The Phloem

All organisms, and in particular multicellular ones, need to maintain functional coherence. They must coordinate activities and processes that occur in their various parts, and integrate a variety of stimuli from the outside to produce meaningful responses. In land plants, the phloem tissue is an essential actor in organismic coordination.

Phloem structure function

In the phloem, tubular cells assemble into so-called sieve tubes which form a continuous microfluidics network. In this network, the products of photosynthesis are distributed throughout the plant body from sources (mature leaves) to sinks (young leaves, roots, fruits etc.). Mass flow is proposed to be driven by osmotically induced gradients of hydrostatic pressure, but direct experimental evidence for this “pressure flow hypothesis” is wanting. The role of the enigmatic sieve tubes as transport routes for assimilates was established in the 19 ^th century, but their extreme sensitivity has hampered the elucidation of the underlying mechanisms and their regulation ever since. Today we know that the phloem also serves as a long distance communication channel. Thousands of small molecules including proteins, RNAs, and phytohormones move within the translocation stream, some of which have important signaling functions. Action potentials, similar to but more slowly propagating than those in animal neurons, are transmitted along the phloem to induce distant reactions. Pathogens such as viruses utilize the phloem for systemic infection. The high content of photoassimilates in sieve tubes attracts an armada of insects, for instance aphids, white flies, or leaf hoppers, that impale their stylets directly into sieve tubes to feed on their contents.

Impact

Given the importance of the tissue, it is astounding that the functional basis is still not understood satisfactorily. Such an understanding, however, would be of fundamental importance for a variety of research areas including plant transport physiology, plant water relations, the physiological control of crop yield characteristics, plant pathogen interactions, etc. To use an analogy – how could we possibly hope to cure diseases like stroke, heart attack, or viral infections if we had no detailed information on the function of the circulatory and nervous systems?

To learn more about sieve tube structure, function, and sieve tube components click the links on the left.

A sieve tube in Arabidopsis thaliana