Remobilization of leaf S compounds and senescence in response to restricted sulphate supply during the vegetative stage of oilseed rape are affected by mineral N availability

Abstract

The impact of sulphur limitation on the remobilization of endogenous S compounds during the rosette stage of oilseed rape, and the interactions with N availability on these processes, were examined using a long-term (34)SO(4)(2-) labelling method combined with a study of leaf senescence progression (using SAG12/Cab as a molecular indicator) and gene expression of the transporters, BnSultr4;1 and BnSultr4;2, involved in vacuolar sulphate efflux. After 51 d on hydroponic culture at 0.3 mM (34)SO(4)(2-) (1 atom% excess), the labelling was stopped and plants were subject for 28 d to High S-High N (HS-HN, control), Low S-High N (LS-HN) or Low S-Low N (LS-LN) conditions. Compared with the control, LS-HN plants showed delayed leaf senescence and, whilst the shoot growth and the foliar soluble protein amounts were not affected, S, (34)S, and SO(4)(2-) amounts in the old leaves declined rapidly and were associated with the up-regulation of BnSultr4;1. In LS-LN plants, shoot growth was reduced, leaf senescence was accelerated, and the rapid S mobilization in old leaves was accompanied by decreased (34)S and SO(4)(2-), higher protein mobilization, and up-regulation of BnSultr4;2, but without any change of expression of BnSultr4;1. The data suggest that to sustain the S demand for growth under S restriction (i) vacuolar SO(4)(2-) is specifically remobilized in LS-HN conditions without any acceleration of leaf senescence, (ii) SO(4)(2-) mobilization is related to an up-regulation of BnSultr4;1 and/or BnSultr4;2 expression, and (iii) the relationship between sulphate mobilization and up-regulation of expression of BnSultr4 genes is specifically dependent on the N availability.

Fig. 1.

Changes in fresh matter of oilseed rape shoots (A) and in dry matter of the leaf blade of the younger leaves (B1) and of leaf ranks 12 (B2), 10 (B3), and 8 (B4) of oilseed rape in control plants (HS-HN), sulphate-limited plants (LS-HN), and combined sulphate- and nitrate-limited plants (LS-LN) for the 28 d of treatments. Vertical bars indicate ±SE of the mean (n=4) when larger than the symbol. Different letters indicate that mean values are significantly different at a given date (P <0.05).

Fig. 2.

Kinetics of the progression of leaf senescence along the axis of oilseed rape (A) in control plants (HS-HN), sulphate-limited plants (LS-HN), and in combined sulphate- and nitrate-limited plants (LS-LN). For a given date, each value refers to the theoretical nodal position of the last leaf rank subjected to senescence (i.e. displaying SAG12 up-regulation and Cab down-regulation, concomitantly, in the leaf blade). Other details as for Fig. 1. Photography of old leaf (leaf rank 8) in combined sulphate- and nitrate-limited plants (LS-LN), control plants (HS-HN), and sulphate limited plants (LS-HN) (B).

Fig. 3.

Changes in the chlorophyll (SPAD) of the leaf ranks 8 (A) and 10 (B) of oilseed rape in control plants (HS-HN), sulphate-limited plants (LS-HN), and in combined sulphate- and nitrate-limited plants (LS-LN) for 28 d of treatments. Vertical arrows indicate the date of the theoretical onset of leaf senescence as described in Materials and methods. HS-HN, black arrow; LS-HN, grey arrow; LS-LN, white arrow. Other details as for Fig. 1.

Fig. 4.

Changes in the S and N amounts of the leaf blade of the leaf ranks 8 (A, B) and 10 (C, D) of oilseed rape in control plants (HS-HN), sulphate-limited plants (LS-HN), and in combined sulphate- and nitrate-limited plants (LS-LN) for 28 d of treatments. Other details as for Fig. 3.

Fig. 5.

Changes in soluble protein amount in the leaf blade of the leaf ranks no. 8 (A) and 10 (B) of oilseed rape in control plants (HN-HS), sulphate-limited plants (HN-LS), and in combined sulphate- and nitrate-limited plants (LN-LS) for the 28 d of treatments. Other details as for Fig. 3.

Fig. 6.

Changes in the sulphate-sulphur amount and concentration in the leaf blade of the leaf ranks 8 (A, B) and 10 (C, D) of oilseed rape in control plants (HS-HN), sulphate-limited plants (LS-HN), and in combined sulphate- and nitrate-limited plants (LS-LN) for the 28 d of treatments. Other details as for Fig. 3.

Fig. 7.

Changes in the ³⁴S amount in the leaf blade of the leaf ranks 8 (A) and 10 (B) of oilseed rape in control plants (HS-HN), sulphate-limited plants (LS-HN), and in combined sulphate- and nitrate-limited plants (LS-LN) for 28 d of treatments. The plants were submitted to a long period of ³⁴S-sulphate labelling until day 0, the date of the beginning of the chase period. Other details as for Fig. 3.

Fig. 8.

Relative expression of BnSultr4;1 and BnSultr4;2 genes expression in the leaf ranks 8 (A, B) and 10 (C, D) of oilseed rape in control plants (HS-HN), sulphate-limited plants (LS-HN), and in combined sulphate- and nitrate-limited plants (LS-LN) for the 28 d of treatments. The results of BnSultr4;1 and BnSultr4;2 expressions are relative to the result obtained in control plants (HS-HN) at day 7. Data were not determined at day 0 for the expanding leaf and at day 28 for the old leaf because of a lack of fresh matter. Other details as for Fig. 3.