Faculty Mentor: Heejin Yoo (School of Biological Sciences, University of Utah)

 

ABSTRACT

As stationary organisms, plants have developed robust immune mechanisms to counteract daily challenges from pathogen attacks. Alongside biotic stressors, abiotic factors such as light significantly influence plant immunity. Within dense canopies, light availability varies, resulting in competition for light not only among different plants but also within the same plant. Upper leaves and taller plants receive more red light, while lower-positioned leaves and smaller plants are exposed to light enriched in far-red wavelengths. This shift in light quality triggers shade avoidance syndrome (SAS)—a response characterized by morphological changes like shoot elongation to improve light capture. Previous studies have reported altered pathogen resistance in shade-grown plants, but immune responses vary among species, suggesting that the impact of light conditions on plant immunity is species-specific. Moreover, the molecular mechanisms governing this relationship remains unclear. In this study, we used pepper plants to investigate how light conditions affect plant immunity. We compared programmed cell death (PCD), bacterial growth, and salicylic acid (SA) levels in plants grown under sun versus shade conditions. While additional data is needed for certain parameters, our study showed that several pepper cultivars exhibit significantly stronger and more rapid PCD under shade conditions compared to sun conditions. Further research will help establish a clearer link between these phenotypic responses and the underlying immune mechanisms.

INTRODUCTION

Plants are stationary organisms and therefore heavily rely on environmental signals to regulate their growth and development. Among these signals, light is a key factor that influences essential processes such as flowering, dormancy, and shade responses (Smith, 2000).

Phytochromes and Shade Avoidance Response

Plants can perceive the light using photoreceptors. A key photoreceptor is phytochrome, which detects red and far-red light allowing plants to respond to changing light conditions. This gene family, including PhyA, PhyB, and PhyC, is widely conserved in flowering plants (Ruberti et al., 2012). Daylight contains more red light than far-red light, but the ratio of red to far-red light can change depending on the environment. Under natural sunlight, light distribution varies within a plant canopy. Upper leaves of the canopy receive more direct red light, while lower leaves experience a different light environment due to light absorption and reflection by the upper leaves. Taller plants absorb most of the red light for photosynthesis, causing far-red light to be more likely transmitted and reflected down to smaller plants. As a result, leaves positioned lower in the canopy receive a reduced red/far-red light ratio, signaling competition for light (Smith, 2000). Phytochromes help plants sense these changes through two forms: Pr (inactive form, absorbs red light) and Pfr (active form, absorbs far-red light). When the red/far- red ratio decreases, it triggers a phenomenon known as shade avoidance syndrome (SAS) (Moreno and Ballaré, 2014). SAS helps plants compete for light by promoting stem elongation, larger leaves, and altered leaf angles (De Wit et al., 2013; Moreno and Ballaré, 2014).

Plant Immune mechanism

Unlike animals, plants lack specialized immune cells and must rapidly switch from growth to defense modes upon pathogen detection (An and Mou, 2011). Their primary defense mechanism includes physical barriers such as waxy cuticles, trichomes, and cell walls (Iqbal et al., 2021). However, when pathogens infect plant cells, plants activate their immune system. The first layer of defense, called pattern-triggered immunity (PTI), allows plants to recognize conserved pathogen-associated molecule patterns (PAMPs) using pathogen recognition receptors (PRR) located on their cell surfaces. This recognition initiates defense responses such as cell wall thickening and stomatal closure to block pathogen entry (Iqbal et al., 2021). In response, pathogens have evolved effector proteins that are delivered into plant cells. To counteract this, plants activate effector-triggered immunity (ETI). ETI is a stronger immune response that occurs when a plant’s intracellular resistance (R) proteins recognize these pathogen effectors. This often leads to localized cell death (hypersensitive response) to block the pathogen spread, as well as the activation of defense hormones like salicylic acid (SA) (An and Mou, 2011; Hua, 2013). During ETI in infected local tissue, uninfected tissue develops a broader resistance known as systemic acquired resistance (SAR) to enhance immunity throughout the plant in anticipation of potential pathogen spread (An and Mou, 2011).

Does Shade Reduce Plant Immunity?

