Maximilian Larter

Numerous plant species can actively pressurize water in their xylem. It has been hypothesized that regular peaks in positive pressure help plants to remove xylem embolism and to replenish water reservoirs. This may happen after rewatering when plants have suffered from soil water deficit (WD) and/or following diurnal cycles under well-watered (WW) conditions. Using X-ray microcomputed tomography (µCT), we tested these hypotheses in tomato plants by quantifying xylem functionality and internal water pools in stems of 35 plants subjected to WD intensities before and after rewatering and 14 WW plants. Positive xylem pressure was detected in only 30% of the WD plants after rewatering, with a majority of this 30% showing no or rare events of xylem embolism repair. However, all WD tomato plants restored their internal water pools after rewatering, independently of their ability to generate detectable positive xylem pressure. In WW plants, positive xylem pressure was consistently generated and allowed for repair of the few embolized vessels. The ability of tomato plants to rehydrate non-xylary parenchyma tissues after WD and to repair embolisms under WW conditions sheds new light on alternative water-use mechanisms of herbaceous species to recover from drought.

As drought events become more severe, understanding drought‐tolerance traits in crops is vital for food security. Brassica oleracea L., a widely consumed vegetable (including cabbage, cauliflower, and broccoli), has seen a rise in cultivation, but the response of different genotypes to drought remains poorly studied. Here, we investigated the mechanisms underlying drought resistance in seven genotypes of B. oleracea (two grandparental lines and five F2 genotypes) by examining stem anatomical and hydraulic traits, as well as stomatal characteristics, leaf dimensions, and stomatal regulation under drought conditions. Detailed measurements of stem anatomical traits were obtained using light and electron microscopy, while stem hydraulic vulnerability was assessed using the Optical Vulnerability method. The seven genotypes displayed contrasting water‐use and hydraulic strategies. Under well‐watered conditions, initial stomatal conductance ( g _sΨ0 ) showed considerable variation independent of their leaf and stomata size and density, while the leaf water potential at 90% stomatal closure ( Ψg _s90 ) was similar among genotypes. Stomatal safety margins (SSM) were positive across genotypes, confirming coordination between stomatal regulation and xylem hydraulics, and mainly driven by stem P ₅₀ , which was approximately 1 MPa lower in the most tolerant genotype. Embolism resistance was best explained by the thickness of the intervessel pit membrane, highlighting the importance of including anatomical traits in plant drought tolerance studies. To conclude, the intraspecific variation observed in anatomical, hydraulic and stomatal traits among our B. oleracea genotypes indicates potential adaptability to drought and suggests that more drought‐tolerant genotypes could be identified within this crop species through targeted, human‐mediated crosses.

Summary: Stomatal closure prevents significant water losses during drought events. Yet, leaves are not perfectly hermetic and dehydration ensues through residual water losses, known as minimum conductance ( g _min ), which is highly relevant since it informs on the water depletion dynamics under stress. We measured g _min on 101 species spanning phylogenetic and ecological diversities, from ferns to flowering plants. Sampling also included different growth forms and life cycles from annual herbs to longevous trees. We used stomatal measurements to estimate operational ( g _{th op} ) and maximum ( g _{th max} ) conductances. Minimum water conductance is highly variable across species, and g _min shows a weak phylogenetic signal across vascular plants. Annual herbaceous plants have greater water loss than woody plants, and deciduous species showed higher g _min rates than evergreen species. Relationships of g _min with g _{th op} and g _{th max} were weak, revealing the lack of a clear tradeoff between maximum potential conductance efficiency and water retention. We found reduced water expenses in species occupying hotter and more seasonal environments. This study integrates g _min in leaf economics, where long‐lived leaves show higher capabilities to retain water under stress. We provide important information in evolutionary physiology to understand the water loss dynamics under drought stress across clades of vascular plants.

