3.6 MT promoted nitrate accumulation and transcription ofMdHY5
Previous studies have shown that exogenous MT treatment enhances N
uptake during N deficiency (Du et al. , 2022; Qiao et al. ,
2019). To investigate the function of MdASMT9 in N absorption,
the 15N-NO3− influx
rate in WT and MdASMT9 -OE plants were measured. We found that
under N-deficiency conditions, the15N-NO3− influx rate
of MdASMT9 -OE plants was substantially higher compared to WT
plants (Figure 5b). Under normal conditions, the
NO3− concentration in the roots of OE
lines were similar to those of WT plants (Figure 5a). However,MdASMT9 -OE plants exhibited more
NO3− in their roots under N-deficiency
conditions. NRT1.1 and NRT2 proteins also play key roles in
NO3− absorption in response to the low
NO3− conditions (Okamoto et
al. , 2003; You et al. , 2022), so we analyzed the
expression of MdNRT1.1 , MdNRT2.1 , MdNRT2.2 ,MdNRT2.3 , MdNRT2.4 , and MdNRT2.5 in the roots of WT
and MdASMT9 transgenic plants under low-N conditions as well
(Figure 5c,d; Figure S1). The results showed that MdASMT9 OE
plants exhibited higher expression of MdNRT2.1 andMdNRT2.4 in roots than that of WT plants during low-N treatment
(Figure 5c,d).
Previous studies have reported that HY5 positively regulates root
nitrate absorption by activating NRT2.1 (Chen et al. ,
2016; Huang et al. , 2015). Yao et al. (2021) found that exogenous
MT and altered MT biosynthesis in Arabidopsis enhance the
expression of HY5 to protect the plant from oxidative stress. Therefore,
we examined whether MT increased the expression of MdHY5 under
N-deficiency conditions (Figure 6a). MdHY5 expression was
detected in apple plants treated with 0.5 μM melatonin (WT+MT) and
overexpressed with MdASMT9 under N-deficiency conditions. The
expression of MdHY5 in the roots of WT, WT+MT, OE-3, and OE-4
plants first increased and then decreased, reaching the maximum value at
16 d of low-N stress. Starting from the fourth day of low-N treatment,
WT+MT, OE-3, and OE-4 plants had higher expression of MdHY5 than
WT. Exogenous MT treatment and overexpression of MdASMT9 in apple
plants thus appeared to enhance the expression of MdHY5 during
low-N treatment.