Several studies suggests that plants grown in shade may become more susceptible to pathogens, viruses, and fungi. For example, Arabidopsis plants grown under low red/far-red light conditions are more susceptible to Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea (Gommers et al., 2017; De Wit et al., 2013). Arabidopsis plants with mutations in the phytochrome B (phyB), a major SAS regulator, have also shown increased susceptibility (De Wit et al., 2013). In addition, Arabidopsis plants grown in shade also produce reduced levels of glucosinolates and camalexin, two important defense-related metabolites (Cargnel et al., 2014). Similar trends have been observed in soybean and alfalfa, where plants grown under low light conditions were more susceptible to infection by Sclerotinia sclerotiorum and Verticillium alboatrum (Roberts and Paul, 2006). In cucumber, far-red light enhanced susceptibility to powdery mildew, while red light suppressed it (Roberts and Paul, 2006). Nicotiana tabacum mutants lacking phytochromes A and B (phyAphyB), which mimic shade conditions, showed increased necrosis following infection with Chilli veinal mottle virus compared to wild-type plants (Fei et al, 2019). Another study found that pepper plants grown under red light showed reduced susceptibility to Phytophthora capsica (Yang et al, 2023). These examples show how shade-grown plants can exhibit increased vulnerability to a range of pathogens, viruses, and fungal parasites. However, not all species respond to shade in the same way. For example, Geranium robertianum exhibit enhanced immunity under shaded conditions (Gommers et al., 2017). Similar observations have been reported in coffee plants infected with coffee rust and oak infected with powdery mildew (Roberts and Paul, 2006). These findings suggest that plant species have evolved distinct strategies to balance growth and defense mechanisms under varying light environments.

Shade, Herbivory, and Defense Hormones

Shade may also affect how plants defend themselves against herbivores. It is thought that plants grown in shaded conditions may be more susceptible to herbivory because shaded plants tend to have thinner leaves and fewer trichomes, making them easier for herbivores to eat. However, some studies show no increase in herbivory under shade (McGuire and Agarwal, 2005; Ballaré, 2014). For example, shade-grown cucumber plants had similar or lower levels of insect damage compared to sun-grown plants, possibly because their leaves were more nutritious, requiring herbivores to eat less (McGuire and Agarwal, 2005). On the other hand, Nicotiana longiflora plants with phyAphyB mutations showed increased susceptibility to herbivory. In this case, it was found that far-red light inhibited the expression and accumulation of phenolic defense compounds against herbivory (Izaquirre et al, 2006). These findings suggest that the link between shade and herbivory varies across species and may depend on species-specific defense strategies.

There are several plant defense hormones, among which jasmonic acid (JA) and SA are two of the most well-studied. JA helps protect against herbivores and fungal pathogens, while SA is important for bacterial and viral resistance (Moreno and Ballaré, 2014). Studies show that shade-grown plants have lower levels of both JA and SA, making them more vulnerable to infections (De Wit et al., 2013; Yang et al., 2023). For example, Arabidopsis phyAphyB mutants exhibit weaker SAR and lower SA signaling (Griebel and Zeier, 2008; Genoud et al., 2002). Similarly, Nicotiana tabacum phyAphyB mutants show reduced JA and SA levels and are more susceptible to Chilli veinal mottle virus (Fei et al., 2019). In peppers (Capsicum annuum), red light has been shown to enhance immunity against Phytophthora capsici by increasing SA levels through the activation of key defense genes (Yang et al., 2023).

Practical Implications for Agriculture

Although shade is often linked to weaker immunity in plants, many farmers use shade netting to improve crop quality and yield. Shade nets provide a cost-effective alternative to greenhouses and have been shown to increase fresh fruit yield in tomatoes and peppers (Masabni et al. 2016, Kittas et al., 2012, Rhoden et al., 1997). Shaded coffee plants also show reduced disease severity, likely because shade changes the microclimate surrounding the plants in a way that disrupts pathogen activity (Mouen Bedimo et al. 2008). In some cases, growing crops under controlled shade may even enhance immunity, depending on the crop species and the light quality provided. Understanding how light conditions affect plant immunity could help farmers balance shading practices to optimize both plant growth and disease resistance.

Objective

This study aims to investigate how light conditions influence plant immunity in Capsicum annuum by measuring programmed cell death (PCD) rates, bacterial growth levels, and SA accumulation in sun- and shade-grown plants.

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