Abstract
Angiosperms are able to respond rapidly to the first sign of dry conditions, a decrease in air humidity, more accurately described as an increase in the vapor pressure deficit between the leaf and the atmosphere (VPD), by abscisic acid (ABA)-mediated stomatal closure. The genes underlying this response offer valuable candidates for targeted selection of crop varieties with improved drought tolerance, a critical goal for current plant breeding programs, to maximize crop production in drier and increasingly marginalized environments, and meet the demands of a growing population in the face of a changing climate. Here, we review current understanding of the genetic mechanisms underpinning ABA-mediated stomatal closure, a key means for conserving water under dry conditions, examine how these mechanisms evolved, and discuss what remains to be investigated.
Highlights
Water availability is a major limiting factor for plant survival and growth, and is one of the most significant constraining factors for crop production
NCED enzymes catalyze the first committed step in abscisic acid (ABA) biosynthesis in plants, the oxidative cleavage of the 9’-cis-epoxycarotenoids neoxanthin and violaxanthin (C40), to produce xanthoxin (C15) [42,43,44]. This pathway is most thoroughly characterized in the model angiosperm Arabidopsis thaliana, wherein NCED3 is the key, rate-limiting gene expressed in leaves in response to water deficit stress [45], and the only gene within the ABA biosynthesis pathway to be significantly upregulated within the time-frame of the stomatal VPD response [21], which occurs within minutes [30]
As these levels are not found endogenously, their biological relevance is debatable, and high levels elicit only minor responses in basal land plants, which contrasts sharply with the even these extremely high levels elicit only minor responses in basal land plants, which contrasts complete stomatal closure induced by considerably lower, biologically relevant ABA levels in seed sharply with the complete stomatal closure induced by considerably lower, biologically relevant plants [152,167,168]. These findings indicate that one or more of the key mechanisms required for ABA levels in seed plants [152,167,168]. These findings indicate that one or more of the key ABA-mediated stomatal closure evolved in a seed plant ancestor, after divergence from lycophyte mechanisms required for ABA-mediated stomatal closure evolved in a seed plant ancestor, after and fern lineages
Summary
Water availability is a major limiting factor for plant survival and growth, and is one of the most significant constraining factors for crop production. To meet the immense challenge of feeding this growing population in the face of a changing climate, it will be necessary to develop crop varieties that can survive in drier and increasingly marginalized environments Achieving this goal will require a detailed understanding of the mechanisms that can enable plants to survive in dry environments [6]. Mechanisms have evolved for regulating the aperture of the stomatal pore, mediated by the hormone abscisic acid (ABA) [8,9], that enable the need for photosynthetic gas exchange to be balanced with minimizing water loss under dry conditions [10]. We summarize our current understanding of the angiosperm mechanisms for ABA-mediated closure in response to low humidity and explore how these mechanisms evolved
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