PlantACT! - How to tackle the climate crisis

Greenhouse gas (GHG) emissions have created a global climate crisis which requires immediate interventions to mitigate the negative effects on all aspects of life on this planet. As current agriculture and land use contributes to up to 25 % of total GHG emissions, plant scientists are at center stage to find possible solutions to a transition to sustainable agriculture and land use. In this article, the PlantACT! initiative of plant scientists lays out a road map in which areas and how plant scientists can contribute to find immediate, mid-term and long-term solutions and what changes are necessary in the way to work out these solutions at the personal, institutional and funding level.


Abstract
Greenhouse gas (GHG) emissions have created a global climate crisis which requires immediate interventions to mitigate the negative effects on all aspects of life on this planet.
As current agriculture and land use contributes to up to 25 % of total GHG emissions, plant scientists are at center stage to find possible solutions to a transition to sustainable agriculture and land use. In this article, the PlantACT! initiative of plant scientists lays out a road map in which areas and how plant scientists can contribute to find immediate, midterm and long-term solutions and what changes are necessary in the way to work out these solutions at the personal, institutional and funding level.

I. The climate emergency
Humanity is facing an unprecedented challenge from climate change [1]. The CO2 concentration in the atmosphere has dramatically increased from 280 ppm (pre-industrial) to 420 ppm within 150 years. As a consequence, the global average temperature has increased by 1.5°C. This anthropogenic climate change is associated with altered rainfall patterns, extreme weather events and less predictable weather patterns. This presents a major challenge to crop production and food security and thus threatens the foundations of human civilization.
The International Panel on Climate Change (IPCC) had set the goal of limiting global warming to less than 1.5°C. [2] Although the goal of 1.5°C is probably not possible any more, achieving climate neutrality is more important than ever, by reducing net CO2 emissions to zero through a 45% reduction in emissions within 10 years [1]. This represents a disruptive goal which demands new thinking, solutions and commitments.
The atmospheric temperature increase caused by rising carbon dioxide concentrations will not decrease significantly even after zero carbon emissions (peak carbon) have been achieved [3]. The climate effects of atmospheric CO2 at peak carbon will remain irreversible for at least 1,000 years, if not counteracted by a net reduction in atmospheric CO2. In reality, anthropogenic climate change is irreversible over the next 10 generations at least, unless rapid measures are taken to sequester carbon dioxide from the atmosphere [3].
The global carbon cycle describes the dynamic cycling of carbon between the atmosphere and marine as well as terrestrial ecosystems ( Figure 1). Overall, terrestrial and aquatic net primary production is in the range of 130 Gt C per year. The vast majority of this assimilated carbon is returned to the atmospheric CO2 pool via respiration. Hence, the natural global carbon cycle (not considering anthropogenic emissions) is nearly balanced [4]. However, human activities perturb the global carbon cycle, leading to a continuous increase of atmospheric CO2 concentration. Net anthropogenic annual carbon emissions are leading to an estimated 5.2 Gt C increase in atmospheric CO2 in 2022 [4] [ Figure 1). All paths towards the 1.5°C goal depend on a rapid reduction of the carbon footprint of agriculture, forestry and land use, combined with the use of bioenergy with carbon capture and storage [5][6][7].

II. Agriculture as a Contributor to Climate Change
Agriculture is both a victim and culprit of global climate change as 20-25% of GHGs are released through agricultural activities. Apart from CO2, significant amounts of methane and nitrous oxide are emitted from agriculture which represent more potent greenhouse gases than CO2 (>30 and 300 times respectively). Methane is produced by rice paddy fields, livestock (via enteric fermentation and manure) and organic waste in landfills [8].
Nitrous oxide emissions are an indirect product of organic and mineral nitrogen fertilizer use. However, both gases have a shorter lifespan than CO2: methane and N2O remain in the atmosphere for 12 and 114 years compared to 300-1,000 years for CO2 [9]. Hence, unlike CO2, reductions in both of these other greenhouse gases would deliver rapid benefits (Box 1).
The N fertilizer supply chain currently contributes >2% greenhouse gas (GHG) emissions [10]. Global use of synthetic N fertilizers is predicted to increase 50% by 2050 [11]. When N fertilizers are applied, significant amounts of N20 are generated through microbial conversion in the soil [10]. In the short term, the most effective strategy is reducing the amount of N applied [12] to avoid over-fertilization through improvements in agronomy, extension advice and management practices. In the short to medium term, a switch to agro-systems utilizing legume crops able to naturally fix nitrogen represents an urgent priority [13]. In the medium to longer term, improvements in nitrogen use efficiency in cereal crops (currently <50%) through breeding for key traits such as root architecture would also provide major gains but might also carry the danger of inducing rebound effects [14]. These plant-based solutions are not reliant on major scientific breakthroughs but exploit existing knowledge that collectively act to reduce fertilizer-related production, usage and emissions.
The majority of CO2 generated by agriculture arises from changes in land use, particularly deforestation for fodder and grazing [15]. Livestock and fodder production each generate more than 3 billion tons of CO2 equivalent. Changes in food and dietary choice will help to reduce GHG emissions [16]. For example, currently 10-30 kg plant proteins are required to produce 1 kg of beef. Increasingly shifting away from animal to alternative protein sources would provide major benefits [17]. In the short term, reducing demand for soya-based animal feed would have major benefits through decreased land conversion [18]. In the mid to long term, adopting plant-based diets remains an efficient option. Plant scientists could contribute to the development of alternative plant-based protein sources by working with food and social scientists.
Given the central importance of food, a reduction of greenhouse-gas emissions from agriculture is a major challenge and will require the implementation of a range of techniques and tools, from capturing or reducing methane emissions at the source, more efficient use of fertilizers, and improved efficiency in meat, dairy and cereal production.
Overall, these measures should be part of a circular agricultural system, integrating crop improvements, mixed crops, field rotations and social interactions with local farming communities.

