Apple fruit periderms (russeting) induced by wounding or by moisture have the same histologies, chemistries and gene expressions

Abstract

Russeting is a cosmetic defect of some fruit skins. Russeting (botanically: induction of periderm formation) can result from various environmental factors including wounding and surface moisture. The objective was to compare periderms resulting from wounding with those from exposure to moisture in developing apple fruit. Wounding or moisture exposure both resulted in cuticular microcracking. Cross-sections revealed suberized hypodermal cell walls by 4 d, and the start of periderm formation by 8 d after wounding or moisture treatment. The expression of selected target genes was similar in wound and moisture induced periderms. Transcription factors involved in the regulation of suberin (MYB93) and lignin (MYB42) synthesis, genes involved in the synthesis (CYP86B1) and the transport (ABCG20) of suberin monomers and two uncharacterized transcription factors (NAC038 and NAC058) were all upregulated in induced periderm samples. Genes involved in cutin (GPAT6, SHN3) and wax synthesis (KCS10, WSD1, CER6) and transport of cutin monomers and wax components (ABCG11) were all downregulated. Levels of typical suberin monomers (ω-hydroxy-C20, -C22 and -C24 acids) and total suberin were high in the periderms, but low in the cuticle. Periderms were induced only when wounding occurred during early fruit development (32 and 66 days after full bloom (DAFB)) but not later (93 DAFB). Wound and moisture induced periderms are very similar morphologically, histologically, compositionally and molecularly.

Fig 1. Time course of change in infiltration of ‘Pinova’ fruit skin patches following wounding by abrasion of the cuticle at 40 days after full bloom (DAFB) using fine sandpaper (‘Wounding’) or by exposure of the fruit skin to moisture for 12 d (‘Moisture’).

Moisture exposure began at 28 DAFB. At 40 DAFB, moisture exposure was terminated and the time-course of change in infiltrated fruit surface area was established. Micrographs from the same surface area were taken under incident white light or incident fluorescent light. The green/yellow fluorescence resulted from localized penetration of the tracer acridine orange through microcracks in the cuticle into the underlying tissues. Scale bar equals 400 μm and is representative for all the images of the composite.

Fig 2. Time course of periderm development following wounding (left panel) and moisture exposure (right panel) of skin patches of ‘Pinova’ apple.

Patches of skin were abraded 40 days after full bloom (DAFB) using fine sandpaper. Microcracks induced by surface moisture served as control (‘Moisture’). Here, the fruit surface was exposed to surface moisture for 12 d from 28 to 40 DAFB. Pairs of micrographs were taken under transmitted white light or incident fluorescent light (filter module U-MWB) after staining with Fluorol Yellow. Fluorol Yellow stains the cuticle and suberized cell walls. Scale bar equals 50 μm and is representative for all images of the composite.

Fig 3. Time courses of change in the expressions of two transcription factors involved in the regulation of the synthesis of suberin (MYB93) and lignin (MYB42), a gene involved directly in the synthesis of suberin monomers (CYP86B1) and a gene involved in the transport of suberin monomers (ABCG20) and two uncharacterized transcription factors (NAC038 and NAC058) in the skin of ‘Pinova’ apple fruit following wounding or following exposure of the fruit surface to moisture.

Patches of fruit skin were wounded 40 days after full bloom (DAFB) by abrading the cuticle using fine sandpaper (‘Wounding’). For comparison, microcracks were induced by exposure of skin patches to surface moisture (‘Moisture’). Here, the fruit surface was exposed to surface moisture from 28 to 40 DAFB. Non-treated fruit served as the respective controls (‘Control’). Expression values are means ± SE of three biological replicates comprising six fruit each. The ‘*’ indicates significant differences between the wounded patch and its control or between the moisture exposed patch and its control, P ≤ 0.05 (Student’s t-test).

