Impact of S availability on thermotolerance acquisition
Firstly, the level of S limitation in our assay was mild enough to
impact only a few nutritional (FA-related, SSPs) and physiological
(ABA:GA3 ratio) quality criteria (Figure 2). By contrast, the indicators
of the stress response displayed contrasting temperature modality
rankings according to the S supply (Table 3). Our results pinpointed
increased antioxidant responses under the 3 late heat peaks and under
natural thermoperiod in plants exposed to limiting S supply (Table 3).
Nevertheless, S nutrition has been reported to improve antioxidant
defenses due to S-containing antioxidant compounds such as the
redox-active cysteine residues of glutathione and thioredoxin (Mukwevho,
Ferreira & Ayeleso 2014) and also specific phytohormones (Xia et
al. 2015; Bashir et al. 2015; Hasanuzzaman et al. 2018).
Therefore, it can be assumed that phytohormones such as IAA, SA and JA
boosted the antioxidant responses and that their biosynthesis and/or
signaling roles (reviewed in Hasanuzzaman et al. 2018) . were
enhanced under S limitation. Indeed, IAA biosynthesis relies on the
effectiveness of S metabolism via adequate levels of glutathione
(Kopriva et al. 2016) and is improved when S is limiting
(Nikiforova et al. 2003); JA signaling is dependent on
S-containing metabolites (Park et al. 2013) and its synthesis is
enhanced under S restriction (D’Hooghe, Escamez, Trouverie & Avice
2013); SA is also known to interact with S during SA homeostasis
regulation (Baek et al. 2010). Likewise, our results showed that
the enhanced antioxidant responses in S-restricting conditions,
especially under the CES and the EMS, correlated with higher
concentrations of IAA and JA but not SA. Altogether, these observations
not only reflect the direct effects of S restriction on phytohormone
synthesis but also the interactions between their specific temperature
threshold for synthesis and the features of the temperature sequences
(i.e. intensity, duration and period), leading to biosynthesis
impairment if these factors are not optimal, regardless of the level of
S.