Atlas of Science

Tag: arabidopsis thaliana

Similar but ectopic factor in chloroplast biogenesis between dicotyledonous and monocotyledonous plants

Seed is one of the successful form of reproduction in evolutionary history of land plants. Compared to primitive plants (ferns, mosses and liverworts) which do not have seeds and require water to propagate themselves, seed plants now dominate biological niches in a wide range of climates. Inside the seed there already existed an embryonic plant prior to germination. A cotyledon is the embryonic leaf which serves as a primary food storage for the seedling to develop true leaves and become self-feeding through photosynthesis. The number of cotyledons is limited to one or two in flowering species, known as monocotyledonous (monocots) and dicotyledonous (dicots) respectively. In some dicots, namely model organism Arabidopsis, cotyledons must turn into green and photosynthetically active because the storage is not sufficient. In such species cotyledons are functionally similar to leaves that are formed from growing point. Both the green cotyledon and leaf have chloroplasts, the specialized plastids for photosynthesis, that are differentiated from proplastids. In cotyledons etioplasts develop from proplastids while seedlings are growing underground. Once the seedling reaches light, it rapidly transforms etioplasts into chloroplasts. By contrast, leaf chloroplasts develop directly from proplastids upon illumination.

Fig. 1. (A) The AtCYO1 knock-out mutant (atcyo1) has albino cotyledons in Arabidopsis. (B) Phylogenetic tree based on CYO1 homologs.

We have previously identified a cotyledon-specific chloroplast biogenesis factor CYO1 (shi-yo-u means cotyledon in Japanese) in Arabidopsis (AtCYO1). Arabidopsis mutant lacking AtCYO1 develops albino cotyledons but normal green leaves (Fig. 1a). AtCYO1 can catalyze formation and breakage of disulfide bonds in proteins, facilitating its correct folding. AtCYO1 coexists with photosynthetic apparatus in the thylakoid membrane where light energy is converted into chemical energy for subsequent CO₂ uptake. We hypothesize that AtCYO1 functions in the assembly of photosynthetic apparatus during the rapid chloroplast transformation. Homology search of amino acid sequence shows that the genes encoding CYO1-like proteins are widely conserved among seed plants including rice, the species in a large family of monocots (Fig. 1b). This is surprising because, in rice, cotyledon does not develop chloroplast. In this work, we examined the role of CYO1 in rice (OsCYO1).

Fig. 2. The OsCYO1 suppression mutant has albino leaves in rice. Control (1), moderately suppressed mutant (2), and severely suppressed mutant (3).

We first confirmed that OsCYO1 had the similar enzymatic characteristics and functions as AtCYO1 does. Genetic transformation with OsCYO1 enabled Arabidopsis mutant lacking AtCYO1 to recover green cotyledons. While the AtCYO1 gene was scarcely expressed in leaves in Arabidopsis the OsCYO1 was expressed in leaves rather than the cotyledon in rice. Then, we genetically modified rice to suppress OsCYO1 gene expression, and found it to be albino seedling (Fig. 2). Evidently, the OsCYO1 takes a critical role in chloroplast biogenesis in leaves and/or whole plant. Relatively moderate OsCYO1 suppression allowed the leaves to be green (Fig. 2), but these green leaves showed lower photosynthetic activities and higher susceptibility to light stress as compared to controls.

This work gives a new insight into the different regulation of chloroplast biogenesis between dicots and monocots. Interestingly, the leaf express less OsCYO1 as it ages. In rice, growing point remained at the shoot base covered by a culm and leaf sheath. While a new leaf is emerging, it may receive a little light as in underground cotyledons in Arabidopsis. CYO1 may be indispensable in assembly/maintenance of chloroplasts in the growing tissue, in the dark or the transition from prolonged darkness to light period.

Jun Tominaga, Atsushi Sakamoto, Hiroshi Shimada
Department of Mathematical and Life Sciences, Graduate School of Science, Hiroshima University, Japan

Publication

Rice CYO1, an ortholog of Arabidopsis thaliana cotyledon chloroplast biogenesis factor AtCYO1, is expressed in leaves and involved in photosynthetic performance.
Tominaga J, Mizutani H, Horikawa D, Nakahara Y, Takami T, Sakamoto W, Sakamoto A, Shimada H
J Plant Physiol. 2016 Dec 1

October 16, 2017
Landscapes of jasmonates during flower development and leaf stresses

