Plant Responses to CO2: Background and Perspectives

For the past 800,000 years, the atmospheric CO2 concentration, [CO2], has fluctuated between 180–200 and 250–280 ppmv during the different glacial and interglacial periods, respectively (Lüthi et al. 2008). Currently, we are in an interglacial period, where [CO2] had been around 280 ppmv for more than 10,000 years and up until the industrial revolution of the 1800s. Since then, [CO2] has been increasing with annual fluctuations peaking in winter, and the most recent peaks exceeding 400 ppmv. The increasing concentration rate has accelerated to the extent that atmospheric CO2 levels may reach twice those before the industrial revolution by the end of this century. This marked increase in [CO2] is the most influential cause of global warming (IPCC, 2013). Higher temperatures and increased [CO2] both pose serious problems to the environment and in particular to plants. This is because plants cannot keep pace with this rapid increase in [CO2] by means of adaptation alone, as most plants are adapted to an atmospheric [CO2] of below 300 ppmv. In 1957, Keeling and his group initiated continuous measurements of atmospheric [CO2] at the summit of Mauna Loa (e.g. Keeling et al. 1996), which continue to show an impressive increasing pattern of [CO2]. Stimulated by the importance of this issue, CO2 doubling growth experiments followed in the 1970s’. Arp (1991) meta-analysed the results of CO2 doubling experiments conducted up to 1990 and showed that enhancement of photosynthetic rates by high [CO2] was only observed in plants grown in very large pots or in fields. In experiments where plants were grown in small pots, photosynthesis was notably downregulated under high [CO2]; this phenomenon became known as the ‘pot size effect.’ From the mid 1990’s, large-scale experiments using open-top chambers or free air CO2 enrichment (FACE) systems were established to circumvent the pot size effect. The results of these large-scale experiments, however, still showed large variations even within the same species (e.g. Hasegawa et al. 2013) and clearly demonstrated that high [CO2] does not necessarily enhance plant growth or crop yield (Makino and Mae 1999, Ainsworth and Long 2005, Luo et al. 2006, Leakey et al. 2009). Thus the precise effects of CO2 on plant growth and performance remain largely unknown. Because [CO2] will continue to rise for at least another several decades even according to the most optimistic predictions by the IPCC (2013), and because many plant functions are downregulated by high [CO2], it is vital that the detrimental effects of high [CO2] on plants are understood, and ways to overcome these effects sought. Since 2011, we have been running a five-year research project entitled ‘Comprehensive studies of plant responses to the high CO2 world by an innovative consortium of ecologists and molecular biologists’ supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). In this special focus issue, some representative outputs from this consortium project have been compiled. It is widely known that in many angiosperm species the stomata close in response to high [CO2]. Several Arabidopsis thaliana mutants showing impaired stomatal responses to CO2 have been isolated using a high-throughput technique employing thermography of rosette leaves, and analysed (Hashimoto et al. 2006, Negi et al. 2008, Negi et al. 2013, Hashimoto-Sugimoto et al. 2013). Negi et al. (2014) review these seminal studies. In addition, mobile water soluble signal(s) derived from the mesophyll of Commelina communis plants were shown to greatly accelerate guard cell responses to CO2 (Fujita et al. 2013), however, their identity remain unknown. Aquaporins have been implicated in regulating stomatal behaviour, mesophyll conductance for CO2 diffusion (Terashima et al. 2006), and leaf hydraulic conductance (Lopez et al. 2013). Although most studies so far have claimed that plasma membrane intrinsic protein 2 (PIP2)-type aquaporins are water channels, on pp. 251–257 Mori et al. (2014) clearly demonstrate that some barley (Hordeum vulgare) HvPIP2 aquaporins, including HvPIP2;1, act as CO2 channels, and identified the amino acid residue that is responsible for CO2 permeability. In addition, it was shown previously that overexpression of HvPIP2;1 in rice (Oryza