Are plants likely to be more heat stressed in the future?
The levels of the so-called “greenhouse gases” – carbon dioxide (CO2) and methane (CH4) – are increasing in Earth’s atmosphere. This, most climate scientists agree, is resulting in global warming. (Though I prefer the term “global weirding”.)
Among the environmental effects of global weirding are higher frequencies of extreme heat-waves.
In a previous post, we explored the effects of heat and drought on photosynthesis. But how do plants, in general, deal with high-temperature stress?
How do plants cope with heat?
Because plants are sessile organisms that can not escape environmental extremes, such as very high temperatures, many plant species have evolved mechanisms for coping with heat stress.
Such mechanisms may be at the structural level, such as eliminating leaves altogether – see cactus below right, for example – to decrease their surface to volume ratio (and to, of course, reduce water loss).
But perhaps the most elegant and complex mechanisms plants use to confront heat stress exist at the cellular level.
Heat Shock Proteins (a.k.a., Chaperone Proteins)
Sustained high temperatures (typically above 100o to 115o F) can have deleterious effects on plant cells for several reasons. Heat tends to “unfold” proteins (think cooking an egg) and to disrupt cellular membranes (think melting butter).
The “two-dimensional oil slicks” or lipid bilayers that constitute biological membranes can be “melted” by high temperatures, which may destroy their functional integrity. In other words, they get “leaky”. The cells may correct for temperature effects on this membrane fluidity by adjusting the components (phospholipids) of the membranes.
The partial “unfolding” or denaturization of proteins may be corrected or stabilized in living cells by so-called heat shock proteins (HSPs), including protein chaperones. Chaperones help to keep partially unfolded proteins from aggregating or clumping, which tends to happen at higher temperatures. Most of these HSPs are made by the cells only at relatively high temperatures. (For a review of plant HSPs, please see ref. 2 below.)
How do hot temperatures induce the production of Heat Shock Proteins in plants?
In plants, the sometimes dramatic production of HSPs in response to heat is regulated by heat shock factors (HSF). HSF are protein transcriptional factors that promote the expression of some HSP genes in the nucleus.
Plants have many more HSF than other organisms, which has presented scientists with a complex puzzle as to how the production of HSPs is regulated in plants.
According to this research, conducted by Dr. Rob Larkin and colleagues at Michigan State University, a protein that promotes the production of critical HSPs is tethered to the endoplasmic reticulum (ER) at normal temperatures. At high temperatures, this protein – called bZIP28 -is released from the ER, and it then acts to turn on genes for these HSPs in the nucleus.
Bottom line: New research has provided important clues to how plants cope with heat stress. This may help plant breeders develop crop varieties that can better tolerate increased heat waves that may occur due to global “weirding”.
1. Ainsworth, E.A. and D.R. Ort (2010) “How do we improve crop production in a warming world?” Plant Physiology, Vol. 154, pp. 526-530. (Full Text)
2. Al-Whaibi, M. H. (2010) “Plant heat-shock proteins: A mini review.” Journal of King Saud University – Science, Vol. 23, pp. 139-150. (Full Text)
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