MicroRNAs coordinate immune response with other agriculturally
important traits
MicroRNAs rarely work in independent manner. Single miRNA often
regulates more than one trait. Recent studies on miRNA in rice listed in
Table 2 showed that role of miRNA is not limited to the regulation of
defense response mechanisms, but they are also involved in regulation of
complex traits (Figure 3). For example, the Osa-miR156 regulates
11 SPL genes that are involved in diverse biological and developmental
process in rice. The Ideal Plant Architecture 1 , which encodes aSQUAMOSA Promoter-binding protein-Like transcription factors is
known to be targeted by Osa-miR156 . Overexpression ofOsSPL14 resulted in enhanced resistance to bacterial blight
accompanied by a substantial improvement in yield mediated by the
reduction of unproductive tillers and enhanced panicle branching (Liuet al. 2019). Mutation in OsSPL14 perturbsOsa-miR156 -mediated regulation of OsSPL14 leading to high
yield maintenance as indicated by the reduction of unproductive tillers
and stronger culm (Jiao et al. 2010). Furthermore, overexpression
of OsSPL14 also enhanced grain productivity by increasing panicle
branching (Miura et al. 2010).
Recently, downregulation of Osa-miR156fhl-3p has been shown to
enhance rice immunity against M. oryzae by increasing the
expression of OsSPL14 and WRKY45 (Zhang et al.2020b). It has also been shown that Panicle blast 1 (Pb1 ),
a panicle blast resistance gene encoding a coiled-coil,
nucleotide-binding site, leucine-rich repeat (CC-NBS-LRR) protein
interacted with WRKY45, a transcription factor involved in induced
resistance through the salicylic acid signalling pathway regulated by
the ubiquitin proteasome system (Inoue et al. 2013). Suppression
of Osa-miR156 promotes seed dormancy by inducing OsSPL14gene and repressing GA pathway (Miao et al. 2019). Expression ofOsSPL13 promotes yield in rice by improving grain size and
panicle length (Si et al. 2016). The OsSPL16 improves
grain size by binding to GW7 (Wang et al. 2012, 2015). TheOsSPL18 binds to the promoter of DEP1 and works negatively
in the regulation of grain number (Huang et al. 2009; Yuanet al. 2019). Overexpression of OsSPL7 negatively affects
tiller number and positively affects plant height (Dai et al.2018). The OsSPL9 involved in the transcriptional regulation ofOsa-miR528 , which promotes rice flowering under long-day
conditions by repressing Red and Far-red Insensitive 2(OsRFI2 ) gene (Yang et al. 2019). The OsSPL9mediated transcriptional regulation of Osa-miR528/L-Ascorbate
Oxidase (AO) module enhances antiviral defense in rice (Yao et
al. 2019). Moreover, Osa-miR528 regulates pollen intine
formation by targeting the uclacyanin gene OsUCL23 (Zhanget al. 2020c). Constitutive expression of Osa-miR528enhances salinity and tolerance to nitrogen-starvation in creeping
bentgrass (Yuan et al. 2015).
Plant hormones play fundamental roles in regulating the trade-offs
between growth and defense mechanisms (Huot et al. 2014). Theauxin response factors (ARFs) genes that are involved in auxin
signalling are targeted by Osa-miR160 and Osa-miR167 . TheOsa-miR160 positively regulates resistance to blast whereasOsa-miR167 acts in antagonistic fashion. In rice, the alteration
in auxin signalling through the upregulation ofOsa-miR160 -resistant OsARF18 has led to severe defects in
growth and reproductive development, seed size and seed-set (Huang, Li
& Zhao 2016; Wang, Wu, Fang, Chu & Wang 2017). The Osa-miR167promotes auxin response, tiller number and grain weight in rice (Yang,
Han, Yoon & Lee 2006; Liu et al. 2012b; Peng et al.2018). The growth regulating factors (GRFs ) targeted byOsa-miR396 regulates diverse biological processes. TheOsa-miR396-OsGRFs module substantially increases the grain size
and yield due to mutation in OsGRF4 (GS2 ) that perturbOsa-miR396 -mediated regulation of OsGRF4 (Che et
al. 2015; Duan et al. 2015; Li et al. 2016).
Additionally, Osa-miR396 promotes panicle branching by regulatingOsGRF6 (Gao et al. 2015).
Several immune-responsive miRNAs play a central role in regulating
abiotic stress and nutrient uptake. For example, the Osa-miR164 ,
which modulates grain yield, and plant architecture in rice, has been
shown to affect drought tolerance (Fang, Xie & Xiong 2014; Jianget al. 2018). The Osa-miR166 controls nutrient ion uptake,
drought tolerance, and cadmium tolerance (Iwamoto & Tagiri 2016; Dinget al. 2018; Zhang et al. 2018a). The pathogen–responsiveOsa-miR169 has been shown to regulate nitrogen uptake and
salinity tolerance in rice (Zhao et al. 2009; Yu et al.2018). The Osa-miR319 regulates leaf morphogenesis, plant height,
and improves cold tolerance (Yang et al. 2013; Wang et al.2014; Liu et al. 2017). The Osa-miR398 not only modulates
panicle length, grain number, grain size but also regulates various
abiotic stresses (Lu, Feng, Bian, Xie & Liang 2011; Zhang et al.2017). The Osa-miR444 modulates MADS box transcription factors
to regulate tillering and nitrate signalling (Guo et al. 2013;
Yan, Wang, Hamera, Chen & Fang 2014; Jiao et al. 2020). The
RSV-responsive Osa-miR528 regulates arsenite tolerance and cold
tolerance in rice (Liu et al. 2015; Tang & Thompson 2019; Yanget al. 2019; Zhang et al. 2020c).