Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2009
Publication Date: 9/28/2009
Citation: Koca, U., Berhow, M.A., Febres, V.J., Champ, K.I., Carillo-Mendoza, O., Moore, G.A. 2009. Decreasing Unpalatable Flavonoid Components in Citrus: The Effect of Transformation Construct. Physiologia Plantarum. 137:101-114. Interpretive Summary: Grapefruit plants accumulate high levels of a bitter flavonoid compound called naringin. If the level of naringin could be lowered or eliminated, this may result in increased consumption of grapefruit. In an effort to determine if the levels of naringin and other flavonoids could be altered by specific genetic transformation, germplasm from grapefuit were transformed with a number of gene constructs that have either a positive effect (sense transformation) or a negative effect (anti-sense transformation) on the biosynthetic pathway leading to naringin. The germplasm was then used to generate seedlings and the leaves of these seedlings were assessed for gene expression and levels of naringin. The efficiency and viability of gene transfer varied considerably in the seedlings obtained; generally, the “antisense” transformants were better than the sense transformants. Many of the antisense transformants had significantly lower levels of naringin than the controls (untransformed plants). While it takes a number of years to grow a transformed plantlet into a fruit bearing tree to determine if the fruit has lower levels of naringin, this work is a first step to show that the method may work.
Technical Abstract: Citrus species accumulate large quantities of flavanone glycosides in their leaves and fruit. The physiological role(s) of these compounds in citrus plants are unknown, but they have been documented to benefit human health upon consumption. Flavanone rutinosides are tasteless, whereas flavanone neohesperidosides, such as naringin, give a bitter taste to fruit and fruit juice products, reducing their palatability. In an effort to alter the types and levels of flavanone neohesperidosides in citrus, an Agrobacterium-mediated genetic transformation approach was employed. Citrus paradisi Macf. (grapefruit) epicotyl stem segments were transformed with sense (S) and antisense (AS) constructs of the target genes chalcone synthase (CHS) and chalcone isomerase (CHI), whose products catalyze the first two steps in the flavonoid biosynthetic pathway. Transformation with each of the individual constructs led to a different and unpredictable combination of viability, phenotypic change, transgene steady-state expression, and alteration in flavonoid content in the resulting transgenic plants. These qualities were consistent within the transgenic plants obtained using any particular construct. Transgenic plants with decreased leaf naringin levels were obtained, particularly when the CHS-AS constructs were employed.