III. Challenges for Future Global Food Production
Growing global populations, shifting dietary patterns towards greater meat consumption, and increased food waste at both the consumer and supply chain levels, are major factors impacting global food systems. It is unclear how an increase of 70-100% in food production to meet global demands can be achieved in either a sustainable or equitable manner. Given the widespread degradation of terrestrial systems, there is no major surplus of arable lands on which to cultivate new crops. Likewise, any further conversion of forests into agricultural land via deforestation threatens biodiversity, contributing a major source of CO2 emissions and further jeopardizes planetary health. To increase food production using current agricultural practices would require more chemical fertilizers and pesticides, with major negative environmental, climate and human health related impacts.
With most of the land suitable for agriculture already in use, fertile agricultural land is increasingly becoming the preserve of wealthy nations and/or industry, heightening economic disparities between the global North and South.
Plants require sunlight, nutrient rich soils and water for optimal growth. Although mildly higher temperatures can prolong the growing seasons in some regions, extreme temperatures inhibit crop growth and impact yields through decreased fertility.
Furthermore, changing weather patterns alter the timing of rainfall as well as the distribution of pests and diseases. To cope with these challenges, short term agronomic solutions include changing farming practices, such as rotating crops to match water availability and/or adjusting sowing dates to temperature and rainfall patterns (Table 1).
Plant scientists can also contribute by identifying microbes and plant traits for generating (in the medium to longer term) crop varieties (Table 1)  Moreover, breeding efforts should consider the need for more diverse and resilient agroecosystems and should benefit from local knowledge related to the adaptation and selection process. Crop varieties that meet these conditions could contribute to efficient adaptation strategies to cope with climate change. In this context, the PlantACT! initiative (Plants for climate ACTion!) will alert, engage and work on solutions to reduce agriculturebased GHG emissions and facilitate a more equitable and sustainable global food production system.

IV. Plants, soil and microbes as actors for mitigation
Soil was long considered solely as an inert growth substrate. If the chemical and physical properties were not suitable, herbicides, fertilizers and pesticides had to be added to soil to provide stable yields. This notion has now changed following recognition that besides the physical and chemical structure of soils, a diverse living community of soil organisms is essential for crop production. Soil microorganisms form beneficial symbiotic associations with plants and help plant roots in nutrient uptake and control of diseases.
Soil microorganisms also play a role in soil water and nutrient holding capacity and can contribute to mitigating climate change by maintaining or increasing soil carbon content.
In the future, holistic approaches of the soil-plant-microbe ecosystem must be considered to achieve sustainable solutions related to climate change [19][20]. In this context, agriculture is not the only target of this approach, but landscaping and land restoration of unused land could provide novel solutions to climate change (Box3). PlantACT! supports the idea that soil restoration could play a key role in improving agriculture and carbon capture as well as long term carbon sequestration.

