Wine Chemistry and Biochemistry

(Steven Felgate) #1

6C Proteins 217


during ripening, it can be presumed thatthe haze forming potential increases as


ripening proceeds (Murphey et al. 1989; Pocock et al. 2000; Tattersall et al. 1997).


6C.4.1 Fungal Infection


While PR protein synthesis in healthy grapes appears to be triggered by v ́eraison,


this does not mean that the classical PR protein inducers – stress, wounding and


pathogenic attack – cannot further modulate the levels of PR proteins in grapes.


Grape PR proteins exhibit antifungal activity in vitro against common fungal


pathogens of grapevines, includingUncinula necator,Botrytis cinerea,Phomopsis


viticola,Elsinoe ampelinaandTrichoderma harzianum(Giannakis et al. 1998;


Jayasankar et al. 2003; Monteiro et al. 2003a; Salzman et al. 1998; Tattersall et al.


2001). It would be tempting to speculate that the antifungal activity observed in vitro


reflects the main function of the PR proteins in vivo, and that their expression in fruit


after v ́eraison represents a preemptive defense mechanism for fruit. Jayasankar et al.


(2003) added further credence to this hypothesis by demonstrating that grapevines


regenerated after in vitro selection withE. ampelinaculture filtrates had greater dis-


ease resistance and high constitutive expression of PR proteins, including VvTL1.


Studies in which the synthesis of PR proteins is modified by gene technology would


allow us to investigate this hypothesis further.


Increased expression of some PR genes and enhanced concentrations of some


PR proteins have been observed in leaves and berries from grapevines infected with


pathogens (B ́ezier et al. 2002; Derckel et al. 1998; Jacobs et al. 1999; Renault et al.
1996; Robert et al. 2002). In glasshouse experiments, Monteiro et al. (2003b)


observed increased levels of thaumatin-like proteins in berries infected withU.


necatorcompared to uninfected berries. Jacobs et al. (1999) demonstrated that


chitinase and beta-1,3-glucanase activity increased in grape berries and leaves in


response to powdery mildew infection, and that expression of genes, VvChi3,


VvGlub, and VvTL2, coding for PR proteins, was also strongly induced. Of these


three putative gene products, only VvTL2 has been detected as a soluble protein


in grape juices and wines (Waters et al. 1996). Girbau et al. (2004) demonstrated


that powdery mildew infection ofV. viniferacv. Chardonnay grape bunches resulted


in increased levels of a grape minor thaumatin like protein, VvTL2, in wine. At


high levels of infection (>30% of berries in a bunch infected), this had a significant


impact on the level of haziness in the wine following a heat test.


Contrary to expectations that fungal diseases would lead to elevated levels of


PR proteins in berries, Marchal et al. (1998) observed that juice from berries


infected byB. cinereashowed reduced protein levels, and suggested that proteolytic


enzymes fromB. cinereawere responsible for this. Secretion of proteases byB.


cinereahas been observed in culture media and on fruits such as apple (Zalewska-


Sobczak et al. 1981) and tomato (Brown and Adikaram 1983). Girbau et al. (2004)


also examined the impact of infection of grapes withB. cinereain the vineyard and


showed that infection resulted in marked decreases in the levels of PR proteins in

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