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2014 ; 114
(6
): 1375-83
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Fruit softening and pectin disassembly: an overview of nanostructural pectin
modifications assessed by atomic force microscopy
#MMPMID25063934
Paniagua C
; Posé S
; Morris VJ
; Kirby AR
; Quesada MA
; Mercado JA
Ann Bot
2014[Oct]; 114
(6
): 1375-83
PMID25063934
show ga
BACKGROUND: One of the main factors that reduce fruit quality and lead to
economically important losses is oversoftening. Textural changes during fruit
ripening are mainly due to the dissolution of the middle lamella, the reduction
of cell-to-cell adhesion and the weakening of parenchyma cell walls as a result
of the action of cell wall modifying enzymes. Pectins, major components of fruit
cell walls, are extensively modified during ripening. These changes include
solubilization, depolymerization and the loss of neutral side chains. Recent
evidence in strawberry and apple, fruits with a soft or crisp texture at
ripening, suggests that pectin disassembly is a key factor in textural changes.
In both these fruits, softening was reduced as result of antisense downregulation
of polygalacturonase genes. Changes in pectic polymer size, composition and
structure have traditionally been studied by conventional techniques, most of
them relying on bulk analysis of a population of polysaccharides, and studies
focusing on modifications at the nanostructural level are scarce. Atomic force
microscopy (AFM) allows the study of individual polymers at high magnification
and with minimal sample preparation; however, AFM has rarely been employed to
analyse pectin disassembly during fruit ripening. SCOPE: In this review, the main
features of the pectin disassembly process during fruit ripening are first
discussed, and then the nanostructural characterization of fruit pectins by AFM
and its relationship with texture and postharvest fruit shelf life is reviewed.
In general, fruit pectins are visualized under AFM as linear chains, a few of
which show long branches, and aggregates. Number- and weight-average values
obtained from these images are in good agreement with chromatographic analyses.
Most AFM studies indicate reductions in the length of individual pectin chains
and the frequency of aggregates as the fruits ripen. Pectins extracted with
sodium carbonate, supposedly located within the primary cell wall, are the most
affected.
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