Shoot orientation affects vessel size, shoot hydraulic conductivity and shoot growth rate in Vitis vinifera L

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.

Xylem anatomy and hydraulic traits in Vitis grafted cuttings in view of their impact on the young grapevine decline

Grapevine grafting is an essential practice in viticulture and over the years, various bench grafting techniques have been developed to mechanize the nursery process and to increase the yield in number of viable cuttings. Bench grafting is a fundamental nursery practice that can potentially affect the quality of propagation material also in young decline associated to grapevine trunk diseases and has been recently reported to influence leaf symptoms development associated with diseases of Esca complex. The study aimed to investigate how three bench grafting methods [i.e., (i) Omega graft as mechanical technique, (ii) Whip and Tongue graft as manual technique and (iii) Full Cleft graft as semi-mechanical technique] can influence these phenomena. Specifically, the different methods were compared for their effect on the anatomical development of the grafting point and the functionality of the xylem, also considering two factors: the grapevine cultivar (Cabernet Sauvignon, Glera and Teroldego) and the scion/rootstock diameter (thin and large). Observations by light microscopy on the anatomical evolution and measurements on the xylem morphology and hydraulic traits were correlated with the grafting methods and the investigated varieties. The anatomical observations revealed that the mechanical (Omega) and semi-mechanical (Full Cleft) grafting methods have a faster callusing response while the manual technique (Whip and Tongue) has a slower but greater vascularization of the differentiated callus. Significant differences between cultivars and/or grafting types were also detected in necrotic area on the grafted tissues. Statistical analysis of the grapevine vessels suggested differences in xylem parameters between cultivars, while grafting type had no significant effects. On the other hand, the grafting type significantly affected the intrinsic growth rate. The study confirms the potential incidence of lesions and dysfunctionalities correlated with the grafting method applied, which can potentially induce grafted vine declines in vineyards due to the necrotic area detected on the grafted tissues.

Cultivation of arbuscular mycorrhizal Broussonetia papyrifera seedlings by planting the mycorrhizal nurse plant downwards

Plant mycorrhization can be achieved by transplanting new seedlings with mycorrhizal nurse plants; however, this method inevitably induces plant interactions. Transplanting nurse plants downwards may prevent light competition among new seedlings and nurse plants in the same pot. We hypothesized that seedling mycorrhization via mycorrhizal provision from plants planted downwards would be a feasible and efficient strategy. We used seedlings cultivated for 6 months after inoculation with arbuscular mycorrhizal fungi (AMF) as nurse plants, and seedlings cultivated for 1 month without AMF as recipient plants, transplanting one nurse plant and three recipient plants together in one pot. We compared two approaches for cultivating mycorrhizal Broussonetia papyrifera seedlings: planting mycorrhizal nurse plants upwards (M-NU) and downwards (M-ND). We also planted non-mycorrhizal nurse plants upwards (NM-NU) and downwards (NM-ND) as controls. We analyzed growth parameters and the mycorrhizal colonization status of recipient plants at 45, 60, and 75 days after planting (DAP). As expected, the plant growth, gas exchange, and root morphological parameters of recipient plants with mycorrhizal nurse plants were higher than those of recipient plants with non-mycorrhizal nurse plants at 60 and 75 DAP. Furthermore, the AMF colonization status and physiological growth status of M-ND recipient plants were improved compared with M-NU recipient plants. Our results demonstrate that inducing seedling mycorrhization by planting mycorrhizal nurse plants downwards is a feasible strategy for achieving AMF symbiosis while mitigating negative interactions among plants.

Behind the curtain of the compartmentalization process: Exploring how xylem vessel diameter impacts vascular pathogen resistance

A key determinant of plant resistance to vascular infections lies in the ability of the host to successfully compartmentalize invaders at the xylem level. Growing evidence supports that the structural properties of the vascular system impact host vulnerability towards vascular pathogens. The aim of this study was to provide further insight into the impact of xylem vessel diameter on compartmentalization efficiency and thus vascular pathogen movement, using the interaction between Vitis and Phaeomoniella chlamydospora as a model system. We showed experimentally that an increased number of xylem vessels above 100 μm of diameter resulted in a higher mean infection level of host tissue. This benchmark was validated within and across Vitis genotypes. Although the ability of genotypes to restore vascular cambium integrity upon infection was highly variable, this trait did not correlate with their ability to impede pathogen movement at the xylem level. The distribution of infection severity of cuttings across the range of genotype's susceptibility suggests that a risk-based mechanism is involved. We used this experimental data to calibrate a mechanistic stochastic model of the pathogen spread and we provide evidence that the efficiency of the compartmentalization process within a given xylem vessel is a function of its diameter.

Water availability dynamics have long-term effects on mature stem structure in Vitis vinifera

PREMISE OF THE STUDY

The stem of Vitis vinifera, a climbing vine of global economic importance, is characterized by both wide and narrow vessels and high specific hydraulic conductivity. While the effect of drought stress has been studied in 1- and 2-yr-old stems, there are few data documenting effects of drought stress on the anatomical structure of the mature, woody stem near the base of the vine. Here we describe mature wood anatomical responses to two irrigation regimes on wood anatomy and specific hydraulic conductivity in Vitis vinifera Merlot vines.

METHODS

For 4 years, irrigation was applied constantly at low, medium, or high levels, or at alternating levels at two different periods during the growing season, either early spring or late summer, resulting in late season or early spring deficits, respectively. The following variables were measured: trunk diameter, annual ring width and area, vessel diameter, specific hydraulic conductivity and stem water potential.

