Sieve¹ elements are a key component² of phloem（筛部，韧皮部）, the conductive tissue through which plants transport carbohydrates³ and a wide range of signalling molecules⁴. Elongated⁵ cylindrical⁶ cells are capped at one end by a sieve plate and arranged end-to-end to form sieve tubes which in turn form a network throughout a plant's body. "Sieve elements are very special cells which play an important role in carbon sequestration, yet so far very little has been known about their differentiation," says Professor Ykä Helariutta from the Institute of Biotechnology, University of Helsinki, Finland. "We've identified several genes⁷ regulating the process and characterized it with unprecedented⁸ precision."

 

The results of the collaboration⁹ between the labs of Professor Ykä Helariutta and principal investigator¹⁰ Eija Jokitalo are published in a pair of papers appearing in Science and Nature Communications.

 

"Understanding how the phloem network develops is a significant aspect of plant development and could have important applications in biotechnology and synthetic¹¹ biology," describes Helariutta.

 

 

Sieve elements lose their nucleus¹³ during the course of normal development. In the first paper, the team described how that happens and identified several genetic¹⁴ factors controlling the process. They used serial¹⁵ block-face scanning electron microscopy to reconstruct a 3D model of developing sieve elements from ultra-thin sections, enabling them to track enucleation（去核） of these cells. The nucleus first deforms¹⁶ from a smooth sphere to a crumpled¹⁷ structure, before shrinking and loosing its contents into the cytoplasm, where they are degraded. This is coupled with the degradation of some organelles and shape changes of others.

 

The researchers identified two transcription factors, NAC045 and NAC086, which are expressed in sieve element cells before enucleation. Plants lacking both genes have defective¹⁸ sieve element formation and die at the seedling¹⁹ stage. Serial block-face scanning electron microscopy showed that sieve elements in the double mutant do not undergo enucleation. Furthermore, by expressing NAC45 in cells where it isn't normally found, the researchers showed that it is sufficient to start the degradation of the nucleus and cytoplasm.

 

The researchers also identified a family of genes, dubbed²⁰ NEN1-4, which act downstream of NAC045 and NAC086. Although the enucleation process starts in plants with mutations in these genes, it doesn't complete properly.

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