Hormones are small compounds which regulate many aspects of life, from organ development to environmental adaptations. Jasmonates are plant hormones controlling various biological processes including flower development and defense against insects and fungi. In order to conserve an inducible and efficient defense, but also to promote coordinated flower development processes, the hormone amounts must be temporally and spatially regulated in plants. JA-Ile (jasmonoyl-isoleucine) is an active form of Jasmonates and its abundance is tightly controlled. On the one hand, the biosynthesis of the hormone JA-Ile contributes to its quick accumulation, and on the other hand the JA-Ile quantity is limited through modifications that lead to its inactivation. In the model plant Arabidopsis thaliana, two pathways inactivating JA-Ile have been identified in wounded leaves. One pathway is based on JA-Ile oxidations by cytochromes P450 enzymes from the CYP94 family (CYP94B1, CYP94B3 and CYP94C1), and the other one consists on the JA-Ile cleavage by the amido-hydrolases enzymes IAR3 and ILL6. These two concomitant pathways modify JA-Ile and thus produce JA-Ile derivatives, including the 12OH-JA, 12OH-JA-Ile and 12COOH-JA-Ile (Fig. 1).

Fig. 1. JA-Ile turnover and profiles of jasmonates in Arabidopsis thaliana. Flower maturation, wounding and Botrytis cinerea fungal infection induce JA production and JA-Ile accumulation through the enzyme JAR1. The jasmonate active hormone promotes stamen development and plant defense responses. The JA-Ile inactivation is realized by cleavage by the amido-hydrolases IAR3 and ILL6, and also by oxidations by the CYP94s. The CYP94B1, CYP94B3 and CYP94C1 oxidations generate the 12OH-JA-Ile and 12COOH-JA-Ile derivatives, CYP94C1 being responsible of most of the 12COOH-JA-Ile production. The 12OH-JA-Ile can be further cleaved by IAR3 and ILL6 to form the 12OH-JA derivative. Pie diagrams show the specific and dynamic proportions of jasmonates according to each biological context. hpw: hour post-wounding; dpi: day post-inoculation.

The inactivation pathway by the CYP94 enzymes has previously been characterized in mechanical wounded leaves (reflecting an insect attack) and in leaves infected by the fungus Botrytis cinerea. This suggests that it could be a general mechanism to inactivate JA-Ile in the context of environmental stress. However, it was unknown if this pathway also occurred when JA-Ile production was induced by the plant itself for developmental purposes. In order to study this question, the role of the CYP94 enzymes on JA-Ile has been investigated during the flower development in this article.

The dosage of jasmonates in close buds and open flowers showed that the abundance of each compound is dynamic during the flower development (Fig. 1). In closed buds, JA-Ile is more accumulated than in open flowers, contrary to the 12COOH-JA-Ile oxidized derivative that is more abundant in open flowers. This indicates that JA-Ile quantity is limited by oxidations during the flower development. In order to know which CYP94 enzymes could be involved in JA-Ile oxidation in flowers, their expression were measured. CYP94B1, CYP94B3 and CYP94C1 are expressed at different levels during the flower development. More specifically, CYP94C1 expression is increased during the flower maturation until full opening and is the most expressed in stamen (where the pollen grains are produced) from open flowers. In order to determine which of the 3 enzymes is/are responsible for JA-Ile oxidation in flower, the jasmonates were analyzed in single, double and triple mutants for CYP94B1, CYP94B3 and CYP94C1. The analysis shows that CYP94C1 is the master enzyme that oxidizes JA-Ile to produce the 12COOH-JA-Ile in flower.

These results were then used to compare the dynamic turnover of JA-Ile and the accumulation of its derivatives between 3 biological conditions: 1) flowers in maturation, 2) mechanical wounded leaves, 3) infected leaves by the fungus Botrytis cinerea. Wounding and infection are external signals inducing JA-Ile biosynthesis, whereas the increase of JA-Ile in flowers is induced by the plant itself. CYP94C1 is the major enzyme controlling JA-Ile oxidation in infected leaves as well as in flowers. However, in wounded leaves, CYP94B3 acts in concert with CYP94C1 for JA-Ile oxidation, CYP94B3 being mostly responsible for 12OH-JA-Ile accumulation. The relative percentages of the measured jasmonates are presented for the 3 systems in the figure 1. Depending on the biological condition, the specific and dynamic expression of each CYP94 and each amido-hydrolase leads to different proportions of JA-Ile derivatives. Like a landscape painter can use the same colors in different proportions to get various outcomes, the plant controls the proportions of each jasmonates to trigger distinct biological effects.

