Cell Wall Metabolism in Ripening Fruit

Abstract

Fractionation of p.ctic polysaccharides from the juice of ripening 'Bartlett' pears (Pyrus communls) gave two general types of polyuronides. The major type was a homogalacturonan (HGA) whose molecular weight decreased upon ripening. The other type comprised heteropolymers composed of various amounts of arabinose, rhamnose, and galactose. Treatment of the major arabinose-containing heteropolymeric fraction of high molecular weight (400,000) with a pear exo-polygalacturonase to degrade contaminating HGA gave a polyuronide which was inert to tomato endopolygalacturonase. Glycosyl-linkage analysis of this arabinosyl-polyuronide gave results expected from a rhamnogalacturonan I-like polysaccharide with large, highly branched araban side chains (RG-1). A linkage between HGA and RG-1 was not found. RG-1, in ripening pears, appeared to be degraded with the initial loss of much of its arabinose. ent metabolic stability. The latter may be due to a lack of suitable hydrolytic enzymes in the ripening fruit or to a peculiar structure which protects the pectic polysaccharides from further degradation. Potent glycoside hydrolyzing activities of pear fruit, namely PG2 (16), and a-L-arabinofuranosidase (2) might be expected to rapidly degrade the watersoluble pectins which appear to accumulate in ripening pears (1).The present paper describes the change in quantity of the major pectic polysaccharide fractions obtained from the juice of progressively more ripe 'Bartlett' pear fruit. The characterization of these pectins in terms of composition, mol wt, linkage, and susceptibility to the isolated enzymes named above shows that the two major pectin types in pears, HGA, and a rhamnogalacturonan I-like polymer with a high arabinose content are not linked in pear juice and are degraded by very different mechanisms as pear fruit ripen. MATERIALS AND METHODSThe loss of flesh texture experienced by pome fruits upon ripening is attributable, in part, to dissolution of the ordered arrangement of cell-wall and middle lamella polysaccharides (12). Cell-wall preparations derived from progressively ripe fruit show a decrease in the content of pectic polysaccharides especially. Accompanying the preferential loss ofgalacturonic acid and of certain neutral sugars, notably D-galactose from apple (11), and L-arabinose from pear (9), cell walls are corresponding increases in the ripening fruit of water-soluble pectic polysaccharides containing these sugars (1,16). These are of interest because they accumulate and are readily isolated in the form in which they existed at the time of extraction. They were not obtained with the use of extraction procedures (chemical or enzymic) that could release from isolated cell wall preparations polymers that could be altered from those present in vivo. The structures ofthe water-soluble pectic polysaccharides result from native biosynthetic and degradative processes and the analysis of these structures will give clues to the structure of pectin in native undegraded cell walls and to the enzymes involved in...

The CDTA-soluble pectic substances from soybean meal are composed of rhamnogalacturonan and xylogalacturonan but not homogalacturonan

“…The enzymatic degradation of Fraction P showed that CDTA‐extractable pectic polysaccharides from soybean are different from pectic polysaccharides extracted from both other leguminous sources, like pea,5 and nonseed tissues, like apple,29, 44, 45 carrot,46 kiwifruit,47, 48 onion,49 pear,50 potato,51, 52 suspension‐cultured sycamore cells,26, 53, 54 and sugar beet 55. All these pectins contain homogalacturonan and rhamnogalacturonan regions, which can be degraded (possibly after saponification) with polygalacturonase and rhamnogalacturonase, respectively.…”

Immediate perception of a reward is distinct from the reward’s long-term salience

“…However, in addition to L-arabinose, L-fucose can also produce memory (Figure 1E); both are components of the pectin in many fruits’ cell walls (Dick and Labavitch, 1989; Ahmed and Labavitch, 1980). It is therefore possible that these sugars may signal some specific attributes of ripening fruit—ripening is accompanied by breakdown of the fruit’s cell walls—although neither of these sugars are present in fruits near the concentrations (1 M) used in memory assays.…”

Pome fruits

“…Pears are a classic source of xylan (Chanda et al, 1951); however, the available information does not exclude the presence of this hemicellulose in apples or the presence of xyloglucan in pears. Like apples, pears contain homogalacturonan and rhamnogalacturonan, and these do not seem to be linked covalently (Dick and Labavitch, 1989). Compositional changes are restricted to the pectic polymers.…”