Faculty Colloquium: Always more to NO

Please join us on Friday, October 12 at 3 p.m. in the Hatfield Room for this week’s Faculty Colloquium being presented by Gary Tallman, Professor of Biology.

TITLE: Always more to NO: Heat blocks nitric oxide accumulation required for plant growth hormone function

ABSTRACT:  Nicotiana glauca (Graham), tree tobacco, is an equatorial perennial plant with a high tolerance to heat stress.  One particular type of cell in this plant, the guard cell (GC), is useful for studying the effects of heat stress on fate-determining hormonal signaling.  At  lower temperatures (< 32ºC) two plant hormones, auxin (1-naphthalene acetic acid; NAA) and cytokinin (6-benzylaminopurine; BAP), cause GC to expand 20-30 fold, regenerate cell walls, dedifferentiate, re-enter the cell cycle, and divide.  At higher temperatures (> 34ºC) GC expand only 5-6 fold; they do not regenerate walls, dedifferentiate, re-enter the cell cycle, or divide.  Heat (38ºC) suppresses activation of an auxin-responsive gene “switch” (promoter) in GC suggesting that inhibition of cell expansion and cell cycle re-entry at high

temperatures is due to suppressed auxin signaling.  A molecular gas, nitric oxide (NO) has been implicated in auxin signaling in other plant systems.  During my sabbatical my lab showed that heat stress
inhibits NO accumulation by GC and that L-NG-monomethyl arginine  (L-NMMA), 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 re-entry, dedifferentiation, and cell division; and suppressing activation of the  auxin-responsive promoter.  We also showed that heat and L-NMMA 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.