Plants grow upward from a tip of undifferentiated tissue called the shoot apical（顶上的） meristem（分生组织）. As the tip extends, stem cells at the center of the meristem divide and increase in numbers. But the cells on the periphery²（外围，边缘） differentiate¹ to form plant organs, such as leaves and flowers. In between these two layers, a group of boundary cells go into a quiescent³（静止的） state and form a barrier that not only separates stem cells from differentiating⁴ cells, but eventually forms the borders that separate the plant's organs. Because each plant's form and shape is determined⁵ by organ formation and organ boundary creation, elucidating⁶ the underlying⁷ mechanisms⁸ that govern these functions could help scientists design the architecture of crop plants to better capture light and ultimately produce more crop yield with less input⁹. New research from two teams led by Carnegie's Zhiyong Wang and Kathryn Barton focuses on the role of the crucial plant hormone¹⁰ brassinosteroid in the creation of plant-shoot architecture. 

 

Their work is published by Proceedings¹¹ of the National Academy of Sciences during the week of December 3. 

 

Like all organisms, plant growth and development is regulated by internally produced chemical signals, including hormones¹² like brassinosteroid, which is found throughout the plant kingdom. The brassinosteroid signaling pathway is involved in regulating more than 1,000 plant genes¹³. Mutant plants that are deficient¹⁴ in brassinosteroid that are grown in the dark show features of plants grown in the light. They also have defects at many phases of the plant life cycle, including reduced seed germination¹⁵, dwarfism, and sterility¹⁶.

 

The new study lead by Wang and Barton uncovered yet another role of brassinosteroid: the formation of boundaries between organs. Plants made hypersensitive to brassinosteroid displayed fused organs. The team included lead author's Carnegie's Joshua Gendron and Jiang-Shu Liu, as well as Min Fan, Mingyi Bai, and Stephan Wenkel, from Carnegie, and Patricia Springer from the University of California Riverside. Their investigations¹⁷ showed that activation¹⁸ of the brassinosteroid pathway represses a group of genes called the cup-shaped cotyledon, or CUC family, which is responsible for organ boundary formation. Using sophisticated techniques the team demonstrated that the protein in the brassinosteroid pathway that is responsible for binding¹⁹ to DNA²⁰ and, in this case, for inhibiting²¹ CUC genes, is present at high levels in the meristem's undifferentiated stem cells and developing organ primordia, but very low in the boundary cells, suggesting that different levels of brassinosteroid activity contribute to the opposite growth behavior of these three types of cells.

 

"This work links the plant steroids to growth and development, organ boundary development, providing a link between the physiology²² of the plant and its architectural design," Wang and Barton said.

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