Genotypic variation in Actinidia deliciosa fruit size and carbohydrate content
Creators: name: Nardozza, Simona <1980>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: AGR/03 Arboricoltura generale e coltivazioni arboree

description: In a global and increasingly competitive fresh produce market, more attention is being
given to fruit quality traits and consumer satisfaction. Kiwifruit occupies a niche
position in the worldwide market, when compared to apples, oranges or bananas. It is a
fruit with extraordinarily good nutritional traits, and its benefits to human health have
been widely described.
Until recently, international trade in kiwifruit was restricted to a single cultivar, but
different types of kiwifruit are now becoming available in the market. Effective
programmes of kiwifruit improvement start by considering the requirements of
consumers, and recent surveys indicate that sweeter fruit with better flavour are
generally preferred. There is a strong correlation between at-harvest dry matter and
starch content, and soluble solid concentration and flavour when fruit are eating ripe.
This suggests that carbon accumulation strongly influences the development of kiwifruit
taste.
The overall aim of the present study was to determine what factors affect carbon
accumulation during Actinidia deliciosa berry development. One way of doing this is by
comparing kiwifruit genotypes that differ greatly in their ability to accumulate dry
matter in their fruit. Starch is the major component of dry matter content. It was
hypothesized that genotypes were different in sink strength. Sink strength, by definition,
is the effect of sink size and sink activity.
Chapter 1 reviews fruit growth, kiwifruit growth and development and carbon
metabolism.
Chapter 2 describes the materials and methods used.
Chapter 3, 4, 5 and 6 describes different types of experimental work.
Chapter 7 contains the final discussions and the conclusions
Three Actinidia deliciosa breeding populations were analysed in detail to confirm that
observed differences in dry matter content were genetically determined. Fruit of the
different genotypes differed in dry matter content mainly because of differences in
starch concentrations and dry weight accumulation rates, irrespective of fruit size. More
detailed experiments were therefore carried out on genotypes which varied most in fruit
starch concentrations to determine why sink strengths were so different.
The kiwifruit berry comprises three tissues which differ in dry matter content. It was
initially hypothesised that observed differences in starch content could be due to a larger
proportion of one or other of these tissues, for example, of the central core which is
highest in dry matter content. The study results showed that this was not the case.
Sink size, intended as cell number or cell size, was then investigated. The outer pericarp
makes up about 60% of berry weight in ‘Hayward’ kiwifruit. The outer pericarp
contains two types of parenchyma cells: large cells with low starch concentration, and
small cells with high starch concentration. Large cell, small cell and total cell densities
in the outer pericarp were shown to be not correlated with either dry matter content or
fruit size but further investigation of volume proportion among cell types seemed
justified. It was then shown that genotypes with fruit having higher dry matter contents
also had a higher proportion of small cells. However, the higher proportion of small cell
volume could only explain half of the observed differences in starch content. So, sink
activity, intended as sucrose to starch metabolism, was investigated.
In transiently starch storing sinks, such as tomato fruit and potato tubers, a pivotal role
in carbon metabolism has been attributed to sucrose cleaving enzymes (mainly sucrose
synthase and cell wall invertase) and to ADP-glucose pyrophosphorylase (the
committed step in starch synthesis). Studies on tomato and potato genotypes differing in
starch content or in final fruit soluble solid concentrations have demonstrated a strong
link with either sucrose synthase or ADP-glucose pyrophosphorylase, at both enzyme
activity and gene expression levels, depending on the case. Little is known about
sucrose cleaving enzyme and ADP-glucose pyrophosphorylase isoforms. The
HortResearch Actinidia EST database was then screened to identify sequences
putatively encoding for sucrose synthase, invertase and ADP-glucose
pyrophosphorylase isoforms and specific primers were designed. Sucrose synthase,
invertase and ADP-glucose pyrophosphorylase isoform transcript levels were anlayzed
throughout fruit development of a selection of four genotypes (two high dry matter and
two low dry matter). High dry matter genotypes showed higher amounts of sucrose
synthase transcripts (SUS1, SUS2 or both) and higher ADP-glucose pyrophosphorylase
(AGPL4, large subunit 4) gene expression, mainly early in fruit development. SUS1-
like gene expression has been linked with starch biosynthesis in several crop (tomato,
potato and maize). An enhancement of its transcript level early in fruit development of
high dry matter genotypes means that more activated glucose (UDP-glucose) is
available for starch synthesis. This can be then correlated to the higher starch observed
since soon after the onset of net starch accumulation. The higher expression level of
AGPL4 observed in high dry matter genotypes suggests an involvement of this subunit
in drive carbon flux into starch. Changes in both enzymes (SUSY and AGPse) are then
responsible of higher starch concentrations. Low dry matter genotypes showed
generally higher vacuolar invertase gene expression (and also enzyme activity), early in
fruit development. This alternative cleavage strategy can possibly contribute to energy
loss, in that invertases’ products are not adenylated, and further reactions and transport
are needed to convert carbon into starch. Although these elements match well with
observed differences in starch contents, other factors could be involved in carbon
metabolism control. From the microarray experiment, in fact, several kinases and
transcription factors have been found to be differentially expressed.
Sink strength is known to be modified by application of regulators. In ‘Hayward’
kiwifruit, the synthetic cytokinin CPPU (N-(2-Chloro-4-Pyridyl)-N-Phenylurea)
promotes a dramatic increase in fruit size, whereas dry matter content decreases. The
behaviour of CPPU-treated ‘Hayward’ kiwifruit was similar to that of fruit from low dry
matter genotypes: dry matter and starch concentrations were lower. However, the CPPU
effect was strongly source limited, whereas in genotype variation it was not. Moreover,
CPPU-treated fruit gene expression (at sucrose cleavage and AGPase levels) was
similar to that in high dry matter genotypes. It was therefore concluded that CPPU
promotes both sink size and sink activity, but at different “speeds” and this ends in the
observed decrease in dry matter content and starch concentration. The lower “speed” in
sink activity is probably due to a differential partitioning of activated glucose between
starch storage and cell wall synthesis to sustain cell expansion.
Starch is the main carbohydrate accumulated in growing Actinidia deliciosa fruit.
Results obtained in the present study suggest that sucrose synthase and AGPase
enzymes contribute to sucrose to starch conversion, and differences in their gene
expression levels, mainly early in fruit development, strongly affect the rate at which
starch is therefore accumulated. This results are interesting in that starch and Actinidia
deliciosa fruit quality are tightly connected.