Genetics of biotic and abiotic stress resistance: basic concepts

26 Pages·2008·6.98 MB·English

Genetics of biotic and abiotic stress
resistance: basic concepts
luigi cattivelli
Disease resistance
Different types of resistance
Identification of sources of disease resistance
The regions of species diversification/evolution also contain most of the sources of
resistance due to the long co-evolution between plants and pathogens
Sources of resistance can be found in cultivated accessions as well as
in landraces, wild accessions and wild related species
Resistance to powdery mildew in Hordeum
Gene Source chromosome
Mla 5S(1HS)‏‏‏‏
mlt 1S(7HS)‏‏‏‏
Mlf Hordeum spontaneum 1L(7HL)‏‏‏‏
Mlg 4L(4HL)‏‏‏‏
Mlj 7L(5HL)‏‏‏‏
Ml(La)‏‏‏‏ Hordeum laevigatum 2L(2HL)‏‏‏‏
MlHb Hordeum bulbosum 2S(2HS)‏‏‏‏
Mutants fromHordeum
mlo 4L(4HL)‏‏‏‏
vulgare
The Efficiency of resistance is based on two parameters:
1) the degree of resistance 2) The duration/stability of resistance
1) Degree of resistance
Resistance with qualitative genetic bases: they confer a full (or
almost full) resistance against specific races of the pathoges. They
behave as mendelian, often dominant, traits
Resistance with quantitative genetic bases: they usually are
independent on the specific race of the pathogen but
show a non complete level of resistance
From evolutionary point of view this condition limits the evolution of
new virulent strains of the pathogen.
Co-evolution between plant and pathogen
In agricultural systems the diffusions of resistant
cultivars might lead to the evolution of new pathogen
strains/races with newvirulences.
To slow down this process three different strategies
might be considered:
1) Pyramiding: 2 o more R genes in the same
genotype
2) Utilization of different cultivars carrying different
R genes in the same region‏
3) Utilization of multi-lines cultivars
R-genepolyculturesare proposed to give more durable resistance (1) Any pathogen race that can overcome
only one R gene will give rise to a much slower epidemic. (2) Any such pathogen race that undergoes an
additional mutation to overcome another R gene is likely to be less fit than a race that can overcome only
one R gene, becauseavrgenes are likely to encodepathogenicityfactors. (3) Highinoculumofavirulent
races is likely to promote systemic acquired resistance, reducing the susceptibility of the plants. (Curr.
Opin. Plant Biol.2001,vol.4,no4pp.281-287)‏
Plant pathogen interaction
The R-Avr combinations leading to
incompatibility are epistatic toward the
combination leading to compatibility.
Signalling networks triggered by R genes
(Curr OpinBiotech 2003, 14: 177-193)‏
PR proteins PR proteins
PDF1.2
The origin of Avr products
The product of Avr genes
may be involved in the
virulence process in host
without the corresponding
R.
A virulence protein can
became Avr once it has
been recognized by a R
gene.
R genes conserved domains
LRR: leucine rich repeats (consensus XX(L)X(L)XXXX);
- sono ripetizioni in serie di circa 24 aminoacidi contenenti Leu o altri residui idrofobici ad
intervalli regolari
- per la specificità sono importanti gli AA idrofilici esposti
- in proteine di Drosophila, uomo e lievito LRR mediano interazioni proteina-proteina
NBS: nucleotide binding sites = Ploop
- dominio per legame di ATP o GTP
- questo legame può alterare la interazione tra gene R e altre proteine dellavia di trasduzione
del segnale
LZ: leucine zipper, un sottotipo della struttura Coiled Coil
- queste sequenze ripetute facilitano la interazione proteina-proteina, favorendo la formazione
di strutture
coiled-coil
TIR:
- similarità al dominio citoplasmatico della proteina Toll di Drosophila e ai recettori
dell’interleukina 1 dei mammiferi
- questi domini, in seguito a legame con un ligando, causano attivazione di fattori di
trascrizione; è quindi verosimile che anche nei geni R svolgano questa funzione.
The LRR domains (often) confers the race specificity
1) L’allele resistente del gene Pi-ta in riso (resistenza a Magnaporthe grisea) differisce
dall’allele suscettibile per un solo aminoacido presente nel dominio LRR (Ala invece di Ser)
(Plant Cell 2000, 12: 2033-2045)Inoltre è stata dimostrata una interazione diretta tra il dominio
LRR del gene Pi-ta e la proteina di avirulenza di M. grisea Avr-Pita (EmboJ 2000, 19: 4004-
4014)‏
2) in molti sistemi pianta patogeno variazioni nella sequenza LRR ovariazioni nel numero di
ripetizioni di copie LRR sono responsabili di diverse capacità di riconoscimento, come al locus
Cf4/Cf9 in pomodoro.
3) gli alleli L6 e L11 in lino, che conferiscono resistenza a razze diverse di ruggine, differiscono
solo a livello di dominio LRR; inoltre quando i domini TIR e NBS di L6 e L10 sono stati fusi al
dominio LRR di L2, la specificità conferita eraquella di L2 (Plant Cell 1999, 11: 495-506)‏
4) malgrado ciò, gli alleli L6 e L7 hanno identiche LRR e la loro sequenza differisce solo a
livello del dominio TIR; per questi due alleli la regione di specificità è a livello del dominio TIR
(Plant Cell 2000, 12: 1367-1377)‏
Positional cloning of R genes: the Mla locus of barley, 11 R
genes homologues clustered together
Mla
175D16-T7 721K19-R1.1 MWG2083 MWG 2197
9X 100 Kb 140 Kb 120 Kb 2X 3600 gametes
175 D16
721 K19 FrYAC120 ID1
714 K1
80 H14
RGH1
BAC clones from Manchuria (No knownMlaspecificity)‏‏‏‏
RGH2
RGH3 YAC clone fromFranka(Mla-6)‏‏‏‏
Comparative mapping of R gene
homologues in the monocot species
rice, barley, and foxtail millet. A circle
diagram was chosen to visualize
syntenic relationships that align the
genomes of barley (green), rice
(red), and foxtail millet (blue). Map
locations of NBS-LRR genes that
could be mapped in at least two of
the three tested species are given.
Syntenic map positions are marked
by bold red spokes and nonsyntenic
R gene homologue loci are boxed in
black. Clusters containing at least
two highly divergent NBS-LRR genes
in rice and foxtail millet (RHC-A to
RHC-D) are highlighted in the
periphery. Barley chromosomes are
numbered 1H to 7H, rice
chromosomes 1 to 12, and foxtail
millet chromosomes I to IX (PNAS
1998, 95: 370-375)‏.
Abiotic stress tolerance
ABIOTIC STRESSES: situations where environmental stimuli that
normally influence plant development, growth, and productivity, exceed
thresholds (species-specific), damaging the plant
drought
cold (chilling and freezing)‏
salt
heavy metals
heatshock
anoxia
nutrient stress
Stress resistance can be developed through exposition to sub-
optimal growing conditions: the acclimation process (hardening)
Barley plants frozen at -10°°°°C
.
Plant response to drought stress
(generally conserved across species)‏
• Development:
– Growth reduction
– Alterations in flowering times
– Increase in the root/shoot ratio
• Morophological adaptation
– Stomatalclosure
– wilty
– Abscission
• Physiological changes
– Decrease in transpiration
– Reduction of water potential