Hydraulic consequences of enzymatic breakdown of grapevine pit membranes

Investigating anatomical traits and molecular mechanisms involved in resistance to Pierce's disease. A commentary on 'Xylem-dwelling pathogen unaffected by local xylem vessel network properties in grapevines (Vitis spp.)'

This article comments on:

Ana Clara Fanton, Martin Bouda and Craig Brodersen, Xylem-dwelling pathogen unaffected by local xylem vessel network properties in grapevines (Vitis spp.), Annals of Botany, Volume 133, Issue 4, 1 April 2024, Pages 521–532 https://doi.org/10.1093/aob/mcae016

Xylem-dwelling pathogen unaffected by local xylem vessel network properties in grapevines (Vitis spp.)

BACKGROUND AND AIMS

Xylella fastidiosa (Xf) is the xylem-dwelling bacterium associated with Pierce's disease (PD), which causes mortality in agriculturally important species, such as grapevine (Vitis vinifera). The development of PD symptoms in grapevines depends on the ability of Xf to produce cell-wall-degrading enzymes to break up intervessel pit membranes and systematically spread through the xylem vessel network. Our objective here was to investigate whether PD resistance could be mechanistically linked to xylem vessel network local connectivity.

METHODS

We used high-resolution X-ray micro-computed tomography (microCT) imaging to identify and describe the type, area and spatial distribution of intervessel connections for six different grapevine genotypes from three genetic backgrounds, with varying resistance to PD (four PD resistant and two PD susceptible).

KEY RESULTS

Our results suggest that PD resistance is unlikely to derive from local xylem network connectivity. The intervessel pit area (Ai) varied from 0.07 ± 0.01 mm2 mm-3 in Lenoir to 0.17 ± 0.03 mm2 mm-3 in Blanc do Bois, both PD resistant. Intervessel contact fraction (Cp) was not statically significant, but the two PD-susceptible genotypes, Syrah (0.056 ± 0.015) and Chardonnay (0.041 ± 0.013), were among the most highly connected vessel networks. Neither Ai nor Cp explained differences in PD resistance among the six genotypes. Bayesian re-analysis of our data shows moderate evidence against the effects of the traits analysed: Ai (BF01 = 4.88), mean vessel density (4.86), relay diameter (4.30), relay density (3.31) and solitary vessel proportion (3.19).

CONCLUSIONS

Our results show that radial and tangential xylem network connectivity is highly conserved within the six different Vitis genotypes we sampled. The way that Xf traverses the vessel network may limit the importance of local network properties to its spread and may confer greater importance on host biochemical responses.

Mechanisms of grapevine resilience to a vascular disease: investigating stem radial growth, xylem development and physiological acclimation

BACKGROUND AND AIMS

Plant vascular diseases significantly impact crop yield worldwide. Esca is a vascular disease of grapevine found globally in vineyards which causes a loss of hydraulic conductance due to the occlusion of xylem vessels by tyloses. However, the integrated response of plant radial growth and physiology in maintaining xylem integrity in grapevine expressing esca symptoms remains poorly understood.

METHODS

We investigated the interplay between variation in stem diameter, xylem anatomy, plant physiological response and hydraulic traits in two widespread esca-susceptible cultivars, 'Sauvignon blanc' and 'Cabernet Sauvignon'. We used an original experimental design using naturally infected mature vines which were uprooted and transplanted into pots allowing for their study in a mini-lysimeter glasshouse phenotyping platform.

KEY RESULTS

Esca significantly altered the timing and sequence of stem growth periods in both cultivars, particularly the shrinkage phase following radial expansion. Symptomatic plants had a significantly higher density of occluded vessels and lower leaf and whole-plant gas exchange. Esca-symptomatic vines showed compensation mechanisms, producing numerous small functional xylem vessels later in development suggesting a maintenance of stem vascular cambium activity. Stabilization or late recovery of whole-plant stomatal conductance coincided with new healthy shoots at the top of the plant after esca symptoms plateaued.

CONCLUSIONS

Modified cropping practices, such as avoiding late-season topping, may enhance resilience in esca-symptomatic plants. These results highlight that integrating dendrometers, xylem anatomy and gas exchange provides insights into vascular pathogenesis and its effects on plant physiology.

