Species-specific stomatal ABA responses in juvenile ferns grown from spores

Abstract

Adjustable stomatal pores in leaves control the balance between CO2 entry for photosynthesis and water loss via transpiration. The drought and low humidity-induced phytohormone abscisic acid (ABA) is the major regulator of active stomatal closure responses in angiosperms. Whether the ABA signalling pathway for stomatal closure functions similarly in older land plant groups, such as lycophytes and ferns, is still unclear: some studies find no stomatal ABA response in ferns, others find that ABA response is present or triggered by specific environmental conditions. Here we analysed steady-state gas-exchange, stomatal density and stomatal response to exogenously applied ABA in nine fern species grown from spores under controlled growth conditions. We find that ABA responses in ferns are species-specific: stomata in four out of nine species closed in response to ABA. The ABA-sensitive species mostly had slow responses of low magnitude, suggesting reduced ABA-sensitivity of ABA signalling pathway in ferns. Species with larger stomatal conductance tended to close stomata in response to ABA, whereas a relatively strong response of ~35% was also found in Cyrtomium falcatum, a fern with low stomatal conductance. Our results show that ferns constitute a diverse group with varying degree of stomatal ABA-sensitivity. Further characterisation of ABA signalling pathway components in diverse fern species is needed to understand the genetic basis for the variable ABA-sensitivity in ferns.

Abscisic acid induces stomatal closure in horsetails

“…In accordance, the values of steady-state pretreatment transpiration of horsetails remained well below 1 mmol m À2 s À1 , whereas Arabidopsis transpiration in the respective conditions was c. 2 mmol m À2 s À1 (Tulva et al, 2023). Relatively low values of transpiration and stomatal conductance of horsetails and ferns have been associated with their lower stomatal density (Franks & Beerling, 2009;K€ ubarsepp et al, 2020;Wuyun et al, 2023), reduced hydraulic conductance (Brodribb et al, 2005) and relatively large investment into secondary metabolites (Cândido-Sobrinho et al, 2022). Lower transpiration of ferns may be a factor explaining why ABA-induced stomatal closure is not detected when ABA is applied via petiole feeding (Brodribb & McAdam, 2011 (g) E. pratense in the gas exchange measurement cuvette.…”

“…However, it is important to mention that kinetics and amplitude of stomatal ABA response of horsetails and ferns is different compared with angiosperms: significant ABAinduced stomatal closure of horsetails was detected only after 24 min of application (Fig. 1b), which is much longer than in Arabidopsis (Merilo et al, 2018; see also Hõrak et al, 2017;Plackett et al, 2021;Wuyun et al, 2023). Reduced extent of stomatal response is partly associated with generally lower values of stomatal conductance in evolutionarily older species.…”

“…S1). Combined with higher stomatal conductance of E. arvense in 2023, its leaves had more open stomata, potentially enabling better ABA uptake as also discussed in Wuyun et al (2023).…”

“…Methodologically planned studies are needed to explain the conditionality of fern and horsetail stomatal ABA response, where plants are grown from spores in controlled environment (e.g. Wuyun et al, 2023) or local growth conditions characterized more specifically. It is possible that differences in stomatal development also play a role: based on 2023 dataset, we found that E. arvense had significantly lower stomatal density than E. pratense (112.7 AE 5.7 vs 142.1 AE 4.1, respectively; Fig.…”

“…Conflicting results have provoked more studies with bryophytes, lycophytes and ferns, summarized by Clark et al (2022) with the conclusion that hydroactive stomatal regulation is an ancestral state present in all lineages. However, as stomatal ABA responsiveness is still obviously absent in some ferns, either species-specific secondary loss of stomatal function has taken place or specific priming conditions to sensitize ABA-signalling cascade were not met in these experiments (Hõrak et al, 2017;Plackett et al, 2021;Clark et al, 2022;Wuyun et al, 2023).…”