Ichiro Terashima
I have been involved in several long-term projects. None have reached completion yet, and many more are in progress, requiring ongoing research and development.
Scaling studies on photosynthetic production
Since the era of IBP (International Biological Program 1965-19XX), studies on the photosynthetic production in plant canopies have been conducted in agricultural fields, grass lands, forests, etc., and vast knowledge has been accumulated. I have been utilizing such knowledge, particularly obtained by Japanese groups, to analyse the photosynthetic system of a single leaf. I have been also trying to understand the canopy level production more deeply based on detailed analyses of behaviors of individual organs. Hence, I have been conducting scaling studies encompassing plant physiology, anatomy/morphology, developmental biology, and ecology, at the levels ranging from molecules to plant canopies, focusing on photosynthetic production.
Organisation of the photosynthetic system of a single leaf
Optical properties of the palisade and spongy tissues: In most bifacial leaves, there are two photosynthetic tissues, the palisade and spongy tissues, locating in the upper and lower parts of the leaf. I cut leaves into the sections parallel to the leaf surface and measured optical properties of these sections to reconstruct the light environment within a single leaf. Absorption coefficient of chlorophyll in situ was greater in the spongy tissue than in the palisade tissue. Regular columnar cells having most chloroplasts along the cell surfaces in the palisade tissue allows light to penetrate to the deeper part of the leaf. On the other hand, in the spongy tissue, cell surfaces face various directions and thereby light is scattered and the light path is lengthened. This increases the chance of light absorption by chlorophylls. The difference in the optical properties in these tissues contribute to moderation of light gradient. However, the light gradient within a leaf is still considerable and the spongy tissue mostly receive weak green light.
Acclimation of chloroplasts to local light environment within a leaf: Chloroplasts acclimate to the intra-leaf light environment. Chloroplasts in the top part of the palisade tissue are most sun-type, and with the depth in the leaf, chloroplast properties gradually change to shade-type ones. In the sun-type chloroplasts, the components contributing to the increase in the maximum rate of photosynthesis are abundant, while shade-type chloroplasts have more antenna chlorophyll proteins for light absorption. This acclimation / differentiation of the chloroplasts is continuous, which was, for example, proved by comparison of the photosynthetic properties of 10 serial paradermal sections of a spinach leaf.
Roles of 1) and 2) in leaf photosynthesis: Both the different optical properties between the palisade and spongy tissues and the intra-leaf gradient of chloroplast properties contribute to shaping the leaf photosynthetic system towards the ideal system. In the ideal system, with the increase in the actinic light, the photosynthetic rates of the all chloroplasts would increase uniformly and be light-saturated at the same time, resulting in the sharpest light response curve. Importance of the sharp curve is apparent by comparing the light response curves of the same leaf, which are different depending on the direction of actinic light. When the bifacial leaf is illuminated from the lower side, the light response curve is blunt and it is necessary to give very intense light, much more than the maximum sun light, to saturate leaf photosynthesis. However, in actual leaves, it appears that the acclimation of chloroplasts to the intra-leaf environment is not perfect and the chloroplasts in the top part are light-saturated at lower light levels than the chloroplasts in the deeper part.
Why are not leaves black but green? Role of green light in leaf photosynthesis: When the upper chloroplasts are nearly light saturated, further addition of blue or red light from the upper side hardly increases leaf photosynthesis, because such light energy is mostly dissipated as heat. On the other hand, addition of green light, which penetrates to deeper parts, would drive photosynthesis in the chloroplasts in the deep parts. The action spectrum of leaf photosynthesis is usually measured with weak monochromatic lights. We developed a new method to estimate quantum yields of monochromatic lights in white light. In our new method, monochromatic lights are applied in the presence of white light. The leaf should have many chloroplasts to use intense sunlight. All these chloroplasts should be excited. Then, green light, which penetrates to the deep part of the leaf, is useful. This effect of green light overcompensates the loss of green light as reflected and transmitted light. This is the solution of the long-lasting ‘enigma’ why plant leaves are green.
CO2 environment within a leaf: We conducted research on CO2 diffusion in the leaf using the same differential equation that is applied to leaf canopies to analyse why sun leaves are thicker than shade leaves. It was concluded that sun leaves should have large internal mesophyll surface area, and the cumulative chloroplast surface area facing the intercellular spaces. To accommodate abundant mesophyll surfaces, sun leaves should be thicker. We also found that CO2-permeable aquaporins (cooporins) are involved in CO2 diffusion through the plasma membrane. Under some conditions, like drought and high CO2, the mesophyll conductance, the conductance for CO2 diffusion from the intercellular spaces to the chloroplast stroma, decreases. Using a tobacco mutant which hardly produces ABA, we confirmed that ABA was involved in the decrease in mesophyll conductance. Regulations of activities and abundance of cooporins should be examined.
Regulation of stomata by mesophyll factors
Cell biological and molecular biological studies have been identifying many components for signal transduction for stomatal opening and closure. However, stomatal responses in such studies are generally much slower and smaller than those measured in gas exchange studies. Thus, some ecophysiologists believe that some factors from mesophyll or vascular system via the guard cell apoplast regulate stomatal behavior. Based on some preceding studies, we developed an epidermis transplant system, in which an epidermal strip from the leaf is transplant on mesophyll and confirmed that water soluble mesophyll factors regulate or enhance stomatal response to CO2 or light. Furthermore, we revealed that the apoplast solution rapidly obtained from a leaf segment by centrifuge exerts opening and closing effects depending on the environmental condition of the leaf. We are currently identifying the signaling substance(s).