Fig 4. Time courses of change in the expression of genes involved in the synthesis of cutin monomers (SHN3, GPAT6) and wax constituents (KCS10, WSD1, CER6) and their transport (ABCG11) in the skin ‘Pinova’ apple fruit following wounding or following exposure of the fruit surface to moisture. Patches of fruit skin were wounded 40 days after full bloom (DAFB) by abrading the cuticle using fine sandpaper (‘Wounding’). For comparison, microcracks were induced by exposure of skin patches to surface moisture (‘Moisture’).

Here, the fruit surface was exposed to surface moisture from 28 to 40 DAFB. Non-treated fruit served as control (‘Control’). Expression values are means ± SE of three biological replicates comprising six fruit each. The ‘*’ indicates significant differences between the wounded patch and its control or between the moisture exposed patch and its control, P ≤ 0.05 (Student’s t-test).

Fig 5. Cross-sections through patches of ‘Pinova’ apple fruit skin at the mature stage (156 days after full bloom (DAFB)) that had been wounded or exposed to surface moisture during early fruit development.

Patches of fruit skin were wounded at 32 DAFB by abrading the cuticle using abrasive paper (‘Wound periderm’). For comparison, microcracks were induced by exposure of skin patches to surface moisture (‘Moisture-induced periderm’) from 31 to 43 DAFB. Non-treated naturally russeted surfaces (‘Native periderm’) and non-russeted surfaces served as control (‘Cuticle’). The cross-sections were stained with Fluorol Yellow. Scale bars in A 10 mm (upper) and 50 μm (lower). Bars are representative for all bright field and all fluorescence images of the composite.

Fig 6. Composition of cutin and suberin of skins of mature apple fruit.

Periderm formation in the fruit skin was induced during early development by abrading the cuticle using abrasive paper (‘Wound periderm’) (A) or by exposing the fruit skin to surface moisture for 12 d between 31 and 43 days after full bloom (DAFB; ‘Moisture-induced periderm’) (B). The treated patches of skin were excised at maturity 156 DAFB. Native periderm from naturally russeted fruit (C) and cuticles from non-treated non-russeted fruit served as controls (D). Data represent means ± SE of two to three replicates comprising periderms and cuticles of five fruit each. The data shown in (B) were taken from Straube et al. [22].

Fig 7. Wax constituents of the skins of mature apple fruit.

Periderm formation in the fruit skin was induced during early development by abrading the cuticle using fine sandpaper (‘Wound periderm’) (A) or by exposing the fruit skin to surface moisture for 12 d between 31 and 43 days after full bloom (DAFB; ‘Moisture-induced periderm’) (B). The treated patches of skin were excised at maturity 156 DAFB. Native periderm from naturally russeted fruit (C) and cuticles from non-treated non-russeted fruit served as controls (D). Data represent means ± SE of two to three replicates comprising periderms and cuticles of five fruit each. The data shown in (B) were taken from Straube et al. [22].

Fig 8. Total masses of suberin (A), wax (B) and cutin (C) in patches of skin of mature apple fruit.

Periderm formation in the fruit skin was induced during early fruit development by abrading the cuticle using fine sandpaper (‘Wound periderm’) or by exposing the fruit skin to surface moisture for 12 d between 31 and 43 days after full bloom (DAFB; ‘Moisture-induced periderm’; [22]). The treated patches of skin were excised at maturity 156 DAFB. Periderm from naturally russeted fruit (Native periderm) and cuticles from non-treated, non-russeted fruit (Cuticle) served as controls. Data represent means ± SE of two to three replicates comprising periderms and cuticles of five fruit each.

Fig 9. Developmental time course of periderm formation following wounding of ‘Pinova’ apple at 32 days after full bloom (DAFB) (‘early’), 66 DAFB (‘intermediate’) and 93 DAFB (‘late’).

For wounding, the cuticle was abraded using fine sandpaper. Cross-sections were prepared 8 d after wounding (left panel) or at maturity (156 DAFB) (right panel). Pairs of micrographs were taken under transmitted white light or incident fluorescent light (filter module U-MWB) after staining with Fluorol Yellow. Fluorol Yellow stains the cuticle and suberized cell walls. Sections were viewed at 20× (scale bar 500 μm, left column) or at 100× (scale bar 100 μm, right column).