Failure to Observe Stomatal ABA Responses in a Fern and a Lycophyte

Area of Science:

• Plant Physiology
• Plant Molecular Biology
• Evolutionary Biology

Background:

• Abscisic acid (ABA) is crucial for plant drought tolerance via stomatal closure, a well-understood process in seed plants.
• Contradictory findings exist regarding ABA's effect on stomatal closure in ferns and lycophytes, necessitating further investigation.
• Understanding ABA signaling in diverse plant lineages is key to comprehending plant adaptation to arid environments.

Purpose of the Study:

• To investigate the response of stomata to abscisic acid (ABA) in a seed plant (Vicia faba), a fern (Nephrolepis exaltata), and a lycophyte (Selaginella uncinata).
• To analyze ABA signaling components, including ion fluxes and vacuole behavior, in the guard cells of these plant types.
• To determine if ABA-induced stomatal closure and downstream signaling occur in ferns and lycophytes.

Main Methods:

• Comparative analysis of stomatal closure in response to ABA across Vicia faba, Nephrolepis exaltata, and Selaginella uncinata.
• Measurement of guard cell potassium ion (K+) and calcium ion (Ca2+) fluxes.
• Assessment of reactive oxygen species (ROS), nitric oxide (NO) accumulation, and vacuole dynamics in response to ABA.

Main Results:

• ABA induced significant stomatal closure (~80%) in Vicia faba, consistent with seed plant responses.
• Nephrolepis exaltata and Selaginella uncinata showed no stomatal closure in response to ABA under the tested conditions.
• Downstream ABA signaling events (Ca2+ influx, K+ efflux, NO accumulation, vacuole disaggregation) were observed in V. faba but largely absent in N. exaltata and S. uncinata, with only ROS accumulation noted.

Conclusions:

• Stomata of the fern Nephrolepis exaltata and the lycophyte Selaginella uncinata do not respond to abscisic acid (ABA) under the studied growth conditions.
• Key ABA signaling pathways appear to be absent or non-functional in these representative lycophyte and fern species.
• This lack of ABA response in ferns and lycophytes suggests divergent evolutionary strategies for drought stress management compared to seed plants.