Figure 1. Percent loss in galactose and arabinose sugar residues in cell walls of susceptible and resistant ripe fruit pericarp tissue.
USDA, ARS, Beltsville Ag. Res. Ctr. Beltsville, MD , 20705
Numerous changes in tomato fruit cell wall structure occur during fruit ripening. A major structural change is the degradation of polyuronides by increasing levels of polygalacturonase (PG) activity and a concomitant loss of galacturonic acid residues from the cell wall. A net loss of arabinose and galactose residues also occurs from cell walls of ripening fruit. Numerous lines of evidence support a role for PG in the fruit ripening process. It's role in fruit softening, however, is unclear in light of recent work in which expression of a chimeric PG gene in transgenic rin tomato fruit allowed normal polyuronide degradation but was not sufficient to induce fruit softening (Giovannoni et al. 1990).
Giovannoni et al. (1990) suggested that PG may play an important role in the developmentally induced susceptibility to pathogens that accompanies tomato fruit ripening. Using transgenic rin tomato fruit, their results demonstrated that PG expression in transgenic fruit was similar to wild type levels and induced susceptibility to the blackmold fungus Alternaria alternata. Similar to A. alternata, the causative organisms of tomato fruit anthracnose, Colletotrichum coccodes and C. dematium, elicit disease symptoms primarily in ripe fruit. By assaying PG activity in ripe fruit of susceptible and resistant tomato lines we sought to test the hypothesis that expression of high levels of endogenous PG in ripe fruit is sufficient to induce susceptibility to anthracnose fruit rot. Changes in cell wall composition were also examined for their effect on anthracnose resistance.
Ripe field grown fruit of four USDA anthracnose resistant breeding lines developed by T.H. Barksdale and six susceptible breeding lines or commercial varieties were puncture inoculated with a C. coccodes spore suspension using the hypodermic inoculation technique described by Robbins and Angell (1970). Anthracnose resistance was assessed by measuring lesion diameters. PG activity and cell wall neutral sugar composition in pericarp tissue of field ripened bulk fruit samples were assayed as previously described (Gross 1982, 1984).
Assessment of anthracnose resistance in ripe tomato fruit via measurement of lesion diameter and scoring of percent infected fruit confirmed resistance to tomato anthracnose in lines 88L147, 625-3-1, 83L237 and 85L126 (Table 1). All fruit of the susceptible checks, 71L2, 90L146, FM6203, Floradade, US141 and UC82B exhibited infection with mean lesion sizes approximately 2.5 to 7 times greater than that noted in the resistant lines.
Analysis of PG activity in ripe tomato fruit indicated relatively high levels of enzyme activity in the susceptible lines 71L2 and 90L146 in comparison to the four resistant lines. In contrast, PG activity in pericarp tissue of the susceptible cultivars FM6203 and Floradade was low and comparable to that found in the resistant lines. Subjective evaluation of FM6203 and Floradade fruit firmness indicated that fruit of these cultivars are firmer than those of 71L2 and 90L146 at comparable stages of maturity and may account for the decreased PG activity in these cultivars. In light of measurements of anthracnose resistance in FM6203 and Floradade, which clearly demonstrate susceptibility to anthracnose, it is unlikely that high PG activity is sufficient to induce anthracnose susceptibility in tomato fruit.
Table 1. Polygalacturonase activity and anthracnose measurements in ripe tomato fruit.
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Line Mean lesion % Infected Anthracnose PG Activity
diameter1 fruit2 rating (micromol/min
(mm) g fresh wt)
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88L147 4.0 38 R 0.029
625-3-1 3.0 15 R 0.045
83L237 7.7 58 R 0.065
85L126 4.1 40 R 0.050
71L2 16.9 100 S 0.231
90L146 17.8 100 S 0.102
FM6203 19.0 100 S 0.062
Floradade 18.9 100 S 0.024
US141 16.3 100 S -
UC82B 18.4 100 S -
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1 n = 122 Fruit lesions greater than 1 mm diameter
R = resistant; S = susceptible; - = not determined
To test the influence on anthracnose resistance of changes in cell wall integrity, which result in part from modifications in the net cell wall composition during fruit ripening, the neutral sugar composition of crude cell walls were analyzed in several lines. Quantification of cell wall monosaccharides revealed a decreased loss of cell wall galactosyl (2.5 to 3.3-fold) and arabinosyl residues (1.5-fold) throughout fruit ripening in resistant line 625-3-1, relative to the susceptible lines US141 and UC82B (Figure 1). No direct evidence is available to indicate that changes in these neutral sugars is related to fruit softening. These neutral sugars may, however, restrict the action of hydrolytic enzymes on pectin and thereby facilitate anthracnose resistance. Other potential mechanisms influencing tomato anthracnose disease development may include phytoalexin production and proteolytic or nonproteolytic inhibition of fungal pectic and proteolytic enzymes.
Figure 1. Percent loss in galactose and arabinose sugar residues in cell
walls of susceptible and resistant ripe fruit pericarp tissue.
Literature cited:
Giovannoni, J.J., D. DellaPenna, C.C. Lashbrook, A.B. Bennet and R.L. Fischer 1990. Expression of a chimeric polygalacturonase gene in transgenic rin (ripening inhibitor) tomato fruit, p.217-227. In: Bennet, A.B. and O'Neil, S.D. (eds.). Horticultural Biotechnology. Wiley-Liss, Inc., New York, NY.
Gross, K.C. 1982. A rapid and sensitive spectrophotometric method for assaying polygalacturonase using 2-cyanoacetamide. HortScience 17:933-934.
Gross, K.C. 1984. Fractionation and partial characterization of cell walls from normal and non-ripening mutant tomato fruit. Physiol. Plant. 62:25-32.
Robbins, M.L. and F.F. Angell 1970. Tomato anthracnose: A hypodermic inoculation technique for determining genetic reaction. J. Amer. Soc. Hort. Sci. 95:118-119.