Sinks for Plant Surplus Carbon Explain Several Ecological Phenomena

Plants engage in many processes and relationships that appear to be wasteful of the high-energy compounds that they produce through carbon fixation and photosynthesis. For example, living trees keep leafless tree stumps alive (i.e. respiring) and support shaded understory trees by sharing carbohydrates through root grafts or mycorrhizal fungal networks. Plants exude a variety of organic compounds from their roots and leaves, which support abundant rhizosphere and phyllosphere microbiomes. Some plants release substantial amounts of sugar via extra-floral nectaries, which enrich throughfall and alter lichen communities beneath the canopy. Large amounts of photosynthetically fixed carbon are transferred to root associates such as mycorrhizal fungi and N-fixing micro-organisms. In roots, some fixed C is respired through an alternative non-phosphorylating pathway that oxidizes excess sugar. Each of these processes is most prevalent when plants are growing under mild-to-moderate deficiencies or nutrients or water, or under high light or elevated atmospheric CO2. Under these conditions, plants produce more fixed carbon than they can use for primary metabolism and growth, and so have ‘surplus carbon’. To prevent cellular damage, these compounds must be transformed into other compounds or removed from the leaf. Each of the above phenomena represents a potential sink for these surplus carbohydrates. The fundamental ‘purpose’ of these phenomena may therefore be to alleviate the plant of surplus fixed C.

Sampling Guidelines and Recommendations for Submitting Samples for Diagnosing Laurel Wilt in Avocado Trees (Persea americana L.)

Laurel wilt (LW) is a vascular disease caused by a fungal pathogen transmitted to avocado trees by several ambrosia beetle species and through root grafts among adjacent avocado trees. A critical part of preventing and controlling plant diseases is determining the causal agent so that the appropriate management practices can be implemented to eradicate or contain the outbreak. Proper sampling is a critical step in disease diagnosis and in the determination of the causal agent of disease. This new 3-page publication of the UF/IFAS Horticultural Sciences Department was written by Jonathan Crane, Romina Gazis, Jeff Wasielewski, Daniel Carrillo, Bruce Schaffer, Fredy Ballen, and Edward Evans.https://edis.ifas.ufl.edu/hs1394

Roots of Cooperation: Can Root Graft Networks Benefit Trees Under Stress?

The occurrence of natural root grafts, the functional union of roots of the same or different trees1–3, is common and shared across tree species2. However, their significance for forest ecology remains little understood. While early research suggested negative effects of root grafting (i.e. increases the risk of pathogen transmission)4,5, recent evidence supports the hypothesis that it is an adaptive strategy that reduces stress6–8 by facilitating resource exchange9,10. Here by analysing mangrove root graft networks, we show evidence of cooperation-associated benefits of root grafting. Grafted trees were found to dominate the upper canopy of the forest, and as the probability of grafting and the frequency of grafted groups increased with a higher environmental stress, the mean group size (number of trees within groups) decreased. While root networks could form randomly (i.e. trees do not actively ‘choose’ neighbours to graft to)11,12, the increased frequency and reduced group sizes in higher-stress environments point to the existence of underlying mechanisms that regulate ‘optimal size’ group selection related to resource use within cooperating groups8,13,14. This work calls for further studies to better understand tree interactions (i.e. network hydraulic redistribution)15 and their consequences for individual tree and forest stand resilience and water-use efficiency.

Rapid Detection of the Laurel Wilt Pathogen in Sapwood of Lauraceae Hosts

Laurel wilt (LW), caused by Raffaelea lauricola (RL), is a vascular fungal disease affecting species in the Lauraceae that has rapidly spread across the United States. This disease has caused significant tree losses in natural forests and Florida’s commercial avocado orchards. RL spreads through ambrosia beetle vectors and root grafts. Early detection and eradication are recommended to contain outbreaks. Therefore, rapid diagnosis is key for the timely implementation of mitigation strategies. Current LW diagnosis can take up to 10 days and involves pathogen isolation and the amplification of two microsatellite regions. To reduce diagnosis time, we optimized the standard PCR-based detection technique and assessed its potential use in the testing of woody samples. We further screened the microsatellite primers IFW and CHK on a higher number of fungal taxa as well as 11 host genotypes. Sensitivity was evaluated using RL-DNA at different concentrations in pure and mixed solutions. There was no cross-amplification in non-RL species. Both primers amplified all tested RL strains (100); however, the IFW primers were more sensitive than the CHK primers. Using the IFW primers, we detected RL in 89% of sapwood samples (76/85). This protocol provides a rapid and effective molecular-based approach that reduces the diagnostic time from 10 days to 24 h. This method can be an important tool for diagnostic laboratories. Altogether, our data and fungal collection, represent a robust foundation for future transferability of this protocol to more sensitive detection technologies (qPCR, LAMP) and for its application to samples from diverse origin (beetles, roots).

Seasonal availability of inoculum of the Heterobasidion root disease pathogen in central Wisconsin

After deposition of airborne basidiospores, the root disease pathogen Heterobasidion irregulare Garbelotto and Otrosina infects fresh conifer stumps and spreads through root grafts or by root contact to adjacent trees. Infection can be prevented, however, by borate application. Because the need for stump protection depends on inoculum availability, spore trapping was conducted (usually biweekly) from September 2009 through December 2011 in three infested plantations of predominantly red pine (Pinus resinosa Aiton) in central Wisconsin. A semiselective medium in Petri plates was exposed for 1 h in daylight at each of four locations in each plantation. After 7–10 days incubation at 20 °C, plates were examined and presence and abundance of colonies of the Spiniger asexual stage were recorded. Heterobasidion irregulare was detected on most dates during the two growing seasons, but colonies were most abundant during late summer, fall, and early winter. Relatively fewer colonies developed on medium exposed during periods of coldest winter temperatures, but colonies of the pathogen did develop frequently on medium exposed at ≤ 5 °C and occasionally on medium exposed at ≤ 0 °C. Biologically based guidelines for stump treatment require additional studies of seasonal factors influencing inoculum availability, in situ spore germination, infection, and establishment of the pathogen.

