Tomato Fruit Cell Wall Synthesis during Development and Senescence

Abstract

The pedicel of tomato fruit (Lycopersicon esculentum Mill., cv 'Rutgers') of different developmental stages from immaturegreen (IG) to red was injected on the vine with 7 microcuries [14C(U)]sucrose and harvested after 18 hours. Cell walls were isolated from outer pericarp and further fractionated yielding ionically associated pectin, covalently bound pectin, hemicellulosic fraction 1, hemicellulosic fraction 11, and cellulosic fraction I. The dry weight of the total cell wall and of each cell wall fraction per gram fresh weight of pericarp tissue decreased after the mature-green (MG) stage of development. Incorporation of radiolabeled sugars into each fraction decreased from the IG to MG3 (locules jellied but still green) stage. Incorporation in all fractions increased from MG3 to breaker and tuming (T) and then decreased from T to red. Data indicate that cell wall synthesis continues throughout ripening and increases transiently from MG4 (locules jellied and yellow to pink in color) to T, corresponding to the peak in respiration and ethylene synthesis during the climacteric. Synthesis continued at a time when total cell wall fraction dry weight decreased indicating the occurrence of cell wall tumover. Synthesis and insertion of a modified polymer with removal of other polymers may produce a less rigid cell wall and allow softening of the tissue integrity during ripening. no detectable PG activity, soften extensively (11) and apples, which contain no endo-PG, soften and show increases in soluble polyuronide during ripening (1) suggesting that softening involves other mechanisms. Using tomato fruit, Gross and Wallner (5) and Gross (6) showed that a net loss of galactosyl residues from the pectic portion of tomato fruit cell wall occurs during ripening. This loss does not result from PG activity (5-7) and strongly suggests that other enzymes or mechanisms, in addition to PG, are involved in cell wall modifications that occur during ripening of tomato fruit.Little research has addressed the synthesis and turnover of cell wall polysaccharides during fruit development and ripening and their potential role in cell wall modifications that lead to softening (14). Cell expansion ceases at the MG stage oftomato fruit development (9). It has generally been assumed that cell wall synthesis also ceases at this time. However, the radiolabeling studies necessary to measure cell wall synthesis have not been conducted. The objective of this study was to measure synthesis of tomato fruit cell wall components at various stages of development from IG to R using a [14C] sucrose pedicel injection technique developed to radiolabel fruit cell walls in vivo.The fruit cell wall has generally been viewed as a static structure which is enzymically degraded during ripening leading to fruit softening. Much emphasis in studies of softening has been placed on the activity of endo-D-galacturonase (EC 3.2.1.15, PG2), an enzyme which cleaves a 1 ,4-galacturonosyl linkages in pectin (2)(3)(4)16). Expression of the PG gene increases at t...

Cell Wall Metabolism in Ripening Fruit (IX. Synthesis of Pectic and Hemicellulosic Cell Wall Polymers in the Outer Pericarp of Mature Green Tomatoes (cv XMT-22)

“…This problem is compounded by the difficulty in separating some of the linkages chromatographically in order to determine levels of incorporation and relative abundances. Nevertheless, evidence has been presented to support previous work (Mitcham et al, 1989;Greve and Labavitch, 1991) that demonstrated the capacity for cell wall synthesis in ripening tomato pericarp, particularly at the MG-breaker transition. Our study has also Plant Physiol.…”

“…He proposed but did not demonstrate that the synthesis could contribute to wall component dissociation and fruit softening. Mitcham et al (1989Mitcham et al ( , 1991 continued the work using [14C]Suc as a precursor to cell wall polysaccharides in tomato (Lycopersicon esculentum Mill.) fruits ripening on the vine.…”

“…Vol. 1 1 4, 1997 transition between the MG and turning stages of ripening (Mitcham et al, 1989;Greve and Labavitch, 1991) prior to perceptible softening.…”

“…They demonstrated incorporation of label into various classes of polysaccharides (defined only by differential extractability) throughout the course of fruit ripening. Greve and Labavitch (1991) used [l3C]G1c as a precursor in excised tomato pericarp discs and expanded on the results of Mitcham et al (1989Mitcham et al ( , 1991 by following label into individual monosaccharides that were identified by GC-MS analysis of hydrolyzed cell wall preparations. In this paper we describe a more detailed investigation of the incorporation of I3C label into specific glycosidic linkages of pectic and hemicellulosic wall fractions extracted from two regions of the outer pericarp of tomato fruit.…”

Pepper β-Galactosidase 1 (PBG1) Plays a Significant Role in Fruit Ripening in Bell Pepper (Capsicum annuum)

“…Flowers were labeled at anthesis, and fruit was harvested according to the number of dpp, or based on surface color using the ripeness stages. 36) ''Mature green'' (MG) indicates that the fruit has ripened fully, approximately dpp 40. ''Turning'' (T) indicates that the areas of green and red color are roughly equal.…”