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