Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 58

The recent immunological observation of
hemicelluloses in the middle lamella,
which disappear during ripening (Ordaz-
Ortiz et al. , 2009), may contribute to the
slight depolymerization of xyloglucans
observed by others (Brummell, 2006).
Much of the research discussed above
was performed simply to discover the
fundamentals and basic biology of fruit
ripening; none the less, the practical goal
of using this knowledge to develop
shipping and storage practices and to
engineer high-quality fruit with a long
shelf-life is never forgotten. In considering
this practical goal, we must also keep in
mind that, although we have selectively
bred fruit for human consumption, the fruit
has evolved for millions of years for
reproduction of the species and not for
commercial practice. Not very many fruits
have evolved for optimal transcontinental
shipping. Seed dispersal by consumption
by an animal is one reason for ripening, but
sometimes the fruit simply drops to the
ground where the seed will then germinate.
Over-ripening may be important to create a
nursery environment for effi cient seed
germination. For example, many fruits,

including tomato, will soften to the point
that the fruit will collapse under its own
weight. However, for human consumption,
we typically want fi rm fruit that can easily
be transported and a delay in the over-
ripening of the fruit. We currently do this
commercially by picking fruit before it is
completely ripe, dipping fruit in calcium,
waxing fruit or storing the fruit in reduced
oxygen atmospheres, but we have also
tried to do this through genetic engin-
eering, such as the Flavr Savr tomato.
Reducing the expression of a single gene
or even two genes may not always produce
the desired phenotype. Cell-wall ripening
is complex. It may be necessary to
suppress multiple genes or identify
regulatory genes (proteins) that modulate
the expression of several genes that affect
the cell wall. None the less, we now have
the tools to identify all the changes in gene
expression and protein accumulation that
occur during ripening and to use this
information to engineer fruit with very
special characteristics that include
delayed softening to improve transport
and delay the over-ripening response to
extend shelf-life.

References

Allen, C.E. (1901) On the origin and nature of the middle lamella. Botanical Gazette 32, 1-34.
Almeida, D.P.F. and Huber, D.J. (1999) Apoplastic pH and inorganic ion levels in tomato fruit: a
potential means for regulation of cell wall metabolism during ripening. Physiologia Plantarum
105, 506-512.
Asif, M., Trivedi, P. and Solomos, T. (2006) Effects of low oxygen and MCP, applied singly or
together, on apple fruit ripening. Acta Horticulturae 712, 253-260.
Awad, M. and Young, R.E. (1979) Postharvest variation in cellulase, polygalacturonase, and
pectinmethylesterase in avocado ( Persea americana Mill, cv. Fuerte) fruits in relation to
respiration and ethylene production. Plant Physiology 64, 306-308.
Bapat, V.A., Trivedi, P.K., Ghosh, A., Sane, V.A., Ganapathi, T.R. and Nath, P. (2010) Ripening of
fl eshy fruit: molecular insight and the role of ethylene. Biotechnology Advances 28, 94-107.
Ben-Arie, R. and Kislev, N. (1979) Ultrastructural changes in the cell walls of ripening apple and
pear fruit. Plant Physiology 64, 197-202.
Bräutigam, A., Mullick, T., Schliesky, S. and Weber, A.P.M. (2011) Critical assessment of assembly
strategies for non-model species mRNA-Seq data and application of next-generation sequencing
to the comparison of C3 and C4 species. Journal of Experimental Botany 62, 3093-3102.
Brookfi eld, P.L., Nicoll, S., Gunson, F.A., Harker, F.R. and Wohlers, M. (2011) Sensory evaluation by
small postharvest teams and the relationship with instrumental measurements of apple texture.
Postharvest Biology and Technology 59, 179-186.
Brummell, D.A. (2006) Cell wall disassembly in ripening fruit. Functional Plant Biology 33, 103-119.