Widemann Emilie
Institut de Biologie Moléculaire des Plantes, CNRS-UPR2357,
associée à l’Université de Strasbourg, 12 rue du Général Zimmer, France

Publication

Dynamics of Jasmonate Metabolism upon Flowering and across Leaf Stress Responses in Arabidopsis thaliana.
Widemann E, Smirnova E, Aubert Y, Miesch L, Heitz T
Plants (Basel). 2016 Jan 6

October 27, 2016
Assessing plant responses to gravity may be the key to unlocking Martian agriculture

For humans to become a multi-planetary species, we must first overcome many technological and biological hurdles. The closest two potential sites for extraterrestrial colonization (apart from low Earth orbit) are the Moon and Mars. The Moon exhibits approximately ⅙ of the gravity we experience on Earth and Mars about 1/3. Plants utilize gravity as a guide for organ growth, a process known as gravitropism. This mechanism directs the roots downward into the soil to anchor the plant and allow for the uptake of water and nutrients, while also guiding the shoots upward to access light for photosynthesis, allow the exchange gases with the air, and to promote efficient reproduction. Thus, if humankind is to create sustainable extra-terrestrial communities, then our species will have to select traits in our companion organisms (plants) that allow them to adapt to these novel alterations in the gravitational field.

Fig. 1. SpaceX founder and CEO Elon Musk has stated he plans to grow a rose on Mars. Image above is an artistic interpretation of what a Prince William Rose may look like on the martian surface.

The plant Arabidopsis thaliana (Mouse Ear cress) represents a powerful tool to begin to define these traits. Aranbidopsis was the first higher organism to have its entire genome sequenced and is now widely regarded as the model plant for genetics research. Like all plants, Arabidopsis changes its growth patterns in response to a variety of environmental stimuli. The easiest way to observe plant response to gravity is by observing root and shoot growth after the plants have been rotated 90 degrees so they are parallel with the ground. Under these circumstances, the root redirects growth to reorient downwards whereas the shoot bends upwards. Time-lapse photography performed using cameras, microscopes, scanners, infra-red camera’s, and even mobile phones can all be used to observe the dynamics of this reorientation and by measuring the speed and directionality of the response, a wealth of insight into how the plant is sensing and responding can be obtained.

Plants grown from different size seeds or that are different ages all have a varying sensitivity to gravity and respond at different rates (Brooks TLD, et al., Plant Physiology, 2010). In fact, even growing plants in differing environments (e.g., different temperatures or varying light quality) can change their sensitivity to a host of stimuli. As there are so many factors that can alter the plant’s sensitivity to gravity, most current research is performed with the plants grown in a constant environment and at a single stage of development. Defining the interaction of these environmental factors on gravity response is a major challenge in current plant research.

The creation of higher throughput tools can potentially greatly accelerate the investigation of these effects of environmental or genetic variations on plant sensitivity to gravity. Recent advances in technology have led to a dramatic reduction in the cost of the mechanical components required to automate scanning cameras across multiple samples and allow control of the quality and direction of light during plant growth. However, the automation of the movement of time-lapse photography systems is not without difficulties and the recent establishment of the ultra low cost educational microcomputer market now makes it economically feasible to have a computer attached to every camera as an alternative method to increase the efficiency of time-lapse image acquisition during plant root tip reorientation. This coupled with the recent availability of cloud based image storage and analyses are now making these research methods available to not only specialist researchers but also to the general public, potentially empowering a new era of citizen science.

With these advances in high throughput technologies we can discover which plant genes and environmental factors are most important for plant survival under the conditions we will encounter during spaceflight or in extraterrestrial colonies, such as altered gravity. With this information, we can then rationally engineer plants that are adapted to, for example, low gravity, and so could provide ecosystem services and food to astronauts. This work will bring us one giant leap closer to deep space exploration and colonizing Mars and beyond.

Richard Barker
Post doctoral researcher in the Botany department of the University of Wisconsin
Madison, USA

Publications

Assessing Gravitropic Responses in Arabidopsis.
Barker R, Cox B, Silber L, Sangari A, Assadi A, Masson P.
Methods Mol Biol. 2016

Evaluating mechano-transduction and touch responses in plant roots.
Swanson SJ, Barker R, Ye Y, Gilroy S
Methods Mol Biol. 2015

April 1, 2016