Discussion
To maximize the fitness, Plant response to these stresses and growth
should be appropriately coordinated, but the related players and
underlying mechanisms remains poorly understood. In the present study,
we provided evidence that CaNAC2c act as crucial regulator in
coordinating pepper growth and response to HTS and RSI.
Our data demonstrated that upon HTS or RSI, CaNAC2c is
transcriptionally upregulated, leading to enhanced basal and acquired
thermotolerance or resistance to RSI, but repression of growth and
development, indicating role of CaNAC2c as negative regulator in pepper
growth but positive regulator in pepper thermotolerance and defense
response to RSI. Noticeably, CaNAC2c silencing did not produced
any effect on pepper resistance to RSI, and its simultaneous silencing
with CaNAC2d significantly decreased resistance to RSI,
indicating thatCaNAC2c acts redundantly with CaNAC2d in
pepper immunity. Similar functional redundancy has been extensively
found in immunity associated genes such as basic Helix-loop-Helix (bHLH)
type transcription factor(Xu et al., 2014). It can be speculated that
functional redundancy in NAC2 subfamily might enable more robust
transcriptional reprogramming upon pathogen recognition. The difference
that CaNAC2c acts specifically in thermotolerance but in redundancy with
other genes in response to pathogen attack might be due to the different
nature of selection pressure derived from HTS and pathogen attack.
During arm race between plants and pathogens, plant immunity has been
frequently overcome by the newly occurred effectors in the pathogens via
targeting specific immune components including transcription
factors(Canonne et al., 2011). Thus this redundancy might be an
important mechanism for the robust of immunity, removal or repression of
a part of the redundant components can be functionally compensated for
by others.
In addition, our data indicated that CaNAC2c acts as a regulator in the
trade-off between pepper growth and thermotolerance or immunity.
Growth-defense trade-offs are thought to occur in plants to maximize
their fitness upon different stresses when resources are limited(Berens
et al., 2019; Huot et al., 2014). The transcriptional levels ofCaNAC2c is lower in the absence of stress in pepper plants, and
is upregulated by RSI or HTS, indicating that the transcriptional levelsCaNAC2c are crucial for derepression of pepper growth in absence
of stress and enhanced defense response upon stress. The transcriptional
modification of CaNAC2c was found to be determined by signaling
mediated by JA, ABA, IAA or GA3, and the data from 5’ series deletion
assay further related the transcriptional modification of CaNAC2cby exogenous applied JA, ABA, IAA or GA3 to specific regions ofCaNAC2c promoter (Figure S3), indicating that ABA and JA are
involved in pepper response to HTS and to RSI, respectively. Similar
observations have been found previously that ABA is involved in plant
response to heat stress(Y. C. Huang et al., 2016), and JA signaling
contribute to plant immunity towards necrotrophic pathogens(Campos,
Kang, & Howe, 2014), and signaling mediated by auxin or gibberellins
(GA3) is involved in the regulation of growth(Denance, Sanchez-Vallet,
Goffner, & Molina, 2013). So, it can be speculated that tradeoff
between growth and immunity or thermotolerance mediated by CaNAC2c is
mainly regulated at transcriptional level through crosstalk between
signaling mediated by ABA, JA, IAA or GA3.
Furthermore, our data showed that CaNAC2c acts as a positive regulator
in thermotolerance and immunity against RSI. This result is similar to
that of our previous studies that CaWRKY6 (Cai et al., 2015),CaWRKY40 (F. F. Dang et al., 2013) and CabZIP63 (Shen et
al., 2016) act positively in pepper response to both RSI and HTS,
supporting the notion that pepper response to RSI is closely linked to
its response to HTS. Although, CaNAC2c is upregulated by both HTS
and RSI, its targeting and transcriptional regulation might be
different. For instance, CaHSFA5 is directly targeted and
regulated by CaNAC2c upon HTS, but not upon RSI (Figure 5, 6 and S7).
Similarly, thermotolerance related CaHSP24 , CaHSP70 andCaHSF2A were upregulated by CaNAC2c upon HTS, but not upon
RSI (Figure 6F and G). By contrast, immunity related DEF1 was
upregulated by CaNAC2c upon RSI but not upon HTS (Figure 6K).
Importantly, immunity positively related(Alvarez et al., 1998; Yoshioka
et al., 2003) and thermotolerance negatively related
H2O2 accumulation(Yu et al., 2019;
Zhuang et al., 2020) (Figure 1 and S6) was decreased by CaNAC2c upon HTS
but enhanced upon RSI (Figure 3E-F). All these data indicate a trade-off
between thermotolerance and immunity against RSI
mediated by CaNAC2c, and that the
targeting of CaNAC2c and its function are modified in a context
dependent manner. It can be speculated that unlike that tradeoffs
between growth and disease resistance or thermotolerance mediated by
CaNAC2c is determined by its transcription, tradeoff between
thermotolerance and immunity against RSI mediated by
CaNAC2c might be mainly determined
by its post-translational regulation, probably in protein-protein
interaction manner(Alves et al., 2014; Chi et al., 2013). As
upregulation of CaNAC2c upon RSI and HTS is regulated by JA- and
ABA-signaling, respectively, potential immune reaction during HTS
response or thermotolerance during RSI response might be blocked by the
antagonism between ABA- and JA-signaling(Anderson et al., 2004; S. Liu,
Ziegler, Zeier, Birkenbihl, & Somssich, 2017; Nahar, Kyndt, Nzogela, &
Gheysen, 2012). HTS and to RSI are most frequently occurred stresses in
subtropical or tropical climates where pepper originally distributed,
the post-translational regulation might benefit rapid and precise
defense response to a specific stress, in particular, from one stress to
another.
Collectively, our data indicate that CaNAC2c acts positively in
thermotolerance and in immunity against RSI while negatively in pepper
growth, CaNAC2c functions differentially in a context dependent manner,
tradeoffs between growth and thermotolerance or immunity mediated by
CaNAC2c is determined by its transcription, while tradeoffs between
thermotolerance and immunity mediated by CaNAC2c is conferred by its
post-translational regulation.