Proteinogenic amino acids are the building blocks of protein, and plants synthesize all of them. In addition to their importance in plant growth and development, growing evidence underlines the central role played by amino acids and their derivatives in regulating several pathways involved in biotic and abiotic stress responses. In the present review, we illustrate (i) the role of amino acids as an energy source capable of replacing sugars as electron donors to the mitochondrial electron transport chain and (ii) the role of amino acids as precursors of osmolytes as well as (iii) precursors of secondary metabolites. Among the amino acids involved in drought stress response, proline and cysteine play a special role. Besides the large proline accumulation occurring in response to drought stress, proline can export reducing equivalents to sink tissues and organs, and the production of H₂S deriving from the metabolism of cysteine can mediate post-translational modifications that target protein cysteines themselves. Although our general understanding of microalgae stress physiology is still fragmentary, a general overview of how unicellular photosynthetic organisms deal with salt stress is also provided because of the growing interest in microalgae in applied sciences.

Sucrose is essential for plants for several reasons: It is a source of energy, a signaling molecule, and a source of carbon skeletons. Sucrose phosphate synthase (SPS) catalyzes the conversion of uridine diphosphate glucose and fructose-6-phosphate to sucrose-6-phosphate, which is rapidly dephosphorylated by sucrose phosphatase. SPS is critical in the accumulation of sucrose because it catalyzes an irreversible reaction. In Arabidopsis thaliana, SPSs form a gene family of four members, whose specific functions are not clear yet. In the present work, the role of SPSA2 was investigated in Arabidopsis under both control and drought stress conditions. In seeds and seedlings, major phenotypic traits were not different in wild-type compared with spsa2 knockout plants. By contrast, 35-day-old plants showed some differences in metabolites and enzyme activities even under control conditions. In response to drought, SPSA2 was transcriptionally activated, and the divergences between the two genotypes were higher, with spsa2 showing reduced proline accumulation and increased lipid peroxidation. Total soluble sugars and fructose concentrations were about halved compared with wild-type plants, and the plastid component of the oxidative pentose phosphate pathway was activated. Unlike previous reports, our results support the involvement of SPSA2 in both carbon partitioning and drought response.

Apocarotenoids are ancient signaling molecules that have played crucial roles in biological communication and adaptation across evolutionary history. Originating in cyanobacteria, these molecules have diversified significantly in plants, where they contribute to stress perception, developmental regulation, and environmental responses. While some apocarotenoids, such as abscisic acid (ABA) and strigolactones (SLs), have been formally classified as plant hormones due to the identification of specific receptors, many others remain functionally enigmatic despite their profound effects on gene regulation and plant physiology. In this study, we focus on β-carotene-derived apocarotenoids that lack identified receptors, shedding light on their potential signaling roles beyond traditional hormone pathways. By synthesizing current knowledge, we highlight key gaps in understanding their biosynthesis, transport, perception, and downstream effects. Addressing these gaps is essential for unraveling the full scope of apocarotenoid-mediated signaling networks in plants. A deeper understanding of these molecules could not only redefine plant hormone classification but also open new avenues for improving crop resilience and stress adaptation in the face of climate change.

Summary: How did a product of photooxidative damage become a signal for stress tolerance? β-cyclocitral (β-cc) promotes acclimation in several angiosperms, yet has detrimental effects in other photosynthetic organisms. We investigated this contrast in the moss Physcomitrium patens to explore the evolution of β-cc signaling. We combined apocarotenoid treatments and excess light exposure with photosynthetic measurements, biochemical analyses and RNA sequencing. Comparative phylogenomics traced the conservation of regulatory and detoxification components associated with β-cc responses. Physcomitrium accumulated β-cc under excess light and mounted a strong transcriptional response to exogenous β-cc, substantially overlapping with its light-stress response. Nevertheless, β-cc impaired photosynthetic performance and provided no protection. At a phylogenetic level, while MBS1 was deeply conserved, the downstream detoxification module showed marked divergence. Notably, ANAC081/ANAC102-like proteins were recovered in terrestrial monocots but not in Physcomitrium or Lemna turionifera , both showing detrimental responses to β-cc. Our findings suggest that the evolution of protective β-cc signaling involved the recruitment of downstream detoxification regulators, with the SCL14/TGAII- and ANAC081/ANAC102-related lineage emerging as a candidate link between an ancient oxidative cue and beneficial acclimation.