Long-distance signaling of iron deficiency in plants

Abstract

In a recent issue of the Planta, we established two points regarding the long-distance signal of iron status in tobacco (Nicotiana tabacum L.). One is that the long-distance signal generated in iron deficient tissues is a major factor in positively regulating the expressions of iron uptake genes in tobacco. The expression of a ferric chelate reductase gene (NtFRO1) and an iron-regulated transporter gene (NtIRT1) in roots decreased by cutting off the leaves grown under the iron-deficient condition. Conversely, the leaf-excision did not cause upregulation of the genes under the iron-sufficient condition. These results indicated that signals sent from shoots regulate iron uptake in roots under the iron-deficient condition. The second point regarding the long-distance signals is that the strength of the long-distance signals depends on the size of plant including roots. Both genes expressed in proportion to the weight of the remaining leaves, until a certain threshold. The gene expressions were observed also in hairy roots cultured under the iron deficient condition. In this paper, we discuss the long-distance signals of iron status in plants, using a newly obtained data.

Signals controlling the uptake of nutrients in roots seem to be different from other long-distance signals such as morphogenic signals and signals responding to environmental stress. With morphogenic signals, for instance, a flowering signal sets a phase in which some particular genes would activate. Signals responding to environmental stress play a role in transmission of the abnormal situation from the occurring point to a whole or a specific organ. The responses to these signals are unnecessary under the normal condition, but must be activated under the abnormal situation. Thus these responses create an ON/OFF signal. The reasons are explained below. Much energy is likely required for the synthesis and transferring of signals to send information between different organs. Consequently, if the most effective signaling mechanism is adopted as the main pathway, it is easy to speculate that the phase in which these long-distance signals are sent is “the period of activating the expression of downstream genes”. In fact, florigen (FT/Hd3a) is transported at the time of flowering and also wounding signals are transmitted as electrical signals when wound stress occurs.^1–3

On the other hand, the response to the nutrient signals is not “ON/OFF” because the nutrient signals must strictly regulate the down stream genes to uptake nutrients as occasion demands. Vert et al. provided two models concerning the mechanism of the long-distance signals for iron uptake in Arabidopsis thaliana.⁴ In the promotive model, the signals are sent from shoots to roots and induce the expressions of iron uptake genes under the iron-deficient condition; however, the iron-sufficient condition in the shoot would prevent release of the signals. In the repressive model, iron-sufficient shoots constitutively send the signals which suppress the root iron uptake response, and the signals disappear under the iron-deficient condition. Both models are thought to be reasonable; however, it has been unclear which signal is a main signal to regulate the expressions of iron uptake genes. Further, we should make clear the mechanism that the iron uptake genes express under the normal condition because iron is an essential element for plants and needed for the normal growth.

We elucidated an important property of the long-distance signal for iron uptake using tobacco plants.⁵ First, we showed that the expressions of NtIRT1 and NtFRO1 in roots under an iron-deficient condition were markedly decreased within 24 hours after leaf-excision, indicating the long-distance signals for iron uptake are sent under the iron-deficient condition. In addition, we recently analyzed the transgenic tobacco plants expressing GUS under the control of a promoter of HvIDS2, which is an iron-deficiency responsible gene of barley.⁶ When all leaves were excised under the iron-deficient condition, the GUS expression in roots decreased similarly to endogenous genes such as NtIRT1 and NtFRO1 though the mechanism of iron uptake of graminaceous plants is different from that of other plants (Fig. 1).⁷ This result suggests that there are common mechanisms, at least partly, of signaling for iron uptake between graminaceous plants and the other plants.

Figure 1.

Influence of leaf-excision and addition of iron on the expressions of NtIRT1, NtFRO1 and HvIDS2pro::GUS in tobacco roots. Four-week-old seedlings grown in an iron-sufficient (50 µM Fe-EDTA) medium were transferred to iron-sufficient (+) or -deficient (−) media. After two weeks, leaves grown in the iron deficient medium were excised by scissors (Ex) or Fe-EDTA was added to the iron deficient medium at a final concentration of 50 µM (Ad). Roots were harvested 24 hours after the treatment. The expressions were quantified by real-time PCR. Means ± SD of the relative values against NtACT1 are shown (n = 5).

Next, we revealed that the long-distance signals for iron uptake were synthesized not in a specific organ but instead in the whole body of a plant. It is known that the expressions of iron uptake genes in roots are serially downregulated with dependence on the iron concentration of medium; in short, the regulation of iron uptake genes is not “ON/OFF”.⁸ We hypothesized that demand of iron increases with growth of leaves-namely the plant size as well as iron concentration in leaves influences the expressions of iron uptake genes in roots. To clear the hypothesis, we conducted a stepwise excision test of iron-deficient leaves (Fig. 2). The results of the stepwise leaf-excision proved our hypotheses. Interestingly, the expressions of NtIRT1 and NtFRO1 in hairy roots which don't have shoots were induced under the iron-deficient condition though we showed that the expressions were mainly regulated by the long-distance signals from shoots. These results suggested that iron concentration is sensed at each cell in shoots and roots, and the activation of iron uptake in roots is correlated with the total amount of the long-distance signals sent from each tissue.

Figure 2.

Schematic representation of the iron deficient signal with dependence of iron status in leaves (A) and the number of leaves (B). Arrows show the direction and amount of the signal of iron deficiency.

In conclusion, the long-distance signals for iron uptake are generated in the whole body of a plant grown under the iron-deficient condition. On the other hand, further investigation is needed to understand the repressive signals, which may act as the long-distance signals to inhibit excess uptake of iron. The information of physiological responses to iron concentration would be useful in comprehensive analysis for clustering of transcripts. We hope that our research helps to find novel signal molecules and to elucidate the mechanism of signaling for iron uptake in plants.

Addendum to: Enomoto Y, Hodoshima H, Shimada H, Shoji K, Yoshihara T, Goto F. Long-distance signals positively regulate the expression of iron uptake genes in tobacco roots. Planta. 2007;227:81–89. doi: 10.1007/s00425007-0596-x.