ESA2014 preview: External chemical defenses in plants

I’ll be presenting this at 1:30 PM on Wednesday in the Plant-Insect Interactions II session in the Compagno room. Fellow GGE student and collaborator Billy Krimmel will follow soon after with an interesting talk on tarweeds. 

I’ve been studying chenopods and their salt bladder system – which is important both physiologically and defensively for the plant – for awhile and with some gentle nudging from my committee, I’ve been trying to place the chenopod system into a broader context. Namely, what ecologically and evolutionarily differs between a plant which sequesters its chemical defenses (alkaloids, tannins, etc.) in its tissues and one which secretes them onto plant surfaces?

Glandular trichomes (secretory and non-secretory) cover the surfaces of Trichostema laxum.

Coming from New England, where plants with copious exudates are less common, the summer in California is a bonanza of sticky, oily, slimy (!) and otherwise exudate-covered plants. Is this pattern driven by rainfall? Many of these species have congeners elsewhere without copious exudates (e.g. Trichostema, Lessingia, etc.), which begs the question: are exudates effective defenses only in arid environments? Are the defenses liable to environmental removal?

I therefore set up a series of experiments examining these questions. In one, I simulated rain on individuals in a population of Atriplex rosea – a chenopod with defensive exudates – while holding other individuals as controls and rainfall controls (which received water at the base, not on the leaves) and assessed herbivory at the end of the season. Perhaps unsurprisingly, I found a significant increase in herbivory in the group which received rainfall, suggesting that instead of helping these arid, water-starved plants, the rainfall and subsequent removal of exudates (which are entirely water-soluble in A. rosea) actually increased its susceptibility to herbivores.

Chenopods with external defenses (Atriplex prostrata and rosea) and without (Chenopodiastrum murale) at my field site.

Come to my talk to hear more!

Chenopod salt bladders

I recently published a paper on a cool plant defense system of certain plants in the Chenopodiaceae.

Three chenopod species at my field site (McLaughlin Reserve, Lake County, CA). In the center the whitish plant is Atriplex rosea, in the front and front left the dark green plant is Chenopodiastrum murale and in the back left the plant with triangular leaves is Atriplex prostrata

The chenopods are a diverse “family” (people can’t really agree whether they are their own family or form a family with the amaranths) found worldwide. They tend to be common in three habitats, dry, salty shrublands, saltmarshes and recently disturbed areas (often roadside or agricultural). Two genera form most of the diversity and have many economically-important species in them. The first is

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Atriplex, the saltbushes (used to refer to perennial species) or oraches (used to refer to annual species).

A sea of Atriplex prostrata at McLaughlin. 

The second is Chenopodium, which includes the food species quinoa (C. quinoa) and lambs-quarters or pigweed (C. album).

Chenopodium neomexicanum, in the greenhouse

The coolest thing about these plants (and certain other chenopods – but not spinach or beets), in my opinion, is that they have these strange bladder cells on their leaf and stem surfaces. Several scientists have studied the salt sequestration of these bladder cells and found they are extremely important in ionic balance of the plant in saline environments. But many, if not most, of the bladdered chenopods are not halophytes (plants which live in salty areas). So what else are these good for?

The leaf of a cultivated variety of Chenopodium album. All the purple balls are salt bladders – the leaf surface below is green. 

I suspected, given their location on the plant surfaces, that they might be part of a defensive system of the plant, as they would be the first tissues contacted by herbivores

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and they would allow the plant to segregate defenses, which are often bad for the plant, away from photosynthetic tissues. So I tested the defensive function of these bladders by removing them from leaves and testing herbivore preference with a choice, assessing herbivore preference without a choice, and removing them in the field and assessing herbivory rates compared to control leaves.

Removed bladders from the C. album leaf above. The purple coloration is due to betalain, a compound shown in other studies of amaranths (closely related) to be an effective defense against insect herbivores. 

I found strong support for a defensive function for these structures. Plants have all sorts of cool structures (domatia, hairs, sticky glands, etc.) which are defensive in function and with this work, I added one more to this list. I’m working on a few further projects on chenopods now, I’ll update with those when they get completed.

Reference: LoPresti, EL (2013) Chenopod salt bladders deter insect herbivores. Oecologia, DOI: 10.1007/s00442-013-2827-0