The function of ABA transporters during low humidity-induced stomatal closure in Arabidopsis thaliana.

Plants are important for our nature and for all living things. These organisms can produce food, energy and several natural products from carbon dioxide and solar energy. Plants take up CO2 from the air in exchange for water via small pores called stomata on the leaf surface. These stomata are formed by pairs of cells, called guard cells. As plants are sessile organisms, they cannot move and need to adapt their physiology to the imposed environmental conditions. Drought is one of the major problems caused by climate change and guard cell physiology plays an important role in regulating stomatal apertures and the plant water content. For stomatal aperture regulation under limiting water conditions, the plant hormone abscisic acid (ABA) plays a major role. To understand guard cell physiology and the role of ABA in the plant response to low air humidity, defined as the Vapor Pressure Deficit (VPD) between plants and the atmosphere, we performed gas exchange analyses using Arabidopsis thaliana wild type plants and mutants in which ABA transporter genes were disrupted. Among the tested mutants, we observed a slower VPD response in a mutant in which the ABCG22 gene was disrupted. However, we did not observe clearly altered high VPD responses of abcg25, abcg31, abcg40, npf4.6 and npf5.2 ABA transporter mutants. This may be due to a high functional overlap between several ABA transporter genes. We also performed gene expression analyses of ABA transporter genes using the Genevestigator tool. This allowed us to propose the testing of additional ABA transporter genes that might be relevant for the high VPD response in Arabidopsis

Genetic basis and adaptive relevance of drought response in Cape Verde Arabidopsis

Climate change is predicted to impact precipitation patterns, leading to shorter growing seasons and increased susceptibility to drought in many regions worldwide. These changes significantly threaten plant populations and may result in ecosystem desertification. Understanding the mechanisms enabling species to adapt to such changes is crucial for effective conservation strategies and developing resilient crop varieties. Plants cope with drought through various strategies, including avoidance, escape, and drought tolerance, which can be canalized or plastic, depending on the genetic and environmental context. Understanding the balance between canalization and plasticity is essential for predicting plant responses to future climate change. Here, we investigated the genetic architecture of drought adaptation in natural Cape Verdean Arabidopsis thaliana populations. In Chapter One, we reviewed the impact of climate change on precipitation patterns and its consequences on plant populations, including increased susceptibility to drought and extinction risk. We discussed various strategies plants employ to cope with drought, such as avoidance, escape, and drought tolerance. We also discussed the importance of understanding the balance between canalization and plasticity for predicting plant responses to future climate changes. We also highlighted the significance of genetic adaptations in enabling species to adapt and persist in rapidly changing environments and the potential insights gained from studying A. thaliana populations on the Cape Verde Islands (CVI), which have experienced rapid adaptation and evolutionary rescue in response to drought-prone climates. In Chapter Two, we investigated the evolution of stomatal conductance and water use efficiency (WUE) in an A. thaliana population that colonized an island with a montane cloud scrubland ecosystem characterized by seasonal drought and fog-based precipitation. We found that stomatal conductance increases and WUE decreases in the colonizing population relative to its closest outgroup population from temperate North Africa. Genome-wide association mapping revealed a polygenic basis of trait variation, with a substantial contribution from a nonsynonymous SNP in MAP KINASE 12 (MPK12 G53R), which explains 35% of the phenotypic variance in WUE in the island population. Furthermore, we reconstructed the spatially-explicit evolutionary history of MPK12 53R on the island and demonstrated that this allele increased in frequency due to positive selection as A. thaliana expanded into harsher regions of the island. The findings showed how adaptation shaped quantitative eco-physiological traits in a new precipitation regime defined by low rainfall and high humidity. In Chapter Three, we examined the genetic architecture of variation in growth rate, leaf color, and stomatal patterning in response to precisely controlled water conditions among CVI A. thaliana populations. Genome-wide association mapping analyses revealed that moderately complex genetic architectures with roles for several major effect variants underlie variation in these traits. Furthermore, we found that several identified genes through genetic mapping have pleiotropic functions for complex traits underlying drought stress, highlighting the intricate nature of plant adaptation to these challenging conditions. In conclusion, this work presents a comprehensive analysis of the mechanisms and genetic basis of plant adaptation to drought stress, focusing on the natural A. thaliana populations in the CVI islands. Understanding these mechanisms is critical for predicting species distribution and adaptive responses to drought stress. Furthermore, our findings expand our knowledge of how drought adaptation results from numerous genetic variants, suggesting polygenic adaptation, and reveal that new mutations arise frequently enough to potentially facilitate rapid adaptation in colonizing populations. Lastly, these findings enrich our understanding of plant responses to drought and provide valuable insights for developing effective conservation strategies and resilient crop varieties

Arabidopsis NPF4.6 and NPF5.1 Control Leaf Stomatal Aperture by Regulating Abscisic Acid Transport

The plant hormone abscisic acid (ABA) is actively synthesized in vascular tissues and transported to guard cells to promote stomatal closure. Although several transmembrane ABA transporters have been identified, how the movement of ABA within plants is regulated is not fully understood. In this study, we determined that Arabidopsis NPF4.6, previously identified as an ABA transporter expressed in vascular tissues, is also present in guard cells and positively regulates stomatal closure in leaves. We also found that mutants defective in NPF5.1 had a higher leaf surface temperature compared to the wild type. Additionally, NPF5.1 mediated cellular ABA uptake when expressed in a heterologous yeast system. Promoter activities of NPF5.1 were detected in several leaf cell types. Taken together, these observations indicate that NPF5.1 negatively regulates stomatal closure by regulating the amount of ABA that can be transported from vascular tissues to guard cells