2.3 Elevated tropospheric O3 stress
Tropospheric O3 is a harmful secondary air pollutant that negatively affects crop productivity through both direct oxidative damage to plant cells, and through its role as a greenhouse gas and subsequent contribution to global warming (Ainsworth, 2017). Tropospheric O3 concentrations ([O3]) have more than doubled since pre-industrial times (Monks et al., 2015) and while there is spatial heterogeneity in global tropospheric [O3], O3 levels remain high in major agricultural growing regions during the crop-growing season (Ainsworth, 2017). It is estimated that on a global scale, O3 pollution has led to a 2-15% decrease in yield for crops such as wheat, rice, maize, and soybean (Avnery et al., 2011; Van Dingenen et al., 2009).
Plant uptake of O3 occurs through both stomatal and non-stomatal pathways (soil and cuticular deposition), with the primary form of O3 entry occurring through the leaf stomata (Ainsworth, 2017). Upon entry into the intercellular air space of leaves O3 reacts to produce reactive oxygen species (ROS), leading to cellular damage (Ahlfors et al., 2009; Grimes et al., 1983; Heath, 1987). Perception, signaling and detoxification of O3 and ROS has been well reviewed (Ainsworth et al., 2012; Ainsworth, 2017; Vainonen & Kangasjarvi, 2015), and includes altered redox balance, increases in cytosolic calcium, mitogen-activated protein kinase (MAPK) signaling cascades, and altered expression of genes involved in hormone and antioxidant metabolism, respiration, and photosynthesis (Ainsworth, 2017).
The physiological response of plant responses to O3 and their agronomic consequences have also been previously reviewed (for list see Ainsworth, 2017; Montes et al., 2022), and demonstrate chronic elevated [O3] decrease photosynthesis and stomatal conductance in C3 plants, and increase rates of respiration (Morgan et al., 2003; Ainsworth, 2008; Feng et al., 2008). Additionally, decreased photosynthetic rates are associated with a decrease in photosynthetic proteins, pigment, and nitrogen (N) content, and increased rates of respiration are associated with changes in leaf antioxidant balance (Dutilleul et al., 2003). These reductions in plant primary metabolism can lead to reduction in plant growth rates, leaf area and biomass accumulation (both aboveground and belowground) (Morgan et al., 2003; Ainsworth, 2008; Feng et al., 2008).
As with other abiotic stresses, the timing of O3 stress can play a role in the overall impact on plant productivity; O3 can directly impact reproductive development in plants (Black et al., 2000; Leisner & Ainsworth, 2012) and O3 exposure during reproductive development can lead to a greater reduction in photosynthesis than during vegetative development in some plants (Morgan et al., 2003; Ainsworth, 2008; Feng et al., 2008). Overall, O3 impacts plants at the community, whole plant, leaf, and cellular level and can lead to reductions in crop yield and overall biomass accumulation (Ainsworth et al., 2012). While there is significant within-species variation in O3tolerance in crops (Ainsworth, 2017; Booker et al., 2009), future work is needed to see successful gains in breeding and biotechnological approaches to improving resiliency to O3 in crops.
2.4 Elevated atmospheric CO2
Anthropogenic CO2 emissions have increased the atmospheric [CO2] since the beginning of the industrial revolution, with the global concentration increasing from 340 ppm in 1980 to 417 ppm in February of 2022 (GML-NOAA, 2022). If CO2 emissions are maintained, the Representative Concentration Pathway model (RCP) 8.5 predicts that atmospheric [CO2] could reach between 550-600 ppm by 2050 (IPCC, 2021). Therefore, it is important to understand the effects of elevated CO2 on crop physiology, yield and its interaction with other abiotic factors as well as biotic stresses such as crop diseases. The effects of elevated CO2 at the crop, plant and genetic level have been widely studied before (for review see Ainsworth & Rogers, 2007; Ainsworth & Long, 2020) but a better understanding of the interactive effects of CO2 with other abiotic and biotic stress is needed.
As a substrate of photosynthesis, elevated concentrations of atmospheric CO2 increase photosynthesis of C3 plants between 20 to 45% by saturating Rubisco and decreasing photorespiration (Ainsworth & Rogers, 2007; Leakey et al., 2009; Walker et al., 2016). Of these, C3 legume crops such as soybean, peanut, and peas (Pisum sativum ) have higher photosynthetic rates than other C3 crops due to the benefits of biological nitrogen fixation on plant N and sugar metabolic status (Ainsworth & Rogers, 2007; Sanz-Saez et al., 2015). In C4 plants, however, elevated CO2 does not increase photosynthesis directly as they already possess a CO2 concentration mechanism that saturates Rubisco and avoids photorespiration (Leakey et al., 2009). For that reason, C4 crops such as maize, sorghum and other C4 grasses such as Panicum coloratumonly show higher photosynthetic rates when grown under elevated CO2 when also exposed to drought stress. This is a result of reduced transpiration that allows the crops to save more water in the soil and maintain higher photosynthetic rates during drought (Leakey et al., 2009).
The major function of stomata is to maximize CO2fixation while minimizing water loss. As atmospheric [CO2] increase, the intercellular CO2 concentration in C3 and C4 plants increases as well, and therefore plants respond by decreasing both stomatal aperture and stomatal density, resulting in lower water loss at the leaf level (Leakey et al., 2009; Ainsworth & Long, 2020). This decrease in stomatal conductance translates to a decrease in canopy evapotranspiration that results in increased soil moisture content (Leakey et al., 2009). In some cases, however, elevated [CO2] stimulate leaf growth and canopy expansion and therefore canopy transpiration is increased (Gray et al., 2016; Parvin et al., 2019). These factors need to be taken in account when studying the interaction with diseases as a reduced stomatal conductance and number could limit the entrance of leaf pathogens and reduce disease severity (Eastburn et al., 2011).
