Thermotolerant guard cell protoplasts of tree tobacco do not require exogenous hormones to survive in culture and are blocked from reentering the cell cycle at the G1-to-S transition

Heat reduces nitric oxide production required for auxin-mediated gene expression and fate determination in tree tobacco guard cell protoplasts

Tree tobacco (Nicotiana glauca) is an equatorial perennial with a high basal thermotolerance. Cultured tree tobacco guard cell protoplasts (GCPs) are useful for studying the effects of heat stress on fate-determining hormonal signaling. At lower temperatures (32°C or less), exogenous auxin (1-naphthalene acetic acid) and cytokinin (6-benzylaminopurine) cause GCPs to expand 20- to 30-fold, regenerate cell walls, dedifferentiate, reenter the cell cycle, and divide. At higher temperatures (34°C or greater), GCPs expand only 5- to 6-fold; they do not regenerate walls, dedifferentiate, reenter the cell cycle, or divide. Heat (38°C) suppresses activation of the BA auxin-responsive transgene promoter in tree tobacco GCPs, suggesting that inhibition of cell expansion and cell cycle reentry at high temperatures is due to suppressed auxin signaling. Nitric oxide (NO) has been implicated in auxin signaling in other plant systems. Here, we show that heat inhibits NO accumulation by GCPs and that L-N(G)-monomethyl arginine, an inhibitor of NO production in animals and plants, mimics the effects of heat by limiting cell expansion and preventing cell wall regeneration; inhibiting cell cycle reentry, dedifferentiation, and cell division; and suppressing activation of the BA auxin-responsive promoter. We also show that heat and L-N(G)-monomethyl arginine reduce the mitotic indices of primary root meristems and inhibit lateral root elongation similarly. These data link reduced NO levels to suppressed auxin signaling in heat-stressed cells and seedlings of thermotolerant plants and suggest that even plants that have evolved to withstand sustained high temperatures may still be negatively impacted by heat stress.

Heat suppresses activation of an auxin-responsive promoter in cultured guard cell protoplasts of tree tobacco

Cultured guard cell protoplasts (GCP) of tree tobacco (Nicotiana glauca) comprise a novel system for investigating the cell signaling mechanisms that lead to acquired thermotolerance and thermoinhibition. At 32 degrees C in a medium containing an auxin (1-naphthaleneacetic acid [NAA]) and a cytokinin (6-benzylaminopurine), GCP expand, regenerate cell walls, dedifferentiate, and divide. At 38 degrees C, GCP acquire thermotolerance within 24 h, but their expansion is limited and they neither regenerate walls nor reenter the cell cycle. These putative indicators of auxin insensitivity led us to hypothesize that heat suppresses induction of auxin-regulated genes in GCP. Protoplasts were transformed with BA-mgfp5-ER, in which the BA auxin-responsive promoter regulates transcription of mgfp5-ER encoding thermostable green fluorescent protein (GFP) or with a similar 35S-cauliflower mosaic virus constitutive promoter construct. Heat suppressed NAA-mediated activation of BA. After 21 h at 32 degrees C in media with NAA, 49.0% +/- 3.9% of BA-mgfp5-ER transformants strongly expressed GFP; expression percentages were similar to those of 35S-mgfp5-ER transformants at 32 degrees C or 38 degrees C. After 21 h at 38 degrees C in media with NAA, 7.9% +/- 1.6% of BA-mgfp5-ER transformants weakly expressed GFP, similar to GCP cultured at 32 degrees C in media lacking NAA. Expression at 38 degrees C was not increased by incubating for 48 h or increasing NAA concentrations 20-fold. After 9 to 12 h at 38 degrees C, BA was no longer activated when cells were transferred to 32 degrees C. Heat-stressed cells accumulate reactive oxygen species, and hydrogen peroxide (H(2)O(2)) suppresses auxin-responsive promoter activation in Arabidopsis (Arabidopsis thaliana) mesophyll protoplasts. H(2)O(2) did not suppress BA activation at 32 degrees C, nor did superoxide and H(2)O(2) scavengers prevent BA suppression at 38 degrees C.