Thermotolerance and Cold Acclimation HORT 301 – Plant Physiology November 13, 2009 Taiz and Zeiger, Chapter 26 (p ), Web Topics 26.3 and 26.4 High temperature stress and thermo-tolerance Low temperature stress, and cold acclimation and tolerance

High temperature stress and thermotolerance Temperatures ≥50 °C are lethal to cells of most plants Some plants are genetically adapted to high temperatures Others tolerate high temperatures by avoidance Seeds and pollen tolerate high temperature extremes

High temperature stress differently inhibits physiological processes Temperate plant Desert plant

Peng et al., 2004 PNAS, Courtesy of Chan Yul Yoo High temperatures reduce growth and biomass accumulation Photo-assimilation is relatively more sensitive to high temperatures than respiration  increasing temperature Grain Yield (g m -2 ) Biomass (tons ha -1 )  increasing temperature Rice Peng et al., (2004) PNAS

Basal and acquired thermotolerance Basal thermotolerance is the innate capacity to tolerate high temperatures Acquired thermotolerance is an acclimation process by which exposure to sub- lethal temperature increases the capacity of a plant to tolerate lethal high temperatures

Transpiration facilitates maintenance of leaf temperature Leaf temperatures at mid-day are maintained near ambient Genetic inheritance of stomatal conductance and yield at high temperatures

Reflective leaf hairs and waxes and vertical leaf movement reduce light (heat) absorption

Basal and acquired thermotolerance mediated by heat shock proteins Plant HSPs categories - five groups based on molecular mass

HSPs are molecular chaperones Buchanan et al (2000) Biochemistry &Molecular Biology Plants

HSP101 (HOT1) is a molecular chaperone ATPase, and determinant of basal and acquired thermotolerance Hong and Vierling (2000) Proc Natl Acad Sci 97: °C

Katiyar-Agarwal et al, 2003 Plant Mol Biol, Courtesy of Chan Yul Yoo HSP 101 (Arabidopsis) over-expression increases high temperature tolerance of rice

Heat shock factors (HSFs) are transcriptional regulators of HSP expression

Control Over- expression Co- suppression Mishra et al., 2002 Genes and Development, HsfA1 controls basal and acquired thermotolerance in tomato

HsfA1a function in acquired thermotolerance Von Koskull-Doring et al (2007

Chilling and freezing temperature stresses Natural variation for chilling tolerance is dependent on the ecosystem

Chilling tolerance correlates with higher ratio of unsaturated:saturated fatty acids in membrane lipids Chilling causes loss of membrane function (fluidity)

Freezing stress results in cellular dehydration and then physical destruction of cellular structure by ice crystals Figure 22.5 Buchanan, Gruissem and Jones (2000)

Freezing tolerance mechanisms Cell wall limits ice crystal penetration into the symplast Proteins that limit the growth of ice crystals in the apoplast and symplast; e.g. LEAs, antifreeze proteins Sugars and proteins have cryoprotective function, protein and membrane stabilization Sugars and other osmotic solutes reduce (more negative) the intracellular solute/osmotic potential (ψ s ) lowering the freezing temperature

WT 23  C -2  C -4  C -6  C -8  C -10  C Temperature siz1-2 Cold acclimation by exposure to low temperature enhances freezing tolerance Non-acclimated Cold-acclimated siz1-3 WT siz1-2 siz1-3

Cold Low temperature signaling in plants ICE1 DREB1A/CBF3 Cold responsive genes Cold tolerance Miura et al (2007) Plant Cell

CBF transcription factor regulates cold-induced expression of genes and freezing tolerance Jaglo-Ottosen et al. (1998) Science 280: warm – nonacclimated, cold – acclimated, -5 C treatment -5 °C

WT siz Relative mRNA levels h COR15A COR Miura et al. (2007) Plant Cell 19: ICE Time of cold treatment (h) h DREB1A/CBF3 Timing of transcriptional regulation in the low temperature signaling cascade

Current model for low temperature signaling in plants Zhu et al. (2007)