Les forêts du monde entier sont menacées de mortalités importantes lors de sécheresses intenses liés au changement climatique. Les conifères en particulier semblent extrêmement vulnérables à la mort par dysfonctionnement hydraulique de leur système vasculaire ou embolie. Le principal objectif de cette thèse est d’étudier la résistance à l’embolie des conifères dans un cadre évolutif. Premièrement, nous avons mis en évidence que la résistance à l’embolie varie d’un facteur neuf sur plus de 250 espèces parmi les 7 familles de conifères, atteignant un nouveau record du monde avec Callitris tuberculata (P50 = -18.8 MPa). Nous avons montré le lien évolutif entre cette résistance et l’anatomie des ponctuations aréolées. En combinant cette base de données unique avec une phylogénie calibrée de plus de 300 espèces, nous avons retracé la diversification des conifères et l’évolution de leur résistance à l’embolie. Nous avons découvert que plusieurs lignées de conifères ont brusquement changé de dynamiques évolutives, avec l’accélération de la spéciation et de l’évolution de résistance à l’embolie. En outre, les conifères plus résistants se sont diversifié plus rapidement, notamment les genres Cupressus, Juniperus et Callitris (Cupressaceae). La diversification de ces derniers s’est accélérée avec l’aridification de l’Australie sur les derniers 30 Millions d’années. Nous montrons que leur xylème a été façonné par la sécheresse, devenant plus résistant à l’embolie mais surtout sans compromettre l’efficience du transport de l’eau ou augmenter son coût de construction. Cette thèse élargit notre compréhension de l’évolution des plantes vasculaire face aux sécheresses intenses.

Summary: Drought and frost stresses play important roles in determining species distributions, especially at range margins. Understanding how stress resistance traits interact to determine vulnerability to climate change is critical. We developed a large global database of published and new measurements of drought resistance (xylem embolism resistance; P ₅₀ ) and frost resistance (electrolyte leakage; LT ₅₀ ), and investigated evolutionary trade‐offs using Bayesian phylogenetic quantile regressions. Across all woody biomes, P ₅₀ ranged from −1 to −19 MPa, and LT ₅₀ from 0 to below −80°C with conifers generally more resistant than angiosperms. We found a weak trade‐off between drought and frost resistance: Drought‐resistant species tend to be less frost hardy, and vice versa. There are few species resistant to both stresses (e.g. junipers). Including the phylogeny reduced the strength of the relationship, reflecting the phylogenetic signal for these traits. We did not find any strong effects of LT ₅₀ on growth‐related traits, but drought resistance is associated with denser wood, smaller conduits, shorter stature and lower specific leaf area. While we show a trade‐off between frost and drought resistance, our study does not support the global fast–slow economics spectrum. Our results have implications for forests experiencing hotter, drier summers and potentially damaging late frosts.

Anthocyanins, the pigments that give rise to blue, purple, red, and pink colors in many flowers and fruits, are produced by the deeply conserved flavonoid biosynthesis pathway. The regulation of this pathway is thus fundamental for species differences in color across flowering plants, and a growing body of evidence implicates MYB transcription factors as key players activating or suppressing the production of different pigments. Nevertheless, the potential role of MYBs genes in determining the type of pigment produced (as opposed to the overall amount) is unknown. Here, we demonstrate that a lineage of R2R3 MYBs that is closely related to well-known flavonol regulators (MYB12 members in subgroup 7) is the primary determinant of the shift from blue to red flowers in the genus Iochroma. Similar to its ortholog in Capsicum, this Iochroma MYB12-like gene controls the expression of flavonoid-3′-hydroxylase, the pathway branch point between red and blue pigments, and when down-regulated, results in redirection of flux toward red pigments. These results underscore the importance of transcription factor evolution in generating phenotypic novelty as well as the competitive nature of interactions among flavonoid pathway branches. In addition, our study demonstrates the effectiveness of RNAseq of segregating populations, in combination with other lines of evidence, for identifying novel functional variation.

Species distribution models are key to evaluate how climate change threatens European forests and tree species distributions. However, current models struggle to integrate ecophysiological processes. Mechanistic models are complex and have high parameter requirements. Some correlative species distribution models have tried to include traits but so far have struggled to directly connect to ecophysiological processes. Here, we propose a new strategy in which species distributions are based on safety margins which represent species' proximity to their physiological thresholds. We derived frost and drought safety margins for 38 European tree species as the difference between physiological tolerance traits and local maximum stress. We used LT ₅₀ and Ψ ₅₀ as tolerance traits for frost and drought, respectively, and local minimum temperature and minimum soil water potential as maximum stress. We integrated these safety margins into a species distribution model, which tests if the probability of species presence declines rapidly when the safety margin reaches zero, when physiological stress exceeds the species' tolerance traits. Our results showed thaet 35 of the 38 studied species had their distribution explained by one or both safety margins. We demonstrated that safety‐margins‐based model can be efficiently transferred to species for which occurrence data are not available. The probability of presence dropped dramatically when the frost safety margin reached zero, whereas it was less sensitive to the drought safety margin. This differential sensitivity may be due to the more complex regulation of drought stress, especially as water is a shared resource, whereas frost is not. Our analysis provides a new approach to link species distributions to their physiological limits and shows that, in Europe, frost and drought safety margins are important determinants of species distributions.

