and plants will clarify the biological importance of transposons.