Prunasin hydrolases during fruit development in sweet and bitter almonds

Area of Science:

• Plant Biochemistry
• Molecular Biology
• Agricultural Science

Background:

• Amygdalin, a bitter component in almonds (Prunus dulcis), is a cyanogenic diglucoside.
• Its precursor, prunasin, can be converted to amygdalin or degraded into hydrogen cyanide, glucose, and benzaldehyde by prunasin hydrolase (PH) and mandelonitrile lyase.
• Understanding the role of PH in amygdalin synthesis is crucial for determining almond bitterness.

Purpose of the Study:

• To investigate the tissue and cellular localization of PH proteins during fruit development in sweet and bitter almond cultivars.
• To identify and characterize PH genes in different almond varieties.
• To correlate PH activity and localization with the development of bitter versus sweet almond traits.

Main Methods:

• Confocal microscopy was used to visualize PH protein localization in various almond tissues (tegument, nucellus, endosperm, embryo) during fruit development.
• Specific antibodies against PH proteins were employed for detection.
• Two PH genes (Ph691 and Ph692) were identified and their nucleotide and amino acid sequences analyzed and compared to known genes.

Main Results:

• PH protein localization patterns differed significantly between sweet and bitter almond cultivars, shifting between apoplast and symplast during fruit development.
• Two distinct PH genes, Ph691 and Ph692, were identified, showing high sequence identity to each other and to Ph1 from black cherry.
• Differences in PH activity and in vivo protein localization suggest varying prunasin availability for amygdalin synthesis, influencing almond taste.

Conclusions:

• The differential localization and activity of prunasin hydrolase (PH) are key factors in determining whether an almond cultivar develops a bitter or sweet phenotype.
• Genetic and protein-level variations in PH contribute to the distinct biochemical pathways observed in sweet and bitter almonds.
• This research provides insights into the molecular mechanisms underlying almond bitterness and sweetness, with implications for almond breeding and production.