sativa) increased mesophyll conductance for CO2 diffusion (Hanba et al. 2004). Thus, it would appear that some aquaporins actually facilitate CO2 transfer and increase mesophyll conductance. In such cases, these aquaporins would be more appropriately called cooporins as previously proposed by Terashima et al. (2006). The next step would be to determine specific regulation mechanisms of leaf aquaporins and cooporins in a comprehensive manner. Plant structural traits such as the shoot to root ratio, leaf thickness, and stomatal density can be modified at high [CO2]. Taking advantage of precise developmental studies of rice leaves (e.g. Yamazaki 1963), Tsutsumi et al. (2014) conducted a detailed developmental study employing transfer experiments, in which plants containing leaves at various developmental stages were transferred from ambient to high CO2 conditions. They succeeded in pinpointing the stages most sensitive to high [CO2] for various leaf morphological traits including leaf length, width, and blade thickness. Stomatal density and index, often reported to decrease at high [CO2] (e.g. Lake et al. 2001), were unchanged, however. These exciting results will form an invaluable basis for the future identification of factors responsible for these morphological and anatomical changes in response to high [CO2]. It is feasible that the shoot to root ratio may respond to changing [CO2], although some meta-analysis studies have not detected such tendencies (Poorter and Nagel, 2000). On pp. 269–280, Hachiya et al. (2014) analysed combined effects of low-medium pH and high CO2, or low nitrogen (N) stress and high [CO2] on growth of A. thaliana. The results clearly indicate that both of these combined conditions caused preferential root growth, but via different mechanisms. In low pH stress and high [CO2], accumulation of sugar caused a change in the auxin and cytokinin balance in a way that favors lateral root growth. Future experiments conducted under several growth conditions and with different relevant mutants (e.g. Hachiya et al. 2012) will be indispensable to further clarify mechanisms of changes in the shoot to root ratio in response to high [CO2]. At high [CO2] plants tend to accumulate sugar and decrease their nitrogen content, leading to a high C/N ratio in the plant body. Takatani et al. (2014), devised a unique experimental system with an A. thaliana mutant showing reduced nitrate uptake activity to study the nature of the C/N balance. At high [CO2] the mutant suffered from N stress when grown with nitrate, and exhibited some phenotypic features similar to those of wild-type plants subjected to N stress, such as a reduced shoot/root ratio, whereas metabolite levels remained unchanged even under high [CO2]. Thus this experimental system is an effective tool to help uncover plant response mechanisms to high [CO2]. In other work focusing on the C/N balance, Sato et al. (2009, 2011) identified the ubiquitin ligase CNI1/ATL31 as a novel C/N regulator. In these studies, C levels were manipulated with glucose or sucrose in the rooting medium. More recently, Aoyama et al. (2014), however, treated A. thaliana plants with high [CO2] instead of sugar and found that ATL31 is involved in promoting leaf senescence at high [CO2] and low N nutrition. On pp. 306–319, Sato and Yanagisawa (2014) compares metabolite contents in A. thaliana grown with various concentrations of nitrate and/or ammonium and at various [CO2] and light levels. When N and light were sufficient, plant growth was enhanced with [CO2] up to 3600 ppmv, indicating A. thaliana’s potential to utilize CO2 at very high concentrations. It was also revealed that ammonium exerted drastic effects on metabolite patterns regardless of [CO2] levels. In a separate study using rice, Kaneko et al. assessed knockout mutants of nucleotide pyrophosphatase/ phosphodiesterase 1 (npp1) grown at high [CO2] and high temperatures. The results indicate that NPP1, a unique glycoprotein (Nanjo et al. 2006, Kaneko et al. 2011), exerts negative effects on growth and starch accumulation in plants grown at high [CO2] and high temperatures. Manipulation of NPP1 expression levels could potentially be a way to improve rice growth in the high CO2 era. On pp. 333–340, Takagi et al. (2014) show that methylglyoxal, a sugar-derived reactive carbonyl, was produced