Host Barriers and Responses to Uncinula necator in Developing Grape Berries

Area of Science:

• Plant Pathology
• Mycology
• Viticulture

Background:

• Grape berries (Vitis vinifera) transition from susceptible to resistant to powdery mildew (Uncinula necator) within weeks of bloom.
• The precise mechanisms conferring ontogenic resistance in grape berries remain largely unknown.

Purpose of the Study:

• To investigate the factors responsible for halting Uncinula necator development on grape berries as they acquire ontogenic resistance.
• To differentiate between pre-formed and induced resistance mechanisms in grape berries against powdery mildew.

Main Methods:

• Microscopic observation of Uncinula necator conidia germination and appressoria formation on grape berries of varying ages.
• Analysis of cuticle and cell wall thickness, papillae formation, and phenolic compound accumulation.
• Gene expression analysis of pathogenesis-related genes (VvPR-1) and germin-like proteins (VvGLP3).

Main Results:

• Pathogen ingress halted before cuticle penetration, irrespective of increased cuticle thickness with berry age.
• Cell wall thickness did not change significantly during the susceptible-to-resistant transition.
• Papillae formation and accumulation of autofluorescent polyphenolic compounds varied with berry age and pathogen response, but did not solely explain resistance.
• VvGLP3 expression was rapid in resistant berries, while VvPR-1 was induced in susceptible berries.

Conclusions:

• Ontogenic resistance in grape berries is not primarily due to increased cuticle or cell wall thickness, nor phenolic accumulation.
• Resistance likely involves a pre-formed barrier near the cuticle or the rapid synthesis of antifungal compounds in older berries.
• The rapid induction of VvGLP3 in resistant berries suggests its role in early defense against powdery mildew.