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Cucurbit Genetics Cooperative Report 19: 30-31 (article 11) 1996

The Relationship between Papilla formation and Resistance of Cucumber to Sphaerotheca fuliginea (Schlect.) Poll.

Qing Ma, Hongwen Cui

Department of Plant Protection, Northwestern Agricultural University, Yangling, Shaanzi,71200, P.R. China, Department of Horticulture, Northwestern Agricultural University, Yangling, Shaanxi, 712100, P.R. China

Introduction. Papillae are the wall oppositions occurring between the cell wall and plasmalemma when fungal infection pegs penetrate a plant host. The function of the papillae with respect to the expression of powdery mildew resistance have been investigated in barley and wheat winsome detail (1-4). However, the results of these studies are not entirely consistent. The mechanism of powdery mildew resistance in cucumber has received limited study. Therefore, a preliminary study was designed to elucidate the mechanism of resistance to powdery mildew in cucumber using germplasm possessing varying disease resistance.

Materials and Methods. Plant and pathogen. Six cultivars [Jinza No. 2 and 3511 (resistant), Jinyan No. 6 (moderately resistant), Heidan No. 1 and Nongcheng No. 3 (moderately susceptible), and Changchun Thorn (susceptible)] were grown in a greenhouse. An isolate of Sphaerotheca fuliginea (Schlecht) Poll. obtained from field-infected cucumber plants was maintained on a growth chamber-grown susceptible cultivar, Changchun Thorn.

Inoculation, sampling, staining and microscopy observation. The day before inoculation, powdery mildew spores on the leaves were knocked off to ensure establishment of fresh inoculum. The second leaf of plants in the 4-leaf stage were inoculated, and then cultured in a greenhouse at 18-25C. Leaf samples were taken at 8, 10, 12, 14, 16, 24 and 48h after inoculation, made transparent with saturated trichloroacetaldehyde monohydrate, and stained with lactophenol-cotton blue solution. Observations were made using Olympus light microscopy.

Results and Discussion. The papillae observed were spherical or semi-spherical in shape. Papilla formation was not observed in any cultivar 8h after inoculation. The papillae appeared within the host epidemic cells in the middle of appressoria 10h after inoculation in resistant cultivars, and 2h later in susceptible cultivars. The papillae were observed in all the cultivars (79.1 to 87%) 48h after inoculation. Thus, there was no functional difference in the reaction of the cultivars examined (Table 1). The frequency of papilla formation was not associated with the degree of host resistance to powdery mildew (Table 1). This result is consistent with those of Wright, et al. (5). The formation of haustoria marks successful penetration. Our observations also indicate that some appressoria produce papillae but fail to form haustoria, and that the number of papillae not forming haustoria increases as the resistant level rises in cucumber varieties. This means that the papillae function to resist penetration (Note: Table 2, 48h after inoculation).

Approximately 6.7% of the papillae in susceptible cultivars did not form haustoria. In contrast, over 20% of the papillae formed haustoria in resistant cultivars. This observation may be related to the time of papilla formation. The papilla appear early in resistant cultivars, thus hindering the formation of haustoria. Conversely, haustoria formation occurs later in susceptible cultivars. In susceptible cultivars the pathogen penetrates the host and immediately forms haustoria. Thus the pathogen escapes the potential beneficial impact of papillae and successfully establishes a parasitic relationship.

These results demonstrate that the papillae are important structures contributing to a host's resistance to powdery mildew. Papillae act to augment plant barriers to penetration of the pathogen in resistant plants. This function increases as the resistance level of the host rises. This type of resistance depends mainly on the interaction of host resistant genes and the susceptible genes of the pathogen. The papillae appear early on the leaves of resistant cucumbers and the pathogen has greater difficulty in penetrating the papillae and developing haustoria. The papillae appear late in leaves of susceptible plants and the pathogen can penetrate host epidermal cells and establish a parasitic relationship before the papillae are fully developed.

The formation and function(s) of the papillae are not completely clear. For example, not all of the appressoria produce papillae and some appressoria fail to penetrate the host. Furthermore, papillae function against a pathogen's penetration in susceptible cultivars. Likewise, some papillae do not function against pathogen penetration in resistant cultivars. Thus, papillae formation and the genetics of papillae production are attractive areas for further research.

Table 1. Papilla formation among different cucumber cultivars.

Cultivar
Resistant level
No. matured appressoria
No. papillae
Papilla formation rate (%)
Jinza No. 2
Resistant
100
87
87.0
3511
122
102
83.6
Jinyan No. 6
Moderately resistant
129
112
86.8
Heidan No. 1
Moderately susceptible
115
91
79.1
Nongcheng No. 3
104
86
82.7
Changchun Thorn
Susceptible
101
83
82.2

Table 2. Papilla ability against penetration among different cucumber cultivars.

Cultivar
Resistant level
Total No. papillae
No. papillae without haustoria
Anti-penetration rate (%)
Jinza No. 2
Resistant
94
223
24.5
3511
102
21
20.6

Jinyan No. 6

Moderately resistant
106
15
14.2
Heidan No. 1
Moderately susceptible
111
9
8.1
Nongcheng No. 3
95
8
8.4
Changchun Thorn
Susceptible
104
7
6.7

Literature Cited

  1. Aist, J.R. 1976. Papillae and related wound plugs of plant cells. Ann. REv. Phytopathol. 14:145-163.
  2. Aist, J..R. and H.W. Israel. 1967. Papilla formation: Timing and significance during penetration of barley coleoptiles by Erisphe graminis hordei. Phytopathology 67:455-461.
  3. Stolzenburg, M.C., J.R. Aist, and H.W. Israel. 1984. The role of papillae in resistance to powdery mildew conditioned by the mi-o gene in barley. I. Correlative evi8dence. Physiol. Plant Pathol. 25:337-346.
  4. Stolzenburg, M.C., J.R. Aist, and H.W. Israel. 1984. The role of papillae in resistance to powdery mildew conditioned by the ml-o gene in barley II. Experimental evidence. Physiol. Plant Pathol. 25:347-361.
  5. Wright, A.J. and J.B. Heale. 1988. Host responses to fungal penetration in Erysiphe graminis f. sp. hordei infections in barley. Plant Pathol. 37:131-140.
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