Under elevated [CO2], plants tend to produce more sugars due to increased photosynthetic rates (Ainsworth et al., 2004). Feedback inhibition occurs if plants are not able to distribute those sugars from the sources (leaves) to the sink organs (roots, flowers developing seeds) at the same pace that sugars are assimilated, which in turn, inhibits the expression of photosynthetic genes and limits photosynthesis (Ainsworth et al., 2004). This photosynthetic limitation at elevated [CO2] produced by a sink limitation is more common in crops that produce more leaves (sources) and not as many seeds (sinks), as reproductive organs are stronger carbon (C) sinks. This has been demonstrated in wheat, where cultivars with higher harvest index (produce more seeds) were more responsive to elevated [CO2] than cultivars that produced more vegetative biomass (Aranjuelo et al., 2013). Additionally, this increase in carbohydrate content is followed by a reduction in the assimilation of N (Rubio-Asensio & Bloom, 2017; Bloom et al., 2020; Adavi & Shathee, 2021) that, combined, results in a dilution of the N content in all plant tissues and an increase in the C/N ratio that could affect disease growth (Ainsworth & Long, 2004).
Due to a reduction of the transpiration stream and the dilution effect produced by an increase in carbohydrate concentration in seeds and leaves, mineral concentrations tend to decrease at elevated [CO2] (McGrath & Lobell, 2013; Myers et al., 2014; Ebi & Loladze, 2019; Loladze et al., 2019; Ebi et al., 2021). The decrease of micronutrients like iron (Fe), zinc (Zn), and selenium (Se) is significant, as they are essential nutrients for human nutrition and its deficiency in diets affects more than 2 billion people in the world (Ebi & Loladze, 2019; Loladze et al., 2019; Ebi et al., 2021). This decrease in nutrient concentration due to a reduced transpiration at elevated [CO2] can be significant for other nutrients that are important for the integrity of membranes and cell walls such as silicone, calcium (Ca) and boron (B), as it could facilitate the infection of some pathogens.
The stimulation of photosynthesis at elevated [CO2] usually results in biomass and yield increases in C3plants (Bishop et al., 2014; Sanz-Saez et al., 2017; Hu et al., 2022). The magnitude of the positive effect depends on the crop species and the interactions between biotic and abiotic factors (Ainsworth & Long, 2020). However, genotypic variation in the biomass and yield response to elevated [CO2] has been found in several crops under controlled and open environments (Aranjuelo et al., 2013; Bishop et al., 2014; Sanz-Saez et al., 2017). Additionally, under elevated [CO2], leaf area is stimulated, and the canopy closes earlier in the season and is denser than at ambient [CO2] (Srinivasan et al., 2017; Sanz-Saez et al., 2017). A canopy that closes earlier in the season and is denser could produce a more humid microclimate that favors the appearance of diseases.
CROSSTALK BETWEEN ABIOTIC AND BIOTIC STRESS
Plants respond to individual biotic or abiotic stresses or simultaneous challenge by biotic and abiotic stresses in a complex and unique manner at the physiological, transcriptional, and cellular levels (Suzuki et al., 2014; Zhang et al., 2022). Common responses of plants to abiotic stresses are stomatal closure, reduced photosynthesis, increased ROS scavenging activity, reduced leaf growth, and increased root length, as described above. In response to biotic stress, plants respond to challenges by bacterial, fungal, and viral pathogens or nematodes in different ways depending on the biotrophic or necrotrophic lifestyle of pathogens. Some commonly observed responses to these biotic stresses include stomata closure, reduced photosynthesis, production of ROS, phytoalexin production, and local cell death. Overlap in the regulatory networks that control plant responses to both abiotic stress, pathogen recognition and defense therefore include ROS signaling, expression of plant hormones, changes in redox status and ion flux, and changes in cell wall integrity (Kissoudis et al., 2014; Rivero et al., 2022), indicating crosstalk and convergence of mechanisms to combat general stress (Walley et al., 2007; Atkinson & Urwin et al., 2012; Kissoudis et al., 2014).
With climate change comes the higher likelihood that plants might be challenged with simultaneous stressors in agricultural fields and accompanied variable levels of pathogen pressure, as changes in the growing environment (i.e., temperature, water availability) can affect plant disease epidemics and other plant-microbe interactions (Rivero et al., 2022). These combined stresses have the potential to pose an even greater threat to global food security and climate resilience than each stress alone (Rivero et al., 2022). Previous work has shown that susceptibility to hemibiotrophic or necrotrophic pathogens is increased under abiotic stress, while susceptibility to biotrophic pathogens is reduced when combined with abiotic stress (Saijo & Loo, 2019). Therefore, to engineer a more sustainable future food supply we need to understand how biotic and abiotic stress combinations act in combination. This includes understanding stress signaling crosstalk in plant signaling, gene expression, metabolism, and development. Below we summarize our current knowledge of crosstalk in signaling and plant metabolic pathways that occurs when plants are exposed to combined abiotic and biotic stresses. In the following section we outline trade-offs between plant responses to combined biotic and abiotic stress conditions with targets for future research efforts.