October 24, 2017 | David F. Coppedge

Plants Use Elaborate Email System

If email is a system for delivering two-way coded messages across a distance, plants had it long before humans thought of it.

Root to Shoot: You’ve Got Mail

Years ago, CEH astonished readers with a report that plants have email (7/13/01). A lot has been learned since this botanical intranet was discovered. Recently in a ‘minireview’ in Current Biology, two scientists described “Shoot-Root Communication in Flowerings Plants.” It’s a pretty amazing system, but readers of this open-access article may need to skip over the journal-obligatory Darwinspeak to appreciate the wonder of what goes on inside your potted plant.

As sessile organisms, terrestrial plants have evolved sophisticated mechanisms to coordinate the growth and development of two distinct systems, the shoot and the root, in response to environmental fluctuations. Adaptive systemic responses are accomplished by shoot–root communication, which involves diverse long-distance signalling molecules. During the last few decades, various genetic, biochemical, molecular, and grafting studies have identified multiple long-distance signalling molecules which are crucial for plants to adapt to external changes. In this minireview, the long-distance signals implicated in systemic responses to various environmental cues are discussed.

the shoot and root deliver messages to each other

Is it appropriate to compare the signaling molecules to email messages? In some ways, yes; in other ways, perhaps not. Some might argue that a signal is not quite like an individualized, coded email message. It could be compared to a pre-ordained process, like “If you see this signal, read this page in your manual and follow the directions.” And yet some of the signaling molecules are composed of ‘text’ of a sort: peptides and RNA molecules whose specific sequence determine the meaning. What is like email is the long-distance communication, the communication channel, and the language convention. Messages travel far and wide, in the plant’s context. Messages travel down prescribed pathways (the xylem and phloem channels). And upon receipt, things happen: they trigger a response. In a plant, some messages are targeted to specific addresses; other messages are more like an ‘e-blast’ newsletter that informs the whole plant. Notice the words message and information in this quote:

Indeed, various signalling substances, including RNA molecules, proteins, peptides and phytohormones, have been detected in the vasculature, and they induce responses in target tissues which they reach via long-distance transport. For instance, shoot growth and architecture are modulated by root-derived phytohormones; similarly, nutrient uptake activity in the root is up-regulated by shoot-derived signals, indicating that the shoot and root deliver messages to each other to induce systemic responses. The adaptive systemic responses and plasticity of plant growth and development regulated by long-distance signals are crucial for plants to thrive on land. In this minireview, the role of long-distance signals in transmitting information between the shoot and root in response to changes in nutrient status, nodulation, abiotic stress, and light is described.

Not all the signals in plant communication are sequence-based like proteins and RNA. Some are chemicals: e.g., calcium ions, auxin and jasmonic acid. Even human email systems, though, include non-textual signals like icons or sounds. The point is that they have meaning. A chemical showing up at a growing plant tip would accomplish nothing unless there were a pre-arranged convention that switched on genes or switched off others. The sequence-based molecules (proteins and RNA) are even more like email, in that corruption of the sequence garbles the meaning.

The paper lists the kinds of signals plants need (hunger, stress, when to sprout, etc.) and the variety of signaling molecules the plant uses to keep in touch with itself and the environment. These signals are summarized in Figure 1. In their conclusion, the authors provide good justification for comparing plant communication to an intranet, without actually using the metaphor of email:

In this minireview, we have described the roles of various long-distance signals involved in systemic responses to environmental changes. Nutrient status in the rhizosphere is perceived by the root, which, in turn, transmits satiety or hunger signals to induce responses not only in the root but also in the shoot. In addition, the number of nodules in the root is tightly regulated by root-to-shoot and shoot-to-root signalling networks in order to balance nutrient and energy status. Abiotic stress in the root initiates shootward transport of stress signals to induce stress responses on the whole-plant level. From the shoot, sugars, light-activated HY5, and light itself translocate to the root to activate root growth and nutrient uptake. These long-distance messengers transmit information regarding environmental and endogenous changes to distant tissues, enabling plants to integrate their responses at the whole-plant level in order to optimize growth and development, which is essential for them to adapt to an ever-changing environment. Characterizing these processes is therefore important not only in order to acquire a basic understanding of plant life but also to breed plants which can grow optimally in diverse cultivation schemes.

So far we’ve described plant communication as an ‘intranet’ comparable to the messaging that goes on within a single building or company. What adds to the fascination is that plants also make use of an ‘internet’ between plants and other species. As described in our 12/15/14 entry, diverse species in a forest can signal one another using an underground network of fungal hyphae (see also 6/17/04 and 10/04/04). Plants can also communicate, like humans do with cell-phone towers, across space—by sending messages into the air. Not to be left out, animals also utilize within-body and between-body messaging systems (e.g., see Phys.org), and also use two-way communication with plants (Nature, The Conversation). In fact, the whole living world is awash in communication. Messaging might well be considered the defining characteristic of life.

That article in The Conversation says that plants employ a number of “amazing nanotechnologies” beyond communication, such as photosynthesis and super-hydrophobicity. They’re so good, engineers want to imitate them. Yet plants have no brains. Did they think these technologies up on their own?

For an interesting read about information and communication, try William Dembski’s book and website Being as Communion. Dembski, who is well versed in the history of ideas and in philosophy, makes the case that the most fundamental entity in the universe is not matter, but information. “It from bit” was physicist John Archibald Wheeler’s succinct phrase to summarize where his thinking had led him to a similar conclusion. How does Dembski’s theorem relate to plant communication? Plants, like programmed systems, may not possess intrinsic intelligence, but can operate with imposed intelligence. Like interacting robots, their signals possess meaning defined for them from external intelligence (a programmer). But if information is the fundamental “stuff” of the universe, as he argues, how is life distinguished from non-life?

 

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