MicroRNAs fine-tune immunity and yield trade-offs
Developing high yielding varieties with resistance to biotic stress is
one of the current major goals of plant breeding (Eichmann & Schäfer
2015). While this is paramount to sustainable crop production, it is
also well known that strong defence response against pathogens often
comes with unexpected trade-offs in terms of significant losses of yield
(Ning et al. 2017). Modern plant breeding approaches such as
marker-assisted selection, transgenics, and genome-editing are being
applied to breed resistant cultivars with minimal penalty to yield. In
the past few decades, many R-genes in rice have been tagged with tightly
linked molecular markers. However, a limited set of genes are deployed
in crop improvement programs due to the fact that many of them exhibit
pleiotropic effects, unwanted linkage drags that undermine agronomic
traits, and low heritability (Tang & Chu 2017). Furthermore, because of
the rapid breakdown of the R-genes and associated fitness cost, miRNAs
controlling rice immunity has attracted more attention as a means to
develop resistant cultivars while minimizing the negative trade-offs to
plant growth and yield.
Recent reports have shown that many miRNAs facilitate the maintenance of
fitness and resistance without compromising yield in rice. For instance,
the Osa-miR156 , Osa-miR162 , Osa-miR396 , andOsa-miR1873 have been found to regulate defense-yield trade-off
in rice (Chandran et al., 2019; Zhou et al., 2019b; Li et al., 2020;
Zhang et al., 2020b). The Osa-miR156 negatively regulates
immunity against the rice blast, bacterial blight, and BPH by targeting
the IPA1 (Ideal Plant Architecture 1) , OsSPLs
(SQUAMOSA Promoter-binding protein-Like transcription factors) and
several OsWRKY genes. Sequestering Osa-miR156 by target
mimicry method exhibited enhanced resistance to blast, BB and BPH
through accumulation of target genes and defence-related genes (Geet al. 2018; Liu et al. 2019; Zhang et al. 2020b).
However, downregulation of Osa-miR156 proved to cause significant
reduction in plant height, tiller number, and delayed seed germination.
Inducible expression of IPA1 gene under control of bacterial
effector-induced promoter displayed enhanced resistance to BB, with
concomitant improvement in grain size, plant architecture and grain
yield (Liu et al. 2019). The Osa-miR162 synergizes the
mechanisms involved in resistance to blast with the genetic potential
for yield through the OsDCL1 that causes the accumulation of
intracellular H2O2. Nevertheless,
overexpression of Osa-miR162 showed significantly narrower
grains, lower seed weight and poor seed set, leading to negative effects
on grain yield. Silencing of Osa-miR162 showed positive effects
on yield by increasing the number of grains per panicle during M.
oryzae infection (Li et al. 2020).
Suppression of Osa-miR396 induces multiple growth
regulating factors (GRF) genes that leads to the enhancement of
blast and BPH resistance (Chandran et al. 2019; Dai et al.2019) 2019). The Osa-miR396 also regulates grain size, grain
yield, inflorescence development, panicle branching, salinity, and
alkali tolerance (Duan et al. 2015; Gao et al. 2015; Zhanget al. 2020a). The Osa-miR1873 has been shown to balance
blast resistance and plant growth in rice. Overexpression ofOsa-miR1873 compromises resistance and negatively impacts yield
potential by lowering seed-set and by reducing the filled grain per
panicle. In contrast, sequestering Osa-miR1873 was shown to cause
no significant effects in yield (Zhou et al. 2019b). Apart from
these miRNA, Osa-miR159 , Osa-miR160 , Osa-miR164 ,Osa-miR167 , and Osa-miR398 have been shown to affect yield
and component traits, independently. However, further studies might
reveal the modulation of yield and component traits by immune-responsive
miRNAs in response to pathogen and herbivore attack.