Phytohormone stimulate plants innate resistance. Salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) are responsible for primary defense against pathogen. Plant growth regulator such as auxin, brassinosteroid (BR), abscisic acid (ABA), gibberellin (GA) and cytokinin (CK) are critical regulator of plant microbe interactions and contribute to plant immunity. GA regulate plant growth and development such as seed germination, stem elongation and flower development (Yamaguchi and Kamiya 2000). Gibberellins was initially identified from a fungal pathogen Gibberella fujikuroi (teleomorph: Fusarium moniliformae) (Darken et al., 1959) which causes the foolish seedling, bakanae disease in rice (Rosales and Mew 1997). Disease symptoms are seedling elongation with slender leaves and a loss in rice yields. Later GA was found to be produced by plant. GA induce plant defense responses and increase plant resistance (Moosavi 2017).
Phytohormones modulate plant immune defences (Pieterse et al., 2012). Gibberellin is implicated in plant defense signaling pathway (Bari and Jones 2009). Wang et al. (2013) suggested GA3 increases the potential capacity of protein stability and cellular stress response. Pathogens effector alter hormone level by interfering with phytohormone signaling pathway, while others shift hormonal balance by producing hormones or hormone mimics (Han and Kahmann 2019). Plant defense responses are regulated by crosstalk within which plant hormone play a role by communicating and regulating signal transduction pathways. GA modulates plant disease resistance by inducing the degradation of DELLAs, a class of nuclear growth repressing protein that act as central suppressor of GA signaling (Navarro et al., 2008). Increase in cytokinin concentration decreases the biologically active gibberellin signaling (Brenner et al., 2005).
GA provides immunity to rice plant against Pythium graminicola (De Vleesschauwer et al., 2012). GA signaling is a critical regulator against biotrophic and necrotrophic pathogens. Navarro et al. (2008) propose that Gibberella might secrete GA as a virulence factor to degrade DELLAs protein and disable JA-mediated necrotroph resistance in plants, resulting in loss of DELLA-mediated growth restraint.
JA and ethylene dependent responses are initiated by necrotrophs and SA dependent response is activated by biotrophic pathogen (Glazebrook 2005). Jasmonate Zim Domain (JAZ) and DELLA are important nodes of convergence in hormone signaling network (Pieterse et al., 2012). ABA, auxin, GA and CK affect SA-JA backbone of the plant immune signaling network resulting in positive or negative effect on biotroph and necrotroph resistance. Ethylene, auxin and ABA antagonizes SA signaling but synergize JA signaling, whereas, GA antagonizes JA signaling but synergize SA signaling (Pieterse et al., 2012). Brassinosteroid (BR) interferes at multiple level with GA metabolism, resulting in indirect stabilization of the DELLA protein and GA repressor SLENDER RICE 1 (SLR1) (De Vleesschauwer et al., 2012).
The interaction between hormone signaling pathways enables plant to use available resource for either growth or defense. JA and GA plant hormone mediate defense and growth. Binding of bioactive JA or GA ligands to cognate receptors leads to proteasome-dependent degradation of specific transcriptional repressors (JAZ or DELLA family protein) which may repress transcription factor involved in defense (e.g. MYC) or growth [e.g. phytochrome interacting factors (PIFs)](Yang et al., 2012). DELLAs regulate GA homeostasis and represents a convergence point for other hormone-signaling (Daviere et al., 2008). GA control plant growth by regulating the degradation of growth-repressing DELLA proteins (Sun and Gubler 2004; Sun 2011). Navarro et al. (2008) observed DELLA promotes susceptibility to virulent biotrophs and resistance to necrotrophs by altering relative strength of salicylic acid (SA) and jasmonic acid (JA) signaling. GA promotes resistance to biotrophs and susceptibility to necrotrophs by degrading DELLA proteins and changing the balance of SA/JA signal. GA suppress the cellular competence to respond to JAs and shift the balance between JA and SA signaling resulting in enhanced SA signaling and biotroph resistance (Pieterse et al., 2012). DELLA proteins integrate plant responses to various hormonal signals and play a key role in plants capacity to maximize growth and protection.
De Vleesschauwer et al. (2016) observed DELLA protein SLR1 promotes resistance against hemibiotrophic rice pathogens. SLR1 amplifies and integrates the action of the rice SA and JA signaling pathways. While differing with the common assumption that DELLA protein promotes necrotrophic resistance by stimulating JA and antagonizing SA. Their observation highlight the importance of DELLA and GA in molding disease and resistance in different plant pathosystems.
SA mediated suppression of GA results in DELLA stability, which may lead to suppression of BR-mediated signaling (Huot et al., 2014). JA and GA signaling pathway can occur at multiple level and demonstrates the dynamic nature of JA-GA cross talk in regulating growth-defense. The crosstalk between BR and GA is mediated by interaction between GA-inhibiting DELLA proteins and the BR-regulated transcription factor BZR1. At the interface of development and defense signaling these proteins serve as central hub for pathway crosstalk and signal integration for plant growth and defense in response to various stimuli (De Bruyne et al., 2014). GAs like other defense plant hormone appear as multifaceted regulator of plant immunity, the effect of which may vary depending on plant species, type of pathogen involved and the specialized feature of each interaction.
See Part V for further information
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