Girdling affects carbohydrate-related gene expression in leaves, bark and roots of alternate-bearing citrus trees

Abstract

Effects of girdling on carbohydrate status and carbohydrate-related gene expression in citrus trees were investigated. Alternate-bearing 'Murcott' (a Citrus reticulata hybrid of unknown origin) trees were girdled during autumn (25 Sep. 2001) and examined 10 weeks later. Girdling brought about carbohydrate (soluble sugar and starch) accumulation in leaves and shoot bark above the girdle, in trees during their fruitless, 'off' year. Trees during their heavy fruit load, 'on' year did not accumulate carbohydrates above the girdle due to the high demand for carbohydrates by the developing fruit. Girdling caused a strong decline in soluble sugar and starch concentrations in organs below the girdle (roots), in both 'on' and 'off' trees. Expression of STPH-L and STPH-H (two isoforms of starch phosphorylase), Agps (ADP-glucose pyrophosphorylase, small subunit), AATP (plastidic ADP/ATP transporter), PGM-C (phosphoglucomutase) and CitSuS1 (sucrose synthase), all of which are associated with starch accumulation, was studied. It was found that gene expression is related to starch accumulation in all 'off' tree organs. RNA levels of all the genes examined were high in leaves and bark that accumulated high concentrations of starch, and low in roots with declining starch concentrations. It may be hypothesized that changes in specific sugars signal the up- and down-regulation of genes involved in starch synthesis.

Fig. 1. Total non‐structural carbohydrates (TNC) (soluble sugar and starch) in leaves (top) and bark (bottom) of girdled and non‐girdled ‘on’ and ‘off’ citrus trees (means ± s.d. of three independent samples, n = 3).

Fig. 2. Iodine staining of shoot (A–D) and root (E–H) cross‐sections. A and E, Girdled ‘off’ trees; B and F, girdled ‘on’ trees; C and G, non‐girdled ‘off’ trees; D and H, non‐girdled ‘on’ trees. The cross‐sections of shoot and roots are 0·5 cm in diameter.

Fig. 3. Relative transcript levels of STPH‐H, STPH‐L, Agps, AATP, PGM‐C and CitSuS1 in leaves and bark of girdled and non‐girdled ‘on’ and ‘off’ citrus trees (A). Total RNAs (20 µg) isolated from leaves and bark were separated and analysed by RNA blot. 18S rRNA from citrus was used as an internal control for loading differences. Relative levels of transcripts (B) were estimated by quantitation of signals on the blotted membranes with a Phosphorimager BAS1000 (Fuji Photo Film, Tokyo, Japan). The results are expressed as a percentage of the respective maximum level for each gene. Results are representative data of two independent experiments, the same as for Fig. 5.

Fig. 4. Total non‐structural carbohydrates (TNC) (soluble sugar and starch) in roots of girdled and non‐girdled ‘on’ and ‘off’ citrus trees (means ± s.d. of three independent samples, n = 3).

Fig. 5. Relative transcript levels of STPH‐H, STPH‐L, Agps, AATP, PGM‐C and CitSuSy1 in roots of girdled and non‐girdled ‘on’ and ‘off’ citrus trees (A). Total RNAs (20 µg) isolated from roots were separated and analysed by RNA blot. 18S rRNA from citrus was used as an internal control for loading differences. Relative levels of transcript (B) were estimated as described in Fig. 3.