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Powdery mildew (Podosphaera xanthii) resistance in melon is categorized into two types based on inhibition of the
M Kuzuya1, K Yashiro, K Tomita
1Plant Biotechnology Institute, Ibaraki Agricultural Center, Iwama, Nishi-ibaraki, Ibaraki, Japan. mkuzuya@agri.pref.ibaraki.jp
Abstract:
Physiological races of powdery mildew (Podosphaera xanthii) cause different symptoms in eight melon lines. Infection by races 1, 2, and 5 was examined in different melon lines. After a compatible reaction, conidia germination, haustorium initiation from the germ tube, germ tube branching, and sporulation occurred within 12, 24, 48, and 120 h, respectively, and the conidia matured within 240 h. In contrast, type i and ii inhibition were identified through incompatible reactions. The germ tube and haustorium were initiated from conidia, but no germ tube branching occurred in the lines with type i resistance within 48-240 h. In type ii resistance, germ tube branching was observed within 120 h, but no sporulation was observed within 240 h. The number of fluorescing epidermal cells was higher within 24 h in type i, and within 48-120 h in type ii resistance lines than in susceptible lines. Callose accumulation around the haustorium was detected in type ii resistance lines within 48-120 h. This suggests that the rapid hypersensitive response (HR) within 24 h has an important role in the type i response, while HR and callose accumulation in the type ii response occur slowly between 48 and 120 h. Of the resistant lines, PMR 45 and WMR 29 showed a type i incompatible response; the PI 414723 response was entirely type ii; and PMR 5, PI 124112, and MR-1 showed different responses depending on the race. Therefore, the two types of incompatible responses were intermixed in the same germplasm.
Insights
Powdery mildew resistance in melon lines involves two distinct incompatible responses: rapid hypersensitive response (HR) and slower HR with callose accumulation. These resistance types can be intermixed within the same melon germplasm.
Area of Science:
- Plant Pathology
- Plant-Microbe Interactions
- Genetics and Breeding
Background:
- Powdery mildew, caused by Podosphaera xanthii, significantly impacts melon crop yield and quality.
- Understanding resistance mechanisms is crucial for developing durable disease management strategies in cucurbits.
- Physiological races of P. xanthii exhibit varying virulence, necessitating detailed studies on host-pathogen interactions.
Purpose of the Study:
- To characterize the physiological races of Podosphaera xanthii and their interactions with different melon lines.
- To elucidate the molecular and cellular events underlying two distinct types of incompatible (resistant) responses in melon.
- To investigate the genetic basis of resistance by examining the intermixing of resistance types within melon germplasm.
Main Methods:
- Infection assays were conducted using three races of P. xanthii (1, 2, and 5) on eight melon lines.
- Microscopic observation of conidia germination, haustorium initiation, germ tube branching, and sporulation.
- Analysis of fluorescing epidermal cells and callose accumulation around haustoria to assess plant defense responses.
Main Results:
- Compatible reactions showed typical disease progression: germination (12h), haustorium (24h), branching (48h), sporulation (120h), and maturation (240h).
- Type i resistance involved inhibited germ tube branching (48-240h) and rapid hypersensitive response (HR) within 24h.
- Type ii resistance showed germ tube branching (120h) but no sporulation (240h), with slower HR and callose accumulation (48-120h).
- Specific melon lines exhibited distinct resistance types (e.g., PMR 45, WMR 29: type i; PI 414723: type ii).
- Some lines (PMR 5, PI 124112, MR-1) displayed a mix of resistance responses depending on the pathogen race.
Conclusions:
- Two distinct incompatible responses, type i (rapid HR) and type ii (slower HR with callose), confer resistance to powdery mildew in melon.
- The hypersensitive response (HR) plays a key role in both resistance types, but timing and associated defense mechanisms differ.
- The intermixing of type i and type ii resistance within the same germplasm highlights the complexity of powdery mildew resistance in melons and offers potential for breeding durable resistance.
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