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ENHANCED SUBERIN PRODUCTION IN NOVEL POTATO SOMACLONES
PROVIDES PROTECTIVE BIO-BARRIER AGAINST TWO KEY SCAB DISEASES
T. Thangavel A , R.S. Tegg A and C.R. Wilson A
A Tasmanian Institute of Agriculture (TIA), University of Tasmania, 13 St. Johns Ave., New Town, 7008, TAS
Email: tamil.thangavel@utas.edu.au
ABSTRACT. Enhanced resistance to common scab of potato was obtained by somatic cell selection. An added bonus
identified concurrent enhanced resistance to powdery scab, however the mechanism of resistance was not known. This study
sought to compare parental cv. Russet Burbank to three disease resistant somaclonal variants. Glasshouse and field
experiments tested the performance of these clones in the presence of two soil-borne pathogens, Streptomyces scabiei and
Spongospora subterranea f.sp.subterranea. Histological (Fluorescent microscopy) and molecular methods (qRT-PCR) were
used to identify the thickness of phellem (periderm) layer, degree of phellem and lenticel suberisation and relative expression
of three suberin synthetic genes (CYP833A, StKCS6 and POP_A) from pathogen treated tubers. The variants showed a
greater degree of phellem suberisation, and up to 7-fold increased expression of the suberin gene StKCS6 compared to the
parent. We hypothesize that this suberisation provides an enhanced physical structure and important barrier for pathogen
invasion into tubers and is important for enhanced resistance to common and powdery scab.
INTRODUCTION
Common and powdery scabs are important soil-borne potato
diseases and impose substantial impediments to sustainable
and profitable potato production world-wide. Common scab
caused by infection of developing tubers with pathogenic
Streptomyces spp. has resulted in losses of ~4% to the
French fry processing sector, within Australia, through seed
and processing tuber rejection and increased processing
costs (1). Powdery scab is caused by infection of tubers by
the protozoan pathogen S. subterranea. In addition to
induction of tuber lesions, it is thought to cause significant
yield reduction through root infection (2). There are
currently no effective and sustainable management strategies
for either disease. Using somaclonal cell selection
techniques we obtained variants of the cv. Russet Burbank
with extreme resistance to common scab (3). Further
screening identified concurrent resistance to powdery scab
within the majority of these variants, however the
mechanism of resistance was not known. The outer cell wall
(periderm or phellem) of potato tubers are the first defence
to soil-borne pathogens and their physical and chemical
components may determine resistance to pathogen invasion.
One important barrier is suberin, a aliphatic and aromatic
bio-polyester (1). In glasshouse and field studies we
examined the impact of exposure to common and powdery
scab pathogens on phellem thickness and suberin synthesis.
MATERIALS AND METHODS
Disease free mini-tubers were used in all trials. S. scabiei
and S. subterranea inoculation methods were adopted from
previous studies (1, 2). The specific stage of tuber
physiological development was determined by direct
examination (1). Tubers of different ages were harvested
and stored at either -80°C or in FAA (4°C) for gene
expression and histological studies respectively.
Histological studies Fluorescent microscopy was used to
determine the number of phellem layers. Suberisation in
tuber phellem was estimated using the rating scale (0-0% no
suberin,1-0-5%, 2-5-15%, 3-15-30%, 4-30-50%, 5-50-75%,
6-75-90%, 7-95% suberin on all cell layers).
Gene expression studies RNA extraction and qRT-PCR
enabled determination of the relative expression of three
genes involved in suberin synthesis in potato tubers
(StKCS6, CYP833A & POP_A) between the variants and the
parent cultivar following pathogen challenge. Data were
normalised to expression of the internal control r18s.
RESULTS AND DISCUSSION
The Russet Burbank parent had significantly less phellem
layers than two of the selected somaclonal variants whilst
degree of suberisation in all three variants was significantly
higher (Table 1).
Table 1. Comparison of the number of phellem cell layers
and suberisation between variants and the Russet Burbank
parent in glasshouse studies following S. scabiei challenge.
Clone
No. of
phellem layers
phellem
suberisation
R. Burbank (parent) 6.2 0.7
A380
A365
TC10C
8.1
5.5
8.0
1.3
1.5
1.4
LSD (P=0.05) 0.11 0.07
Molecular comparison of elite clone A380 with the parent R.
Burbank shows a 7-fold greater expression of the StKCS6
gene in A380 (Fig. 1). This enhanced expression of a
suberin expression gene supports the histological results in
Table 1.
StKCs6 Cyp833A Pop_A
Suberin Gene
Figure 1. Relative mean fold change in the gene expression
of suberin synthetic genes (CYP863A, StKCS6 & POP_A)
in tubers harvested from variant line A380 and the Russet
Burbank parent.
The observed histological and molecular responses were
seen at key stages of tuber development, when infection is
thought to occur. These results suggest that enhanced
suberisation in disease resistant variants may be important in
restriction of pathogen infection thorugh the peridermal
region and provide resistance to these two tuber diseases.
ACKNOWLEDGEMENTS
TIA (UTAS) for scholarship support for TT.
REFERENCES
1. Khatri, B.B., Tegg, R.S., Brown, P.H. & Wilson, C.R.
(2011) Plant Pathology 60: 776-786.
2. Shah, F.A., Falloon, R.E., Butler, R.C. & Lister, R.A. (2012)
Australasian Plant Pathology 41: 219-228.
3. Wilson, C.R., Tegg, R.S., Wilson, A.J., Luckman, G.A.,
Eyles, A., Yuan, Z.Q., Hingston, L.H. & Conner, A. J.
(2010) Phytopathology 100: 460-467.
7th Australasian Soilborne Diseases Symposium 40
Relative Gene expression
8
7
6
5
4
3
2
1
0
Russet Burbank (Parent)
A380