Natural root grafting in Picea mariana to cope with spruce budworm outbreaks

Spruce budworm (Choristoneura fumiferana Clem.) outbreaks cause extensive mortality and growth reductions throughout boreal forests in eastern North America. As tree vulnerability to defoliation remains partially unexplained by tree and stand attributes, we hypothesized that root grafting might attenuate the negative impact of severe defoliation in tree growth. Two experimental sites in the Abitibi-Témiscamingue region dominated by black spruce (Picea mariana Mill.) were harvested and hydraulically excavated to study tree growth in 36 trees in relation to root grafting and the last spruce budworm outbreak using dendroecological methods. Root grafts reduced the negative effects of defoliation by maintaining stable growth in connected trees during epidemic periods. Among dominant trees, growth releases immediately after the outbreak were uniquely observed in grafted trees. Among suppressed trees, grafted trees tended to grow more than non-grafted trees when defoliation severity was the highest. Carbohydrate transfers through root grafts and enhanced efficiency to acquire resources may explain the better performance of grafted trees under scenarios of limited carbon supply. This study reinforces the growing body of literature that suggests root grafting as a cooperative strategy to withstand severe disturbances and highlights the key role of root grafting in stand dynamics to cope with periodic outbreaks.

Intraspecific Root Grafts and Clonal Growth Within Ailanthus altissima Stands Influence Verticillium nonalfalfae Transmission

Verticillium nonalfalfae, causal agent of Verticillium wilt, is being considered as a biocontrol for the highly invasive Ailanthus altissima in Pennsylvania. This soilborne fungus is extremely virulent on Ailanthus and rapidly transmitted from diseased to healthy trees within Ailanthus stands. The rapid transmission of the fungus could be facilitated by root grafts, but neither root graft formation in Ailanthus nor Verticillium transmission by root grafts in trees has been reported. Here, V. nonalfalfae transmission between diseased and healthy Ailanthus trees via intraspecific root grafts and clonal growth is evaluated. Using air-spade excavation, dye translocation, and root graft inoculations, functional root grafts were detected between Ailanthus trees and transmission of V. nonalfalfae across root grafts demonstrated. Inoculation of one Ailanthus parent stem resulted in 187 root sprouts showing Verticillium wilt symptoms 12 months after inoculation. This study revealed that clonal growth and root grafts, normally advantageous growth habits, leave Ailanthus stands vulnerable to widespread V. nonalfalfae infection. This study also broadens the understanding of the Ailanthus-Verticillium pathosystem, growth strategies of invasive Ailanthus, and epidemiology of Verticillium wilt within trees.

A Geometric Model of the Normal Human Aortic Root and Design of a Fully Anatomic Aortic Root Graft

Objective Available aortic root grafts generally flare outward in the sinus region, and this feature improves procedural ease. However, no current device is based on normal aortic root geometry, and a fully anatomic aortic root graft could further facilitate valve-sparing root operations. Methods To develop a model of the normal human aortic root, high-resolution computed tomographic angiogram images from 11 normal human aortas generated high-density x, y, z coordinates of valve and root structures in Mathematica. Three-dimensional least-squares regression analyses assessed geometry of the aortic valve and root. Shapes and dimensions were quantified, and minor variations in geometry were simplified during graft design. Results Normal aortic valve and root geometry was represented as three leaflet-sinus general ellipsoids nested within a cylindrical aorta. Sinotubular junction diameter was 5 mm larger than the valve base—with a slight funnel-shaped outward commissural flare but cylindrical geometry above the midvalve. The valve base was elliptical, but the midvalve and the sinotubular junction were circular above the midvalve level. Commissural locations on the base circumference were equidistant. On the basis of average three-dimensional geometry, a root graft was designed for root remodeling procedures—to be used with an internal geometric annuloplasty ring of the same design. Conclusions An aortic root graft was designed on the basis of mathematical analyses of computed tomographic angiogram images. The design incorporated three anatomic sinuses, commissural symmetry, and compatibility with geometric ring annuloplasty. The anatomic graft may prove useful for restoring aortic root geometry toward normal during aortic valve and root surgery.

Frequency of root grafting in naturally and artificially regenerated stands of Pinus banksiana: influence of site characteristics

We investigated the frequency of root grafting in naturally and artificially regenerated stands of jack pine ( Pinus banksiana Lamb.) in the western boreal forest of Quebec, Canada. Twelve 30–60 m2 plots were hydraulically excavated to determine effects of site characteristics on frequency and timing of root grafting. Naturally regenerated stands had grafted tree percentages similar to artificially regenerated stands (21%–71% across plots) but greater numbers of root grafts per tree (naturally regenerated, 0.73 graft·tree–1; artificially regenerated, 0.52 graft·tree–1). Mean percentages of grafted trees, number of grafts per tree, and the speed of graft formation were greater in sandy soils (61%, 0.71 graft·tree–1 and 2.43 years, respectively) compared with clay soils (44%, 0.54 graft·tree–1 and 2.97 years, respectively). Proximity of trees was a better predictor of root grafting than stand density, despite many root grafts being found with distant trees (>2 m) in artificially regenerated stands. Our results suggested that root grafts form early in stand development. Even if trees are initially separate entities, this relatively high level of root grafting produces stands where trees are extensively interconnected.