Iron is an essential and commonly limited nutrient for plants. To increase the uptake of
iron during times of low
iron supply, plants, except the grasses, activate a set of physiological and morphological responses in their roots that include
iron reduction, soil acidification, Fe(II) transport and proliferation of root hairs. It is not known how root cells sense and transduce the changes that occur after the onset of
iron deficiency. This work presents evidence that
nitric oxide (NO) is produced rapidly in the root epidermis of tomato plants (Solanum lycopersicum) that are grown in
iron-deficient conditions. The scavenging of NO prevented
iron-deficiency-induced upregulation of the
basic helix-loop-helix transcription factor FER, the
ferric-chelate reductase LeFRO1 and the Fe(II) transporter LeIRT1 genes. On the other hand, exogenous application of the NO donor
S-nitrosoglutathione enhanced the accumulation of FER, LeFRO1 and LeIRT1
mRNA in roots of
iron-deficient plants. The activity of the root
ferric-chelate reductase and the proliferation of root hairs induced by
iron deficiency were stimulated by NO supplementation and suppressed by NO scavenging.
Nitric oxide was ineffective in inducing
iron-deficiency responses in the tomato fer mutant, which indicates that the
FER protein is necessary to mediate the action of NO. Furthermore, NO supplementation improved plant growth under low
iron supply, which suggests that NO is a key component of the regulatory mechanisms that control
iron uptake and homeostasis in plants. In summary, the results of this investigation indicate that an increase in NO production is an early response of roots to
iron deprivation that contributes to the improvement of
iron availability by (i) modulating the expression of
iron uptake-related genes and (ii) regulating the physiological and morphological adaptive responses of roots to
iron-deficient conditions.