Information Technology Reference
In-Depth Information
Papaya (
Carica papaya
) Fruit Ripening I -
Pectinmethylesterase (PME) cDNA Cloning
and Expression during Fruit Development
and Ripening
Aladje BALDÉ
1
; Manuela M.C. GOUVEIA.
2
and Maria Salomé PAIS
1
1
Laboratory of Plant Biotechnology, ICAT; Ed. ICAT, Campo Grande, 1749-016 Lisboa,
Portugal
2
Departamento de Biologia, Univ. Madeira, Largo do Município, P-9050, Funchal,
Portugal
Abstract.
Pectinmethylesterase, an enzyme involved in cell wall softening of
papaya fruit was isolated. The structure of this cDNA and its expression during
development and ripening of the fruit was analysed. Northern Blotting and was used
to determine the expression of pectinmethylesterase genes along fruit development
and ripening. PME is differentially expressed in the inner and outer mesocarp. The
levels of PME activity increase gradually with maturation until day 7 of ripening.
The pectinmethylesterase activity increases differentially from the outer mesocarp to
the inner mesocarp along ripening. These values are similar for 7 days ripened fruits
which corresponds to 70% ripening. After that ripening stage there are no significant
differences between PME in inner and outer mesocarp and the PME activity is
reduced of about 10%. The phylogram generated using an alignment of the deduced
amino acid sequences of PME and of 10 PMEs homologues from other plant species
revealed that pectinmethyl esterase from papaya fruits presents higher similarity
with tomato PME sequences than with the other PMEs sequences available. The
amount of total RNA in the mature ripe fruit duplicated the amount of total RNA in
the green fruit. All the cDNAs were expressed at similar levels at the inner and outer
mesocarp tissues during the different stages of fruit ripening. However, its
expression was highest for ripening stages 1, 3, 5 and 7 decreasing thereafter to
lower levels of expression. These results show that increase in mRNA translation
parallels the increase in PME activity until day 7 of ripening.
Introduction
Papaya exportation has problems associated to long distance transport, due to adulteration
of the pulp whenever the fruits are caught as soon maturation is achieved. To solve this
problem, the current practice is to collect fruits, for exportation, in very precocious phases
of maturation, which brings, as consequence, the adulteration of the organoleptic
characteristics of this fruit. The process of fruit softening is commercially important
because it often dictates early harvest of fruit to avoid damage in subsequent handling,
which can result in failure to develop optimum flavour and colour. Finally, excessive
softening and the associated enhancement in pathogen susceptibility limit the post-harvest
life of papaya.
Papaya fruit is susceptible to chilling injury with critical temperatures ranging
between 10-15 C. In papaya, the symptoms of chilling injury are more evident upon
