Sieve Element Structure and Function
Mullendore, Daniel Leroy
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The distribution of photosynthetically derived assimilates through plants occurs in specialized cells called sieve elements that are connected to each other forming a microfluidic system within the plant. Sieve elements were discovered over 150 years ago, however their in vivo structure and complete assimilate transport properties eludes clarification. Microscopic investigation of sieve elements requires tissue manipulations that disrupt the natural state of translocating sieve elements resulting in interpretations of sieve element ultrastructure. Construed sieve element structures accommodate the osmotic pressure flow hypothesis. Assimilates are postulated to passively flow through sieve elements as a result of osmotically generated pressure differences from source to sink. The assimilate stream characteristics are considered to adhere to physical parameters of the sieve element geometry. For this theory to hold the sieve elements must be free major of obstructions. Presented below is a five year compilation on the investigations on sieve element structure and how the structure may relate to function. Cell wall geometries of the sieve elements were investigated with newly developed microscopy methods and the geometries were compared to phloem flow, implicating that phloem flow may not adhere to the dogma of osmotic pressure flow hypothesis. Sieve element occlusion mechanisms were also reviewed and investigated. Sieve element occlusion related proteins were investigated by means of newly developed microscopic methodologies. One new microscopic method demonstrates unprecedented preservation of sieve element ultrastructure that has never been witnessed.