Tokyo City University clarifies that plants that stop photosynthesis keep their leaves for flower formation

Professor Tatsuya Fukuda and his team from the Department of Natural Sciences, Faculty of Science and Engineering at Tokyo City University have revealed that the size of leaves (scale leaves) of Monotropastrum humile, a mycoheterotrophic plant that stopped photosynthesis during evolution, correlates with the size of each flower organ. Individuals with small scale leaves have small flowers, while individuals with large scale leaves have large flowers, demonstrating that leaves must be retained for flower formation. Their results were published in Plant-Environment Interactions on June 5.

M. humile of the Ericaceae family is a plant that has completely stopped photosynthesis and evolved to obtain carbon sources solely from fungi. It has mycorrhizae that live symbiotically with fungi and indirectly obtains nutrients from trees through fungi that parasitize on trees.

Although scale leaves are smaller than typical plant leaves, fungi-derived resources are invested in scale leaf formation. Until now, the reason for deliberately retaining these seemingly useless leaves was unknown.

In this study, the research group measured the size of M. humile scale leaves along with various other organs. The specimens were obtained from individuals living in forests within Tenryu Field, the Center for Education and Research in Field Sciences, Faculty of Agriculture, Shizuoka University.

The aboveground parts of individuals were disassembled into each organ for measurement, and dry weight comparisons were also conducted. The results showed that scale leaf weight accounts for approximately 17% of the total aboveground weight. Additionally, significant correlations were shown between the size of each flower organ and the scale leaves.

Generally, each flower organ is known to be an organ formed by leaf transformation, and leaves are known to be necessary as a foundation for creating each flower organ. It is also known that insects such as bumblebees are involved in pollination and fertilization of M. humile flowers.

The measurement results showed that when scale leaf size approaches zero, the size of each flower organ also approaches zero, revealing that formation of scale leaves of a certain size is necessary to achieve the flower size required to attract insects for pollination. To create flowers of a size that matches insect size, even without photosynthesis, the plants were compelled to create leaves of a corresponding size.

Fukuda commented: "Many of the research activities in our laboratory are conducted outdoors. When we encounter Monotropastrum humile, which can be seen from spring to summer, we feel somewhat fortunate. Because of this, until summertime, wherever we go, we forget our original purpose and end up searching for it, thinking there might be some around. It's a plant that really captures our attention. We proceeded with research on this plant because it has large scale leaves, but based on this research, we plan to challenge ourselves with plants that have even smaller scale leaves."

Journal Information
Publication: Plant-Environment Interactions
Title: Why Does Non-Photosynthetic Monotropastrum humile (Ericaceae) Have Scale Leaves?
DOI: 10.1002/pei3.70060