Citrate participates in nitrate-alleviated iron deficiency
Since citrate is known to function in the transport of Fe from root to
shoot (Grotz & Guerinot. 2006), it seems likely that it may participate
in the nitrate-regulated iron deficiency response in apple. To assess
this possibility, we measured the contents of organic acids, including
citrate (CIT) and malate (Mal), in apple roots. When Fe was adequate, HN
treatment promoted citrate accumulation (Fig. 4a) but had no effects on
malate content (Fig. 4b) in roots. Under Fe deficiency conditions,
LN-treated roots showed a higher citrate content compared to HN-treated
roots (Fig.4a), while no significant difference was observed in malate
content (Fig. 4b). Citrate and malate content showed no significant
difference in leaves under HN-Fe and LN-Fe treatment (Fig. S4a, b).
These results indicated that low nitrate affects citrate content but not
malate accumulation in roots.
To further verify whether citrate participated in LN-alleviated Fe
deficiency, exogenous citrate was applied to Fe deficient plants. As
shown in Fig. 4, exogenous citrate noticeably alleviated HN-mediated Fe
deficiency, resulting in an increase in chlorophyll content (Fig. 4c,
d), increased total Fe content in newly growing leaves (Fig. 4e), and
decreased total Fe content in roots (Fig. 4f). The above results
suggested that exogenous citrate was able to alleviate Fe deficiency by
facilitating Fe transportation from root to shoot.