Yoshimi Oshima (AIST Postdoctoral Researcher), Nobutaka Mitsuda (Senior Researcher) and co-workers of Plant Gene Regulation Research Group, the Bioproduction Research Institute (Director: Yoichi Kamagata) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), have identified the key regulatory genes of cuticle formation, which covers plant surfaces, in collaboration with the Institute of Floricultural Science (Director: Yuriko Murakami) of the National Agriculture Food Research Organization (President: Takeshi Horie).

Cuticle consists of lipid polymers, which makes plant surfaces glossy and protects plants at their outermost layer from external environmental stresses including water, drought, ultraviolet rays and disease. The researchers have identified the key regulatory genes (MYB106, MYB16) which promote cuticle formation. They also found that cuticle formation regulated by these genes is coordinately regulated with tissue formation and cell expansion.

These genes encode transcription factors which regulate multiple gene activities and therefore manipulation of them makes it easy to modify plant wax included in cuticle and surface structure. The genes are also expected to be used in a variety of fields, including improvement of stress tolerance and disease resistance, breeding of crops producing valuable wax, and development of flowers with attractive textures.

Details have been published in a US scientific journal, The Plant Cell (DOI: 10.1105/tpc.113.110783) on May 24, 2013 (EDT).

Plant surfaces with different cuticle formation (Photos are Arabidopsis)

Plants have been utilized as foods, cloths, and housing materials and garden plants have healed people. Recently, plants attract people's attention because of their importance in production of Chinese medicine, biofuel and industrial materials, and their applications are increasing. In addition, plant wax, one of cuticle components, is widely used in cosmetics, foods, lubricants, paint and fuel. Cuticle covering plant surfaces is known to have an important role in the protection from drought and strong light, since it becomes thicker in plants growing in equatorial or dried areas or in dry season. However, cuticle has not been targeted as a trait for breeding because of its complex components. Due to recent development of molecular biology, the possibility has been raised that cuticle would be a good target of the manipulation to develop plants with tolerance to disease and environmental stresses, valuable wax and attracting texture in their flowers.

AIST has long been studying plant genes, particularly the transcription factors that control the expression of many genes, in order to apply them to the production of industrial materials, medicines and food. The Chimeric Repressor Gene Silencing Technology (CRES-T) and the library of chimeric repressor developed by AIST are used worldwide as valuable tools for a variety of basic and applied transcription factor studies. In addition to the development of such general methodologies, AIST is also studying functions of individual transcription factors involved in various aspects of plants, such as morphology, size, and metabolite production. In the present study, the researchers investigated the mechanism regulating the production of substances on the plant surface and the formation of the surface itself.

This work was supported by the Program for Promotion of Basic and Applied Researches for Innovations in Bio-oriented Industry from the Bio-oriented Technology Research Advancement Institution, Japan, entitled "Practical applications of the floral trait regulation technology based on CRES-T (FY2008-2010)."

Since plants step on land from the ocean, cuticle has been protecting plants in the atmosphere. Hydrophobic compounds such as wax and cutin are deposited on the surface and form cuticle which prevent water evaporation from inside, repel water, and are a barrier to protect the plant from strong light and pathogens. Additionally, they protect the plant from adhesion between tissues when new leaves and flowers grow. The continuous deposition of cuticle during the expansion of tissue and in the changing environment is important for proper plant growth (Fig.1).

Figure 1: Examples of phenomena involving plant cuticle

The researchers have identified transcription factors promoting cuticle formation (MYB106, MYB16) from a model plant, Arabidopsis. These transcription factors are closely related, and both of them were found to be involved in the deposition of cuticular wax and cutin. Overexpression of Arabidopsis MYB106 and MYB16 increased deposition of cuticular wax in leaves. Similar result was obtained in torenia overexpressing MYB106. By contrast, inhibition of MYB106 and MYB16 activities decreased deposition of cuticular wax and induced adhesion of growing tissues. MYB106 activated the other known transcription factor WIN1/SHN1, regulating cuticular wax and cutin, and they coordinately promote cuticle formation (Fig. 2).

MYB106 and MYB16 are highly related to the known transcription factor MIXTA determining the steric shape such as trichomes formed on the surface of plant leaf, stem and flower, and petal conical cell. In this study, the researchers revealed that inhibition or enhancement of these two transcription factors not only decreased or increased cuticlar wax but also induced incomplete formation of epidermal cell shape. These results demonstrated that the formation of surface cell shape and cuticle are coordinately regulated.

Figure 2: Regulation of surface cuticle and cell shape formation

The researchers are planning to develop a technology to improve environmental stress resistance and modify surface structure by enhancing or inhibiting the functions of MYB106, MYB16 and WIN1/SHN1 and to apply the technology to crop breeding. They are going to promote the artificial large-scale production of plant wax and valuable lipids by manipulation of these three factors in future.