Abstract
Phosphate (Pi) and jasmonic acid (JA) play critical roles in plant growth and development. In particular, crosstalk between JA and Pi starvation signaling has been reported to mediate insect herbivory resistance in dicot plants. However, its roles and mechanism in monocot-bacterial defense systems remain obscure. Here, we report that Pi starvation in rice activates the JA signaling and enhances resistance to Xanthomonas oryzae pv. oryzae(Xoo ) infection. The direct regulation of OsPHR2 on theOsMYC2 promoter was confirmed by yeast one-hybrid, electrophoretic mobility shift, dual-luciferase, and chromatin immunoprecipitation assays. Molecular analyses and infection studies using OsPHR2-Ov1 and phr2 mutants further demonstrated that OsPHR2 enhances JA response and antibacterial resistance via transcriptional regulation of OsMYC2 expression, indicating a positive role of OsPHR2-OsMYC2 crosstalk in modulating the JA response and Xoo infection. Genetic analysis and infection assays using myc2 mutants revealed that Pi starvation-induced JA signaling activation and consequent Xoo resistance depends on the regulation of OsMYC2 . Together, these results reveal a clear interlink between Pi starvation signaling and the JA signaling in monocot plants, and provide new insight into how plants balance growth and defense by integrating nutrient deficiency and phytohormone signaling.
Introduction
Phosphorus is an essential macronutrient and indispensable element for plant growth and development in both natural and agricultural ecosystems (Conget al. , 2020). The plant-accessible form of inorganic phosphate (Pi) is highly insoluble in soil and thus Pi starvation is one of the most common nutrient deficiencies, resulting in loss of plant productivity (Raghothama & Karthikeyan, 1999; López-Bucio et al. , 2003; Neumann & RÖmheld, 2012). Plants respond to Pi starvation though reduction of primary root growth, formation of additional lateral roots and root hairs, replacement of phospholipids by sulfolipids and galactolipids, release and uptake of phosphatases from organic sources, increased expression of Pi transporter genes, and accumulation of starch and anthocyanins (Yuan & Dong, 2008; He et al. , 2021). In recent decades, considerable progress has been made regarding the components of the Pi signaling pathway that drives these responses (Franco-Zorrillaet al. , 2004; Wu et al. , 2013; Crombez et al. , 2019). PHOSPHATE STARVATION RESPONSE proteins (PHRs) are the key transcription factors governing Pi starvation response, which they do through binding to a cis -element PHR1 binding sequence (P1BS, sequence GNATATNC) in the promoters of Pi starvation-induced (PSI) genes (Rubio et al. , 2001; Zhou et al. , 2008; Bustos et al. , 2010; Ruan et al. , 2016). The SPX protein family, which is named after syg1 (suppressor of yeast gpa1), Pho81 (the yeast cyclin-dependent kinase inhibitor), and XPR1 (the human xenotropic and polytropic retrovirus receptor 1), negatively regulates Pi signaling through interacting with PHRs and suppressing their transcriptional activities (Lv et al. , 2014; Puga et al. , 2014; Wanget al. , 2014; Wild et al. , 2016; Ruan et al. , 2017; Zhong et al. , 2018; Ruan et al. , 2019). In addition, phytohormones have been reported to be involved in the adaptation of plants to Pi starvation signaling under biotic and abiotic stress (Niuet al. , 2013; Baek et al. , 2017). In Arabidopsis(Arabidopsis thaliana ), Pi deficiency activates jasmonic acid (JA) signaling and enhances herbivory resistance (Khan et al. , 2016). However, the functional and regulatory mechanisms how Pi starvation activates the JA pathway, especially in monocotyledonous plants such as rice, remain to be elucidated.
JA and its derivatives are lipid-derived hormones that regulate plant growth and development, along with defenses against pests and pathogen infections (Kramell et al. , 2009; Aurélie et al. , 2010; Wasternack & Hause, 2013; Vidhyasekaran, 2015; Chini et al. , 2016; Rohit et al. , 2016; Howe et al. , 2018). The main enzymes involved in JA biosynthesis are lipoxygenase (LOX), allene oxide synthase (AOS) and allene oxide cyclase (AOC) in plastids, and OPDA reductase in peroxisomes (Wasternack & Hause, 2013; Ruan, J et al. , 2019). MYC2, a bHLH transcription factor, serves as the key regulatory hub of JA signaling (Kazan & Manners, 2013). When JA levels are low, the transcription activity of MYC2 is suppressed by JASMONATE-ZIM DOMAIN (JAZ) proteins together with NOVEL INTERACTOR OF JAZ (NINJA) and TOPLESS (Chini et al. , 2007; Chini et al. , 2009; Pauwels et al. , 2010). When JA is present, high levels of JA-Ile lead to SCFCOI1-dependent ubiquitination and degradation of JAZ proteins through the 26S proteasome, which in turn activates the expression of MYC2-regulated JA-responsive genes (Chiniet al. , 2007; Thines et al. , 2007; Howe et al. , 2018). MYC2 additionally links JA and other signaling pathways such as those associated with other phytohormones, light, secondary metabolism, and circadian signaling (Kazan & Manners, 2011; Hong et al. , 2012; Kazan & Manners, 2013). MYC2 also mediates the JA-dependent defense against herbivory and pathogen infection (Lorenzo et al. , 2004; Dombrecht et al. , 2007; Zhai et al. , 2013; Vidhyasekaran, 2015; Uji et al. , 2016; Du et al. , 2017). In rice, MYC2 has been reported to be involved in resistance againstXanthomonas oryzae pv. oryzae (Xoo ), the causal agent of rice bacterial blight, devastating rice diseases worldwide (Taoet al. , 2009; Uji et al. , 2016). Together with the documented influence of Pi starvation on the JA pathway inArabidopsis , these findings indicate a role for Pi starvation in the JA-Xoo interaction in rice.
In this work, we demonstrated that OsPHR2, the rice homolog of AtPHR1, bound to the OsMYC2 promoter, thus activated the expression of OsMYC2 and promoted downstream MeJA production, thereby enhanced the rice anti-Xoo defense. Together, our findings revealed that OsPHR2 modulates JA-induced resistance to bacterial blight during Pi starvation via transcriptionally regulating expression of OsMYC2 .
Materials and methods