Soil water deficit (WD) significantly impacts plant survival and crop yields. Many gaps remain in our understanding of the synergistic coordination between molecular and ecophysiological responses delaying substantial drought-induced effects on plant growth. To investigate this synergism in tomato leaves, we combined molecular, ecophysiological, and anatomical methods to examine gene expression patterns and physio-anatomical characteristics during a progressing WD experiment. Four sampling points were selected for transcriptomic analysis based on the key ecophysiological responses of the tomato leaves: 4 and 5 days after WD (d-WD), corresponding to 10% and 90% decrease in leaf stomatal conductance; 8 d-WD, the leaf wilting point; and 10 d-WD, when air embolism blocks 12% of the leaf xylem water transport. At 4 d-WD, upregulated genes were mostly linked to ABA-independent responses, with larger-scale ABA-dependent responses occurring at 5 d-WD. At 8 d-WD, we observed an upregulation of heat shock transcription factors, and two days later (10 d-WD), we found a strong upregulation of oxidative stress transcription factors. Finally, we found that young leaves present a stronger dehydration tolerance than mature leaves at the same drought intensity level, presumably because young leaves upregulate genes related to increased callose deposition resulting in limiting water loss to the phloem, and related to increased cell rigidity by modifying cell wall structures. This combined dataset will serve as a framework for future studies that aim to obtain a more holistic WD plant response at the molecular, ecophysiological and anatomical level.

Soil water deficit (WD) is one of the most important abiotic stresses affecting plant survival and crop yield. Despite its economic relevance, many gaps remain in our understanding of how crops respond to WD, especially concerning the synergistic coordination of molecular and ecophysiological adaptations delaying plant damage and mortality. In this study, we investigated the gene expression imposed by a progressive WD and combined it with measurements pointing to key ecophysiological thresholds in leaves of tomato plants. We uncovered the transcriptomic changes in mature leaves at four stages defined by physiological markers relating to different WD intensities: partial stomatal closure, complete stomatal closure, after leaf wilting, and beginning of embolism development in the veins. By identifying key transcription factors (TFs) across these progressively worsening WD stages, we investigated the timing and impact of ABA-(in)dependent gene regulatory pathways during WD. In addition, we compared the transcriptome in young developing versus mature leaves and explored the physiological mechanisms that may explain the higher tolerance to dehydration in younger leaves. By correlating the transcriptomic changes to precise ecophysiological measurements, the combined dataset will serve as a framework for future studies comparing leaf molecular and physiological responses to WD at specific intensities. Highlight Integrated ecophysiological and gene expression analyses identify key mechanisms underlying the different thresholds of tomato responses to water deficit

The synergy between drought-responsive traits across different organs is crucial in the whole-plant mechanism influencing drought resilience. These organ interactions, however, are poorly understood, limiting our understanding of drought response strategies at the whole-plant level. Therefore, we need more integrative studies, especially on herbaceous species that represent many important food crops but remain underexplored in their drought response. We investigated inflorescence stems and rosette leaves of six Arabidopsis thaliana genotypes with contrasting drought tolerance, and combined anatomical observations with hydraulic measurements and gene expression studies to assess differences in drought response. The soc1ful double mutant was the most drought-tolerant genotype based on its synergistic combination of low stomatal conductance, largest stomatal safety margin, more stable leaf water potential during non-watering, reduced transcript levels of drought stress marker genes, and reduced loss of chlorophyll content in leaves, in combination with stems showing the highest embolism resistance, most pronounced lignification, and thickest intervessel pit membranes. In contrast, the most sensitive Cvi ecotype shows the opposite extreme of the same set of traits. The remaining four genotypes show variations in this drought syndrome. Our results reveal that anatomical, ecophysiological, and molecular adaptations across organs are intertwined, and multiple (differentially combined) strategies can be applied to acquire a certain level of drought tolerance.