Linking Turgor with ABA Biosynthesis: Implications for Stomatal Responses to Vapor Pressure Deficit across Land Plants.

Scott A.M Mcadam,Timothy J Brodribb

doi:10.1104/pp.16.00380

Abstract

Stomatal responses to changes in vapor pressure deficit (VPD) constitute the predominant form of daytime gas-exchange regulation in plants. Stomatal closure in response to increased VPD is driven by the rapid up-regulation of foliar abscisic acid (ABA) biosynthesis and ABA levels in angiosperms; however, very little is known about the physiological trigger for this increase in ABA biosynthesis at increased VPD Using a novel method of modifying leaf cell turgor by the application of external pressures, we test whether changes in turgor pressure can trigger increases in foliar ABA levels over 20 min, a period of time most relevant to the stomatal response to VPD We found in angiosperm species that the biosynthesis of ABA was triggered by reductions in leaf turgor, and in two species tested, that a higher sensitivity of ABA synthesis to leaf turgor corresponded with a higher stomatal sensitivity to VPD In contrast, representative species from nonflowering plant lineages did not show a rapid turgor-triggered increase in foliar ABA levels, which is consistent with previous studies demonstrating passive stomatal responses to changes in VPD in these lineages. Our method provides a new tool for characterizing the response of stomata to water availability.

Highlights

The plant hormone abscisic acid (ABA) mediates a range of physiological processes in plants, from seed dormancy (Finkelstein et al, 2002) through to resource allocation (Sharp and LeNoble, 2002), yet arguably the most critical role for this hormone is that high levels close stomata during desiccation (Mittelheuser and Van Steveninck, 1969)
We found in angiosperm species that the biosynthesis of ABA was triggered by reductions in leaf turgor; and in two species tested, that a higher sensitivity of ABA synthesis to leaf turgor corresponded with a higher stomatal sensitivity to vapour pressure deficit (VPD)
Major differences in the external pressure required to trigger a significant rise in foliar ABA levels were observed between species, with A. thaliana showing increases in foliar ABA level after exposure of leaves to an external pressure of only 0.5 MPa whereas the other herbaceous angiosperm Pisum sativum had a significant increase in foliar ABA level only when leaves were exposed to an external pressure of 1 MPa (Figure 1)

Summary

Introduction

The plant hormone abscisic acid (ABA) mediates a range of physiological processes in plants, from seed dormancy (Finkelstein et al, 2002) through to resource allocation (Sharp and LeNoble, 2002), yet arguably the most critical role for this hormone is that high levels close stomata during desiccation (Mittelheuser and Van Steveninck, 1969). Given that ABA is critical for the effective closure of seed plant stomata during drought (Iuchi et al, 2001; Wilkinson and Davies, 2002), and stomatal closure during drought stress largely coincides with Ψtlp (Brodribb and Holbrook, 2003; Brodribb et al, 2003), it is reasonable to assume that turgor loss in leaf cells provides the endogenous signal for increasing ABA biosynthesis at Ψtlp leading to stomatal closure during water stress. A number of early studies investigating the triggers for ABA accumulation in excised leaves or drought stressed plants observed up to, or greater than, a 20% increase in ABA levels before Ψtlp was reached (Pierce and Raschke, 1980; Henson, 1982; Creelman and Mullet, 1991; Dingkuhn et al, 1991). Very few studies have commented on the trigger for this more subtle increase in ABA level that occurs in dehydrated leaves at positive turgor pressures, yet these increases may be very important for the regulation of stomatal aperture in well watered plants

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Discussion

Conclusion