Background

How succulent plants thrive in dry growth habitats, and which developmental adaptations to plant anatomy and physiology were essential for the expansion into such harsh environments are crucial questions in plant biology.

Succulence, the ability to store and remobilize water in specialized tissues during water-scarce growth periods, is one of those fascinating adaptations. Additionally, most succulent plants also evolved Crassulacean Acid Metabolism (CAM), the most water-use efficient mode of photosynthesis, where atmospheric carbon uptake is temporally decoupled from fixation into carbohydrates (Fig. 1). More precisely, CAM photosynthesis can be classified into four phases over 24-hours (Fig. 2). The magnitude, duration, and even the presence of those phases can be highly plastic and depend on water availability.

Simplified scheme of CAM photosynthesis in the mesophyll cells and simplified scheme of four phases of CAM photosynthesis

In phase I, during the dark period, stomata open to take up CO2 from the atmosphere, which is fixed during a primary fixation step into malate and stored as malic acid in the large vacuoles of the mesophyll cells. The enzyme PEP-Carboxylase (PEPC) is crucial for this primary fixation. In the light period, a secondary fixation of CO2 is conducted by the enzyme Rubisco in the Calvin-Benson-Bassham cycle, resulting in the production of sugars. Depending on the time of the day, however, the CO2 for this secondary fixation step stems either from the decarboxylation of malate, the atmosphere, or a combination of both. In the morning during phase II, stomata remain open for additional CO2 uptake from the atmosphere. Simultaneously, the stored malic acid gets remobilized, malate is decarboxylated, and CO2 is released. During the hotter and drier middays in phase III, stomata remain closed, and CO2 is only obtained through decarboxylation of malate to supply Rubisco. In the cooler evenings during phase IV, when malic acid reserves are depleted, stomata may reopen to take up atmospheric CO2 that is directly fixed by Rubisco.

The predominant stomatal opening during the dark period with lower temperatures and higher relative humidity reduces evapotranspiration and, thus, water loss, making CAM photosynthesis the most water-use-efficient mode of photosynthesis.

Objectives

To understand the anatomical requirements of leaf anatomy to accommodate CAM photosynthesis, evolutionary and comparative studies identified important correlations between succulent leaf anatomy and CAM photosynthesis. However, due to the vast phenotypic spectrum of CAM plant anatomy and CAM expression (facultative vs constitutive CAM), the causality of those correlations remains elusive.

To test the functional link between succulent leaf anatomy and CAM photosynthesis, we use developmental genetic approaches to alter distinct leaf traits and quantitatively determine the functional consequences of such modifications on CAM physiology.

The Model

Kalanchoë laxiflora (accession Oxford Botanical Garden) plant with 10 leaf pairs

We have established the leaf succulent and constitutive CAM plant Kalanchoë laxiflora (Fig. 3) as an experimental model system to genetically dissect how different aspects of leaf development support the constitutive CAM lifestyle. Interestingly, Kalanchoë laxiflora presents an ontogenetic transition from C3 photosynthesis in young leaves to CAM photosynthesis in mature leaves, enabling anatomical, physiological, metabolic, and transcriptomic comparisons between C3 and CAM leaves in a single plant.

Methodology

We want to identify key developmental regulators associated with succulent development and CAM induction using single-cell transcriptomics, reverse genetics and forward genetic screens to disrupt leaf succulence, followed by anatomical, physiological, and metabolic analyses to test the consequences of changed leaf anatomy on the establishment of CAM.

Significance

Our research leverages the experimental accessibility and available genetic and biotechnological protocols in the leaf succulent CAM model Kalanchoë laxiflora to determine the anatomical requirements for CAM photosynthesis. Through manipulation of leaf anatomical development and the quantitative assessment of its consequences on CAM physiology, we will mechanistically decode how anatomy affects CAM physiology and, with that, understand the manifold strategies of succulents to survive prolonged drought periods.

Mission

The Functional Anatomy of Succulents Group is part of the Plant Genetics & Development Department at the Institute of Plant Sciences. We are an international, social, and collaborative lab that strongly believes that diversity advances our scientific discoveries, but also our personalities. We strive to create a welcoming, respectful, and open-minded work environment, where discrimination of any kind is outlawed. Our goal is to share our enthusiasm for science and train the next-generation scientists, where new ideas are appreciated and mistakes are part of the learning process.  If you are interested in working with us, please reach out to heike.lindner@unibe.ch.