V. Conclusions
Given the complexity of the effects of climate change at all levels of planetary life, it is highly unlikely that exclusive disciplinary thinking will provide solutions that will hold up to their promises. Current thinking needs to be readjusted both at the institutional, funding, as well as subject levels to enable multidisciplinary scientific approaches. The presentday scientific culture of exclusive scientific exchange in specific fields needs to be broken down and new forms of interdisciplinary conferences and communication need to be established (e.g. ideas labs, workshops, grass root level proposals that compete with each other for prizes). Information access to farmers, scientists and decision makers via open access platforms is needed to find and evaluate different approaches and solutions.
Solutions must be fact-checked not only in terms of global carbon but also in terms of social and societal impact. The time constraints for proposed solutions (e.g. launching breeding programs for crop adaptations, introducing genes into elite crop varieties takes a decade) have to be considered and weighed against immediate solutions (e.g. changes in agricultural practices, ready microbe-induced crop resilience). Overall, one solution for all will not be possible. Solutions will need to be shaped and targeted differently to reflect geographical and local needs and contexts and will have to be continuously assessed for their impact. For example, solutions need to be targeted differently to the EU and US compared to Sub-Saharan Africa where population growth will be highest this century.
Moreover, land in many countries is limited, but less in Africa, where agriculture suffers from low yield and hence supporting intensification in a sustainable manner could have an immediate impact. Overall, if we want to preserve a livable planet, we must leave our well-trodden disciplinary paths and search for novel inter-disciplinary solutions and approaches. Moreover, not only national but overarching transnational funding programs need to be implemented to develop or adapt solutions to local specificities. PlantACT! aims to urgently accelerate these new inter-disciplinary interactions and solutions by stimulating new forms of working and funding (Box 4). Figure 1. Schematic representation of the terrestrial carbon cycle. Annual growth rate of atmospheric carbon pool (blue arrow) is the differential of emissions from fossil fuels (9.6 Gt C), land use change (1.2 Gt C) and uptake of carbon into terrestrial (3.1 Gt C) and oceanic (2.9 Gt C) carbon pools. Only land-based carbon fluxes are shown here. Data for carbon emissions from agriculture have been taken from FAO (Food and Agriculture Organization of the United Nations, https://www.fao.org/3/cb3808en/cb3808en.pdf). The FAO data includes greenhouse gases other than CO2, converted to CO2 equivalents. Adapted from [26] with data from [4] and [11] (created with BioRender.com).

Box 1. Reducing methane emissions from rice production.
Methane is the second most important greenhouse gas after CO2 and is >20 times more potent than CO2. Rice paddy fields emit 10g CH4/m 2 [21] and this forms 15-20% of anthropogenic methane emissions. Methane arises from the decomposition of organic matter in anoxic conditions by soil methanogenic archaea. Changes in agronomical practices are already available to significantly reduce methane production in rice agrosystems (short term solution). This includes water management practices such as alternate wetting and drying or aerobic rice that act to conserve water. However, transitioning from irrigated rice systems often leads to a yield penalty and greater interannual yield variability because of reduced access to water, weed competition and changes in nutrient availability [22]. To tackle this, plant scientists (working together with agronomist, hydrologist, microbial ecologist and agro-socio-economists) could contribute by developing (medium term) solutions that include new crop varieties for water-saving and low methane rice agrosystems. Traits include early vigor to deal with weed competition and root traits to improve water and nutrient acquisition in aerobic conditions [23] but also the use of perennial rice varieties.

Box 2. Enhancing water use efficiency and carbon capture
Carbon gain in photosynthesis is a water consuming process as fixing one molecule of CO2 requires hundreds of molecules of H2O lost by transpiration. However, there is substantial natural variation of water use efficiency (WUE) among plant species, and this holds great potential to improve this trait in crops. Improved WUE can be achieved by using microbes collected from plants able to cope with extremely low water availability and contributing to this phenotype (short-term solution) and by breeding water-saving crops (mid-term solution). Reducing water loss by narrowed stomatal aperture can lead to decreased CO2 concentration inside the leaves and hence increased photorespiration, in particular at higher temperatures. To avoid a possible penalty on growth in WUE crops, carbon capture efficiency could be improved. Potential advantages of C4 plants (and/or C3-C4 intermediates) can perform photosynthesis at lower stomatal aperture. Examples from breeding for WUE has pointed to genes involved in stomatal patterning, abscisic acid homoeostasis and CO2 signaling [24].

Box 3. Building up soil inorganic carbon (SIC) in arid regions
Soil organic carbon (SOC) represents a major form of terrestrial C storage (Figure 1). The importance of SIC is less appreciated. Oxalogenic plants that secrete oxalate and associate with microbes in the soil show great promise for capturing CO2 in an inorganic form that is highly stable. Fungi and bacteria associated to these plants (called oxalotrophs) can use oxalate as their sole carbon and energy sources. In a soil that is rich in Ca 2+ or Mg 2+ , these microbes can produce Ca 2+ -or Mg 2+ -carbonates which thereby increase the soil inorganic carbon (SIC) content [25]. These natural CO2 trapping systems that are primarily found in arid and hyper-arid regions could provide novel and important C sequestration alternatives. Such systems do not compete with agricultural land and can fix carbon in the soil for decades to centuries.

Box 4. Re-designing the way plant-based climate solutions are funded
The time required to develop plant-based climate solutions is rapidly running out. One major challenge is the research grant funding systems currently operating in many countries which impose delays of up to 12 months between submission of an idea to eventually starting a project. There is an urgent need to re-design and accelerate the way plant-based climate solutions are assessed, initially tested and then rolled out. New formats to catalyze trans-disciplinary research solutions are also urgently needed. The Belmont Forum (https://belmontforum.org) provides an example for how such a change can be designed, which involves funding organizations, international science councils, and regional consortia committed to International transdisciplinary research to provide knowledge for understanding, mitigating and adapting to global environmental change.
PlantACT! aims to urgently accelerate new trans-disciplinary interactions and solutions by stimulating new forms of working and funding. Colors indicate estimated timeframes to implementation: ■ short-term (within a decade), ■ mid-term (one to several decades), ■ long-term (centennial).