Probing the in situ volumes of Arabidopsis leaf plastids using three-dimensional confocal and scanning electron microscopy

Shrink or expand? Just relax! Bidirectional grana structural dynamics as early light-induced regulator of photosynthesis

Light-induced structural changes in thylakoid membranes have been reported for decades, with conflicting data regarding their shrinkage or expansion during dark-light transitions. Understanding these dynamics is important for both fundamental photosynthesis research and agricultural applications. This research investigated the temporal sequence of thylakoid structural changes during light exposure and their functional significance. We combined high-resolution structural approaches (transmission electron microscopy, confocal microscopy with 3D modeling, and small-angle neutron scattering) with spectroscopic and electrophoretic analyses of the photosynthetic apparatus of Arabidopsis thaliana and Ficus elastica plants. A meta-analysis of published ultrastructural data complemented our experimental approach to resolve existing contradictions. We discovered a three-phase response pattern: initial shrinkage, expansion, and relaxation to dark-state equilibrium. The initial shrinkage specifically regulated the cyclic/linear electron transport ratio, providing rapid photoprotection. We also showed that plants' acclimation to different light regimes modulates the kinetics of this response, with constant-light-grown plants exhibiting faster structural adaptations than those acclimated to glasshouse conditions. This work challenges the traditional binary model of light-induced thylakoid structural dynamics, revealing a sophisticated temporal regulatory mechanism, with the dark-adapted state serving as a relaxed equilibrium. The discovered three-phase response reconciles decades of conflicting observations and reveals how plants achieve rapid photoprotection before engaging longer term adaptive responses.

Increased chloroplast occupancy in bundle sheath cells of rice hap3H mutants revealed by Chloro-Count: a new deep learning-based tool

There is an increasing demand to boost photosynthesis in rice to increase yield potential. Chloroplasts are the site of photosynthesis, and increasing their number and size is a potential route to elevate photosynthetic activity. Notably, bundle sheath cells do not make a significant contribution to overall carbon fixation in rice, and thus, various attempts are being made to increase chloroplast content specifically in this cell type. In this study, we developed and applied a deep learning tool, Chloro-Count, and used it to quantify chloroplast dimensions in bundle sheath cells of OsHAP3H gain- and loss-of-function mutants in rice. Loss of OsHAP3H increased chloroplast occupancy in bundle sheath cells by 50%. When grown in the field, mutants exhibited increased numbers of tillers and panicles. The implementation of Chloro-Count enabled precise quantification of chloroplasts in loss- and gain-of-function OsHAP3H mutants and facilitated a comparison between 2D and 3D quantification methods. Collectively, our observations revealed that a mechanism operates in bundle sheath cells to restrict chloroplast occupancy as cell dimensions increase. That mechanism is unperturbed in Oshap3H mutants but loss of OsHAP3H function leads to an increase in chloroplast numbers. The use of Chloro-Count also revealed that 2D quantification is compromised by the positioning of chloroplasts within the cell.

Metabolic modeling reveals distinct roles of sugars and carboxylic acids in stomatal opening as well as unexpected carbon fluxes

Guard cell metabolism is crucial for stomatal dynamics, but a full understanding of its role is hampered by experimental limitations and the flexible nature of the metabolic network. To tackle this challenge, we constructed a time-resolved stoichiometric model of guard cell metabolism that accounts for energy and osmolyte requirements and which is integrated with the mesophyll. The model resolved distinct roles for starch, sugars, and malate in guard cell metabolism and revealed several unexpected flux patterns in central metabolism. During blue light-mediated stomatal opening, starch breakdown was the most efficient way to generate osmolytes with downregulation of glycolysis allowing starch-derived glucose to accumulate as a cytosolic osmolyte. Maltose could also accumulate as a cytosolic osmoticum, although this made the metabolic system marginally less efficient. The metabolic energy for stomatal opening was predicted to be derived independently of starch, using nocturnally accumulated citrate which was metabolized in the tricarboxylic acid cycle to malate to provide mitochondrial reducing power for ATP synthesis. In white light-mediated stomatal opening, malate transferred reducing equivalents from guard cell photosynthesis to mitochondria for ATP production. Depending on the capacity for guard cell photosynthesis, glycolysis showed little flux during the day but was crucial for energy metabolism at night. In summary, our analyses have corroborated recent findings in Arabidopsis guard cell research, resolved conflicting observations by highlighting the flexibility of guard cell metabolism, and proposed new metabolic flux modes for further experimental testing.

Chloroplast ion homeostasis - what do we know and where should we go?

Plant yields heavily depend on proper macro- and micronutrient supply from the soil. In the leaf cells, nutrient ions fulfill specific roles in biochemical reactions, especially photosynthesis housed in the chloroplast. Here, a well-balanced ion homeostasis is maintained by a number of ion transport proteins embedded in the envelope and thylakoid membranes. Ten years ago, the first alkali metal transporters from the K+ EFFLUX ANTIPORTER family were discovered in the model plant Arabidopsis. Since then, our knowledge about the physiological importance of these carriers and their substrates has greatly expanded. New insights into the role of alkali ions in plastid gene expression and photoprotective mechanisms, both prerequisites for plant productivity in natural environments, were gained. The discovery of a Cl- channel in the thylakoid and several additional plastid alkali and alkali metal transport proteins have advanced the field further. Nevertheless, scientists still have long ways to go before a complete systemic understanding of the chloroplast's ion transportome will emerge. In this Tansley review, we highlight and discuss the achievements of the last decade. More importantly, we make recommendations on what areas to prioritize, so the field can reach the next milestones. One area, laid bare by our similarity-based comparisons among phototrophs is our lack of knowledge what ion transporters are used by cyanobacteria to buffer photosynthesis fluctuations.