as a by-product of the triosephosphate isomerase reaction in the Calvin-Benson cycle in wheat (Triticum aestivum) chloroplasts and intact leaves. The production rate of methylglyoxal was dependent on light intensity and [CO2] and could lead to serious oxidative stress, called ‘plant diabetes’ (Saito et al. 2011). In the high CO2 era, it will be necessary to enhance protection mechanisms against this type of stress. Distinction of respiratory characteristics of A. thaliana shoots grown at 390 ppmv and those grown at 780 ppmv CO2 was the focus of a study by Watanabe et al. (2014). Respiration was limited by either substrate supply or demand for ATP in ATP-utilising reactions. At high [CO2], especially towards the end of the dark period, the rate of respiratory O2 uptake was limited by the demand for ATP, suggesting that respiration at high [CO2] tends to be limited by some anabolic processes. In this context, the alternative respiratory pathway may contribute to balancing the NAD(P)H/ATP ratio (Gandin et al. 2013). Duan et al. (2014) examined the impact of high [CO2] on sugar translocation systems in A. thaliana plants, such as abundance of plasmodesmata, and gene expression levels of the sucrose effluxer SWEET12 and sucrose transporters SUC2 and SUT4. They also examined the rates of assimilation and translocation using a 14CO2 tracer. The sugar translocation system changed considerably in response to high [CO2], with differences being observed between the first and third rosette leaves. These results highlight the plastic nature of the translocation system. In 1998, a team led by Kobayashi and Okada initiated the FACE project for paddy field rice (Kobayashi et al. 2006), and the low-cost light-weighted FACE facility operating in a pure CO2 injection mode developed by this team (Okada et al. 2001) has now been adopted in most FACE experiments using herbaceous plants. This team has been systematically producing data (e.g. Hasegawa et al. 2013) that will form a fundamental basis for sustainable rice production in the high CO2 era. Adachi et al. (2014) report the combined effects of high [CO2] and an increase in soil and water temperature by 2°C on rice production. High soil and water temperatures under high [CO2] accelerated downregulation or aging of the leaf photosynthetic capacity. In a different study, Chen et al. (2014) compared leaf photosynthetic properties between ambient and elevated [CO2] levels in a high-yielding indica variety (Takanari) and a popular japonica variety (Koshihikari). Takanari showed greater leaf photosynthesis and stomatal conductance as well as increased mesophyll conductance at both [CO2] levels. It is worth noting that leaf N on an area basis was not suppressed by high [CO2] in Takanari. These results indicate that Takanari will be an invaluable resource for rice breeding in the high CO2 era. Within this century, we will soon face a high CO2 era. It is highly likely that the majority of plants, which are adapted to an atmospheric [CO2] of below 300 ppmv, will suffer various detrimental effects. Thus, it is vital to continue research into this field, namely through large-scale growth experiments as well as precise molecular physiological studies, to clarify the respective mechanisms responsible for the various downregulations that occur under high CO2 conditions. Information gleaned from such studies could be used to optimize plant performance under various environmental conditions in a tailor-made manner, and generate valuable materials for breeding. To this aim, effective collaboration between ecologists, agronomists and molecular biologists is indispensable. Without these concerted efforts, we will not be able to increase biomass production, feed the 10 billion people estimated to populate the planet (Evans 1998), or maintain ecosystems that efficiently fix CO2 in the high CO2 era. This work and the studies compiled in this special issue were supported by a Grant-in-Aid for Scientific Research on Innovative Areas for “Comprehensive studies of plant responses to the high CO2 world by an innovative consortium of ecologists and molecular biologists.” We thank professor Tomoyuki Yamaya, Editor-in-Chief of Plant and Cell Physiology, for providing us with the opportunity to showcase some of our work in this Special Focus Issue on Plant Response to CO2.