Abscisic acid induced a negative geotropic response in dark-incubated Chlamydomonas reinhardtii

Chlamydomonas reinhardtii-A Reference Microorganism for Eukaryotic Molybdenum Metabolism

Molybdenum (Mo) is vital for the activity of a small but essential group of enzymes called molybdoenzymes. So far, specifically five molybdoenzymes have been discovered in eukaryotes: nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and mARC. In order to become biologically active, Mo must be chelated to a pterin, forming the so-called Mo cofactor (Moco). Deficiency or mutation in any of the genes involved in Moco biosynthesis results in the simultaneous loss of activity of all molybdoenzymes, fully or partially preventing the normal development of the affected organism. To prevent this, the different mechanisms involved in Mo homeostasis must be finely regulated. Chlamydomonas reinhardtii is a unicellular, photosynthetic, eukaryotic microalga that has produced fundamental advances in key steps of Mo homeostasis over the last 30 years, which have been extrapolated to higher organisms, both plants and animals. These advances include the identification of the first two molybdate transporters in eukaryotes (MOT1 and MOT2), the characterization of key genes in Moco biosynthesis, the identification of the first enzyme that protects and transfers Moco (MCP1), the first characterization of mARC in plants, and the discovery of the crucial role of the nitrate reductase-mARC complex in plant nitric oxide production. This review aims to provide a comprehensive summary of the progress achieved in using C. reinhardtii as a model organism in Mo homeostasis and to propose how this microalga can continue improving with the advancements in this field in the future.

Systems-wide analysis revealed shared and unique responses to moderate and acute high temperatures in the green alga Chlamydomonas reinhardtii

Different intensities of high temperatures affect the growth of photosynthetic cells in nature. To elucidate the underlying mechanisms, we cultivated the unicellular green alga Chlamydomonas reinhardtii under highly controlled photobioreactor conditions and revealed systems-wide shared and unique responses to 24-hour moderate (35°C) and acute (40°C) high temperatures and subsequent recovery at 25°C. We identified previously overlooked unique elements in response to moderate high temperature. Heat at 35°C transiently arrested the cell cycle followed by partial synchronization, up-regulated transcripts/proteins involved in gluconeogenesis/glyoxylate-cycle for carbon uptake and promoted growth. But 40°C disrupted cell division and growth. Both high temperatures induced photoprotection, while 40°C distorted thylakoid/pyrenoid ultrastructure, affected the carbon concentrating mechanism, and decreased photosynthetic efficiency. We demonstrated increased transcript/protein correlation during both heat treatments and hypothesize reduced post-transcriptional regulation during heat may help efficiently coordinate thermotolerance mechanisms. During recovery after both heat treatments, especially 40°C, transcripts/proteins related to DNA synthesis increased while those involved in photosynthetic light reactions decreased. We propose down-regulating photosynthetic light reactions during DNA replication benefits cell cycle resumption by reducing ROS production. Our results provide potential targets to increase thermotolerance in algae and crops.

A systems-wide analysis of the single-cell green alga Chlamydomonas reinhardti reveals shared and unique responses to moderate and acute high temperatures using multiple-level investigation of transcriptomics, proteomics, cell physiology, photosynthetic parameters, and cellular ultrastructure.

Light- and hormone-mediated development in non-flowering plants: An overview

Light, hormones and their interaction regulate different aspects of development in non-flowering plants. They might have played a role in the evolution of different plant groups by conferring specific adaptive evolutionary changes. Plants are sessile organisms. Unlike animals, they lack the opportunity to abandon their habitat in unfavorable conditions. They respond to different environmental cues and adapt accordingly to control their growth and developmental pattern. While phytohormones are known to be internal regulators of plant development, light is a major environmental signal that shapes plant processes. It is plausible that light-hormone crosstalk might have played an important role in plant evolution. But how the crosstalk between light and phytohormone signaling pathways might have shaped the plant evolution is unclear. One of the possible reasons is that flowering plants have been studied extensively in context of plant development, which cannot serve the purpose of evolutionary comparisons. In order to elucidate the role of light, hormone and their crosstalk in the evolutionary adaptation in plant kingdom, one needs to understand various light- and hormone-mediated processes in diverse non-flowering plants. This review is an attempt to outline major light- and phytohormone-mediated responses in non-flowering plant groups such as algae, bryophytes, pteridophytes and gymnosperms.

On the biosynthesis and evolution of apocarotenoid plant growth regulators

Carotenoids are an important source of metabolites with regulatory function, which include the plant hormones abscisic acid (ABA) and strigolactones (SLs), and several recently identified growth regulators and signaling molecules. These carotenoid-derivatives originate from oxidative breakdown of double bonds in the carotenoid polyene, a common metabolic process that gives rise to diverse carbonyl cleavage-products known as apocarotenoids. Apocarotenoids exert biologically important functions in all taxa. In plants, they are a major regulator of plant growth, development and response to biotic and abiotic environmental stimuli, and mediate plant's communication with surrounding organisms. In this article, we provide a general overview on the biology of plant apocarotenoids, focusing on ABA, SLs, and recently identified apocarotenoid growth regulators. Following an introduction on carotenoids, we describe plant apocarotenoid biosynthesis, signal transduction, and evolution and summarize their biological functions. Moreover, we discuss the evolution of these intriguing metabolites, which has not been adequately addressed in the literature.

The evolving role of abscisic acid in cell function and plant development over geological time

Abscisic acid (ABA) is found in a wide diversity of organisms, yet we know most about the hormonal action of this compound in the ecologically dominant and economically important angiosperms. In angiosperms, ABA regulates a suite of critical responses from desiccation tolerance through to seed dormancy and stomatal closure. Work exploring the function of key genes in the ABA signalling pathway of angiosperms has revealed that this signal transduction pathway is ancient, yet considerable change in the physiological roles of this hormone have occurred over geological time. With recent advances in our capacity to characterise gene function in non-angiosperms we are on the cusp of revealing the origins of this critical hormonal signalling pathway in plants, and understanding how a simple hormone may have shaped land plant diversity, ecology and adaptation over the past 500 million years.