PHOTOINHIBITION and FARRED Biology
Damages to photosystem I in continuous light at low temperatures or in fluctuating light: Although it is well known that photosystem (PS) II is susceptible to strong light, we found that photosystem I is readily damaged under two conditions. When kept at low temperatures, photosystem I of chilling sensitive plants such as cucumber is susceptible to even moderate to weak light. Photosystem I is also very susceptible to fluctuating light in which strong light and weak light alternate. Moreover, repair of the damaged PSI is slow. Light fluctuates in natural environments, and thereby it could be expected that many plants suffer from chronic PSI photoinhibition. However, it appears that plants do not suffer from PSI photoinhibition.
Far-red light not only suppresses PSI photoinhibition but also enhances photosynthesis: PSI photoinhibition is ready inducible by fluctuating red light. Considering that the sun light contains considerable far-red light, we examined effects of far-red light on fluctuating light induced PSI photoinhibition. Presence of far-red light completely suppresses PSI photoinhibition. Examinations of performances of Arabidopsis mutants revealed that the NDH pathway, which is one the PSI cyclic transport pathways and has been regarded as a minor pathway, is responsible for the suppression of PSI photoinhibition. Moreover, we found that far-red light enhances the photosynthetic rate in the weak light by accelerating switching off the heat.
Systemic signaling
Sink and Source relationships: Photosynthetic production of the plant individual is limited either by the photosynthetic capacity of the leaves (source capacity) or by the capacity of other non-photosynthetic organs to consume or store photosynthetic products (sink capacity). With the increase in the atmospheric CO2 concentration, photosynthesis is enhanced by CO2 and thereby the sink capacity will be more limiting. We have been trying to elucidate mechanisms that controls the sink-source balance with the grafted plants which have the above- and below-ground parts from different varieties of radish. We also manipulated sink-source relationships in legume plants to examine species/varietal differences in the sensitivity to photosynthate accumulation.
Systemic regulation: Development of young leaves which will soon become source leaves are affected not only by the immediate environment of the young developing leaves but also by the photosynthesis and/or photosynthetic environment of the mature leaves. For example, stomatal density and the number of cell layers in the palisade tissue were clearly affected by the mature leaves. We also showed that even in the leaves developed under the same environmental conditions, photosynthetic capacities and/or chloroplast type (sun type or shade type) shade, differ considerably depending on the conditions of neighbouring mature and younger leaves. We have been studying the optimum photosynthetic system organisation of not only the single leaf but also that for the plant individual. To construct the optimum photosynthetic system at the plant individual level, communication among the leaves and sink organs appears to be indispensable.
I believe that basic studies are important even for the ‘applied science.’ To increase the plant photosynthetic production to feed ten billion, we must solve many basic problems. Once such basic solutions are obtained, these can be readily applied to many aspects.
Chuan-Ming Yeh
I mainly focus on two research projects of “Plant nutrient homeostasis and adaptive mechanism” and “Genomics and functional genomics of orchids”. In addition, I pay some attention to “Identification of plant growth-promoting microbes by Metagenomic approach” and “Identification of plant transcription factors that regulate biotic and abiotic stress tolerance”.
1. Plant nutrient homeostasis and adaptive mechanism
Our laboratory employs the following strategies to study the transcriptional regulatory mechanisms in response to nutrient deficiency and environmental stress in plants:
1. We investigate the functions of transcription factors in Arabidopsis and rice by screening CRES-T (Chimeric REpressor gene-Silencing Technology) transgenic lines that exhibit tolerance or high sensitivity to nutrient deficiency or various environmental stresses.
2. Through a chemical genetics approach, we screen for compounds that can alter responses to nutrient deficiency or environmental stresses. These candidate compounds are then used to treat the CRES-T seed library to screen for CRES-T transgenic lines responsive to these compounds, thereby further elucidating the transcriptional regulatory pathways promoted or inhibited by the target compounds.
3. We treat model plant seeds with the selected compounds, obtained through screening or reported in the literature, to test whether they possess stress-priming capabilities to enhance the plant's resistance to stress.
4. RNA sequencing (RNA-seq) is employed for transcriptomic analysis by comparing the transcriptomes of wild-type and transgenic plants, aiming to elucidate the downstream pathways and genes regulated by the target transcription factors.
5. High-efficiency and high-throughput yeast one- and two-hybrid systems (Y1H/Y2H) are utilized to study the upstream transcription factors of genes responsive to nutrient deficiency and environmental stresses, or those transcription factors that form homodimers or heterodimers with the target transcription factors.
2. Genomics and functional genomics of orchids
Our research group is involved in the international orchid genome sequencing project and has published the genomes of the medicinal orchid Dendrobium catenatum and the common ancestor of orchids, Apostasia shenzhenica, in the international journals Scientific Reports and Nature, respectively. Additionally, we have contributed to the publication of the genome of the medicinal plant Lycium barbarum in Communications Biology. These genome publications not only aid in understanding the evolution of orchid species but also identify genes involved in the regulation of medicinal polysaccharide biosynthesis pathways, potentially contributing to the traditional Chinese medicine industry. In the future, we aim to establish a faster and more stable orchid transformation system and combine technologies such as CRES-T and high-efficiency, high-throughput yeast hybrid systems to prioritize the functional exploration of orchid transcription factors.
