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Various heat stress response in plants

MBF1c: A transcription factor that functions as a key regulator of basal thermotolerance. It may function upstream to salicylic acid, ethylene and trehalose signaling. In addition, it may regulate expression of heat response transcripts including Hsfs and Dreb2A.

Systemic signaling: When small part of a plant is exposed to heat stress, long-distance signals can propagate through entire plant, leading to enhancement of heat tolerance. ROS producing enzyme RBOHD was shown to be required for the propagation of this systemic signals. In addition, a temporal-spatial interaction between ROS and ABA regulates rapid propagation of heat-induced systemic signal in plants.

CNGC2: Plasma membrane-localized Ca2+ channel. It was proposed to be one of the putative heat sensors in Arabidopsis. KO of CNGC2 showed enhanced heat tolerance in seedlings, but, more susceptible to heat stress in reproductive tissues. These growth stage-dependent differences in heat tolerance may be due to differences in ROS regulatory systems.

bZIP28 & HSFA2: These transcription factors were proposed to be putative heat sensors in Arabidopsis. bZIP28 functions as a key regulator of unfolded protein response in ER (ER-UPR). In contrast, HSFA2 was shown to be a key regulator of cytosolic UPR (CPR). Recent study demonstrated that functions of bZIP28 and HSFA2 might be integrated via ROS signaling.

Memory of 5 min heat stress: When plants are exposed to 45 min heat stress followed by recovery, expression of transcripts up-regulated by heat stress was sustained for 30 min or 1 h then declined during recovery. In contrast, expression of heat response transcripts spiked at 30 min or 1 h, but dramatically declined at 3 h during recovery following exposure to 5-min heat stress. In addition, 5-min heat stress followed by 3 h recovery was efficient to activate acquired thermotolerance of plants, although spike of transcript expression was observed at 1 h during recovery. These results suggest that plants possess the ability to quickly memorize heat stress and reset cellular states during recovery to adapt to subsequent severe heat stress.

Reproductive organs: We compared detailed morphological characteristics of reproductive organs in Arabidopsis thaliana grown under control conditions or subjected to heat stress and found that heat stress enhanced enlargement of stigma via elongation of stigmatic papillae. This morphological alteration is associated with failure of pollen attachment to the stigma, rather than direct effects of heat stress. In addition, Ca2+ derived from pollen together with O2- might be involved in morphological alteration of stigma depending on the patterns of pollen attachment.

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