Variation in xylem characteristics of botanical races of Persea americana and their potential influence on susceptibility to the pathogen Raffaelea lauricola

Abstract

Avocado (Persea americana), an important fruit crop, is under threat from an invasive disease, laurel wilt. The pathogen, Raffaelea lauricola, spreads rapidly in the xylem of infected trees and causes a lethal vascular wilt. A previous study showed that variation in susceptibility to the disease exists among different races of avocado, with the West Indian race being most susceptible. To help elucidate potential explanations for differences in susceptibility, xylem characteristics were examined for fourteen avocado cultivars from the Guatemalan, Mexican, and West Indian botanical races. Samples of each cultivar were assessed for vessel size, vessel density, vessel aggregation, and xylem-specific potential hydraulic conductivity. The West Indian race had significantly greater mean vessel diameters, mean maximum vessel diameters, and xylem-specific potential hydraulic conductivities than the Guatemalan and Mexican races (p < 0.05), which in turn did not differ for any of these variables (p > 0.05). There were no significant differences among the races for vessel aggregation or vessel density. Cultivars of the Mexican and Guatemalan races generally had smaller mean vessel diameters, mean maximum vessel diameters, and mean xylem-specific potential hydraulic conductivities than the West Indian race; however, there was considerable variation among cultivars of the Mexican race. Statistically significant differences in vessel grouping indices and vessel solitary fractions were evident among some cultivars but to lesser extents than were found for vessel size. This study indicates that larger vessel diameters and greater potential hydraulic conductivities exist in the West Indian, compared with the Guatemalan and Mexican races. We suggest that these attributes may be contributing factors in the greater susceptibility to laurel wilt that is evident in the West Indian race.

Different Responses in Vascular Traits between Dutch Elm Hybrids with a Contrasting Tolerance to Dutch Elm Disease

“…As several previous studies suggest, the survival of infected trees appears to be connected with the size of the vessels [ 45 , 46 , 47 ]. An assessment of constitutive traits, such as vessel characteristics before or at the time of infection, is needed and should be supplemented with the study of induced responses in cambial activity and the production of new vessels that were differentiated under specific stimuli.…”

“…lauricola show different vessel diameters. The most sensitive cultivars have both larger vessel diameters and hydraulic conductivity than the less susceptible cultivars [ 46 ]. However, when comparing swamp bay, avocado and another closely related species, Persea borbonia , swamp bay is less sensitive to the pathogen despite possessing vessels with large diameters.…”

Xylem Embolism and Pathogens: Can the Vessel Anatomy of Woody Plants Contribute to X. fastidiosa Resistance?

“…Even if in some pathosystems the relationship between xylem structure and susceptibility to vascular pathogens is well documented (such as in elm [ 107 , 180 ] and grapevine [ 103 , 125 ]), for other host plants, this correlation is weakly sustained due to several reasons: the lack of literature on emerging pathogens, the remarkable inter- and intraspecific diversity of plants, the environmental conditions that can modify the structure of xylem vessels [ 181 , 182 ], and the pathogens’ variability. However, it is known that plants evolve their defenses against vascular pathogens [ 183 ], such as the ability to confine microorganisms and their eventual toxic compounds via compartmentalization [ 102 , 125 ]. Among the many features of a xylem vessel, diameter is the main element that can influence tolerance or susceptibility to wilt diseases [ 125 ] also because a low amount of tyloses [ 104 ] (and energy) is required for pathogen compartmentalization in small vessels [ 125 ].…”

“…Even on avocado trees ( Persea americana Mill. ), it was found that the cultivar susceptible to the fungus Raffaelea lauricola (Harr., Fraedrich and Aghayeva) possesses vessels with a larger average diameter than the resistant cultivars , but no significant differences were found in vessel aggregation and density [ 102 ] ( Figure 2 C). The hypothesis is that smaller vessels are quicker compartmentalized by the host with gums and tyloses, and a small number of pits with a lower area can limit the spread between vessels of the pathogen [ 108 ].…”

“… Schematization of the pathogen spread (represented in brown color) in the xylem tissue of susceptible or resistant plants: ( A ) xylem of the Phaeomoniella chlamydospora –susceptible grapevine cultivar Thompson Seedless, characterized by wide and long xylem vessels (photo from [ 100 ], bar = 1 mm); ( B ) xylem of the Ophiostoma ulmi –tolerant elm hybrid Dodoens, capable of a fast reaction to infection that includes the formation of much smaller vessels and an increase of their density (photo from [ 101 ], bar = 500 μm); ( C ) comparison of the xylem between the Raffaelea lauricola –susceptible avocado cultivar Simmonds (I and II) and the resistant one Duke (III and IV), in which besides differences in vessel length and diameter (C I and C III), any significant difference in the vessel’s density and organization is reported (photo from [ 102 ], bar = 1500 μm); ( D ) vessel ending (red) at the petiole level, which acts like filter for emboli and pathogens, especially at the junction between leaf and stem; adjacent long and open vessels (blue) can facilitate the circulation of emboli or pathogens. …”