Ageing-induced shrinkage of intervessel pit membranes in xylem of Clematis vitalba modifies its mechanical properties as revealed by atomic force microscopy

Bordered pit membranes of angiosperm xylem are anisotropic, mesoporous media between neighbouring conduits, with a key role in long distance water transport. Yet, their mechanical properties are poorly understood. Here, we aim to quantify the stiffness of intervessel pit membranes over various growing seasons. By applying an AFM-based indentation technique "Quantitative Imaging" we measured the effective elastic modulus (E effective) of intervessel pit membranes of Clematis vitalba in dependence of size, age, and hydration state. The indentation-deformation behaviour was analysed with a non-linear membrane model, and paired with magnetic resonance imaging to visualise sap-filled and embolised vessels, while geometrical data of bordered pits were obtained using electron microscopy. E effective was transformed to the geometrically independent apparent elastic modulus E apparent and to aspiration pressure P b. The material stiffness (E apparent) of fresh pit membranes was with 57 MPa considerably lower than previously suggested. The estimated pressure for pit membrane aspiration was 2.20+28 MPa. Pit membranes from older growth rings were shrunken, had a higher material stiffness and a lower aspiration pressure than current year ones, suggesting an irreversible, mechanical ageing process. This study provides an experimental-stiffness analysis of hydrated intervessel pit membranes in their native state. The estimated aspiration pressure suggests that membranes are not deflected under normal field conditions. Although absolute values should be interpreted carefully, our data suggest that pit membrane shrinkage implies increasing material stiffness, and highlight the dynamic changes of pit membrane mechanics and their complex, functional behaviour for fluid transport.

Susceptible and resistant olive cultivars show differential physiological response to Xylella fastidiosa infections

Olive quick decline syndrome (OQDS) is a severe disease, first described in Italy in late 2013, caused by strains of Xylella fastidiosa subsp. pauca (Xfp) in susceptible olive cultivars. Conversely, resistant olive cultivars do not develop OQDS but present scattered branch dieback, which generally does not evolve to severe canopy decline. In the present study, we assessed the physiological responses of Xfp-infected olive trees of susceptible and resistant cultivars. Periodic measurements of stomatal conductance (gs) and stem water potential (Ψstem) were performed using a set of healthy and Xfp-infected plants of the susceptible "Cellina di Nardò" and resistant "Leccino" and "FS17" cultivars. Strong differences in Δgs and ΔΨstem among Xfp-infected trees of these cultivars were found, with higher values in Cellina di Nardò than in Leccino and FS17, while no differences were found among healthy plants of the different cultivars. Both resistant olive cultivars showed lower water stress upon Xfp infections, compared to the susceptible one, suggesting that measurements of gs and Ψstem may represent discriminating parameters to be exploited in screening programs of olive genotypes for resistance to X. fastidiosa.

Pathogen Adaptation to the Xylem Environment

A group of aggressive pathogens have evolved to colonize the plant xylem. In this vascular tissue, where water and nutrients are transported from the roots to the rest of the plant, pathogens must be able to thrive under acropetal xylem sap flow and scarcity of nutrients while having direct contact only with predominantly dead cells. Nevertheless, a few bacteria have adapted to exclusively live in the xylem, and various pathogens may colonize other plant niches without causing symptoms unless they reach the xylem. Once established, the pathogens modulate its physicochemical conditions to enhance their growth and virulence. Adaptation to the restrictive lifestyle of the xylem leads to genome reduction in xylem-restricted bacteria, as they have a higher proportion of pseudogenes in their genome. The basis of xylem adaptation is not completely understood; therefore, a need still exists for model systems to advance the knowledge on this topic.

Pathogen-induced hydraulic decline limits photosynthesis and starch storage in grapevines (Vitis sp.)

Xylella fastidiosa (Xf) is the bacterial pathogen responsible for Pierce's Disease (PD) in grapevine (Vitis vinifera L.) and numerous diseases in agriculturally and ecologically important species. Current theory suggests that localized inoculations via insect feeding lead to bacterial spread through the xylem, reducing water transport capacity, leading to declines in productivity, and ultimately death. Yet, the underlying mechanisms of Xf-induced mortality are not fully understood. In this study, we documented the development of PD symptoms over 12-13 weeks postinoculation. Subsequently assessed photosynthetic capacity, starch storage, and stem hydraulics in four grapevine genotypes (two PD-resistant and two PD-susceptible), comparing those physiological changes to control plants. PD-susceptible genotypes showed a coordinated decline in photosynthesis, starch storage, and stem hydraulics, whereas Xf-inoculation led to no change in starch and stem hydraulics in the PD-resistant genotypes. Together these data support the idea of a link between loss of hydraulic conductivity due to tylosis production with a downstream photosynthetic decline and starch depletion in the PD-susceptible genotypes. Our data support the theory that hydraulic failure and carbon starvation underlie plant mortality resulting from PD.