KEY RESULTS

High water availability early in the season (late deficit) resulted in vigorous vegetative growth (greater trunk diameter, ring width and area), wider vessels and increased specific hydraulic conductivity. High water availability early in the season caused a shift of the vessel population towards the wider frequency classes. These late deficit vines showed more negative water potential values late in the season than vines that received low but relatively constant irrigation.

CONCLUSIONS

We concluded that high water availability during vegetative growth period of Vitis increases vessels diameter and hydraulic conductivity and causes the vines to be more vulnerable to drought stress late in the season.

Hormonal signals involved in the regulation of cambial activity, xylogenesis and vessel patterning in trees

The radial growth of plant stem is based on the development of cribro-vascular cambium tissues. It affects the transport efficiency of water, mineral nutrients and photoassimilates and, ultimately, also plant height. The rate of cambial cell divisions for the assembly of new xylem and phloem tissue primordia and the rate of differentiation of the primordia into mature tissues determine the amount of biomass produced and, in the case of woody species, the wood quality. These complex physiological processes proceed at a rate which depends on several factors, acting at various levels: growth regulators, resource availability and environmental factors. Several hormonal signals and, more recently, further regulatory molecules, have been shown to be involved in the induction and maintenance of cambium and the formation of secondary vascular tissues. The control of xylem cell patterning is of particular interest, because it determines the diameter of xylem vessels, which is central to the efficiency of water and nutrient transport from roots to leaves through the stem and may strongly influence the growth in height of the tree. Increasing scientific evidence have proved the role of other hormones in cambial cell activities and the study of the hormonal signals and their crosstalking in cambial cells may foster our understanding of the dynamics of xylogenesis and of the mechanism of vessel size control along the stem. In this article, the role of the hormonal signals involved in the control of cambium and xylem development in trees and their crosstalking are reviewed.

Self-organisation at the whole-plant level: a modelling study

Within-plant light and nutrient environments are spatially and temporally heterogeneous. The development of different parts of a plant is highly coordinated, which enables the efficient capture and use of resources in such heterogeneous environments. The physiological mechanisms underlying the correlative control of distantly located plant tissues and organs are still not fully understood. In this study, a mathematical model based on a self-organisation mechanism for resource allocation mediated by polar auxin transport is proposed to explain the origin of correlative effects among shoot branches. In the model, the shoot system of an individual plant is treated as a collection of relatively independent modular subunits competing for root-derived resources. The allocation of root-derived resources to different parts of the shoot is determined by their relative vascular contacts with the root system. The development of the vascular network is specified by the polar transport of auxin produced by various parts of the shoot in response to their immediate internal and external environments. The simulation results show that, by altering the amount of auxin they release individually in response to the local environment and modifying their relative vascular contact with the root system, subunits of a shoot are able to coordinate without a central controller and self-organise into functional and structural patterns such as light foraging and correlative dominance. This modelling study suggests that morphological dynamics at the whole-plant level can be understood as the sum of all modular responses to their local environments. The concept of self-organisation holds great promise for an in-depth understanding of the organisational laws that generate overall plant structure and functions.

Mercury hinders recovery of shoot hydraulic conductivity during grapevine rehydration: evidence from a whole-plant approach

This experiment aimed to test whether recovery of shoot hydraulic conductivity after drought depends on cellular metabolism in addition to xylem hydraulics. We rehydrated droughted grapevines (Vitis vinifera) after treating intact plants through the root with 0.5 mm mercuric chloride (a metabolic inhibitor) at the end of the stress period, before rehydration. The contribution of mercury-inhibited water transport in both shoot and root, and the extent of shoot vessel embolization, were assessed. Drought stress decreased plant water potential and induced embolization of the shoot vessels. The rehydration in Hg-untreated plants re-established both shoot water potential and specific shoot hydraulic conductivity (Kss) at levels comparable with watered controls, and induced recovery of most of the embolisms formed in the shoot during the drought. In contrast, in plants treated with HgCl2, recovery of Kss and root hydraulic conductance were impaired. In rehydrated, Hg-treated plants, the effects of Hg on Kss were reversed when either the shoot or the root was treated with 60 mM beta-mercaptoethanol as a mercuric scavenger. This work suggests that plant cellular metabolism, sensitive to mercuric chloride, affects the recovery of shoot hydraulic conductivity during grapevine rehydration by interfering with embolism removal, and that it involves either the root or the shoot level.

Are xylem radial development and hydraulic conductivity in downwardly-growing grapevine shoots influenced by perturbed auxin metabolism?

Downwardly-growing grapevine shoots have smaller and more frequent vessels than upwardly-growing ones and, as a consequence, a lower hydraulic conductivity. Here, grapevine (Vitis vinifera L.) shoot growth orientation was manipulated to test whether downward shoot orientation negatively affects vessel growth in the apex via a shortage of water and nutrients. The orientation of the central vine shoot portion was inverted by two consecutive 135 degrees bends, resulting in double-bent N-shaped vines; the central downward shoot portion was of different lengths in the experimental treatments to induce increasing reductions of shoot conductivity. These treatments reduced shoot conductivity and water flow, but had no effects on vessel development and frequency in the apex. In a second experiment, auxin concentration was assessed in shoots of upwardly- and downwardly-growing plants. IAA concentration at the apical internodes was higher in downwardly-oriented shoots than in shoots growing upwards. In addition, a higher density and a lower vessel diameter were observed in the lower, than the upper side, of the downwardly-oriented shoot, suggesting increased accumulation of auxin in the lower side. These results suggest that the downward orientation induces accumulation of auxin in the apex, which in turn affects the density and the size of the xylem vessels, causing reduction of hydraulic conductivity.