The developmental process of xylem embolisms in pine wilt disease monitored by multipoint imaging using compact magnetic resonance imaging.

Toshihiro Umebayashi,Kyoichi Otsuki,Ryo Sotooka,Tomoyuki Haishi,Sule Zuhair,Kenji Fukuda

doi:10.1104/pp.110.170282

Toshihiro Umebayashi, Kyoichi Otsuki + Show 4 more

Open Access

https://doi.org/10.1104/pp.110.170282

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Abstract

In pine wilt disease (PWD), embolized tracheids arise after virulent pine wood nematodes (PWN), Bursaphelenchus xylophilus, invade the resin canal of pine tree; infected pine trees finally die from significant loss of xylem water conduction. We used a compact magnetic resonance imaging system with a U-shaped radio frequency (rf) probe coil to reveal the developmental process of the xylem dysfunction in PWD. Multiple cross-sectional slices along the stem axis were acquired to periodically monitor the total water distribution in each 1-year-old main stem of two 3-year-old Japanese black pines (Pinus thunbergii) after inoculation of PWN. During the development of PWD, a mass of embolized tracheids around the inoculation site rapidly enlarged in all directions. This phenomenon occurred before the significant decrease of water potential. Some patch-like embolisms were observed at all monitoring positions during the experimental period. Patchy embolisms in a cross-section did not expand, but the number of patches increased as time passed. When the significant decrease of water potential occurred, the xylem dysfunctional rate near the inoculation point exceeded 70%. Finally, almost the whole area of xylem was abruptly embolized in all cross-sections along the stem. This phenomenon occurred just after water conduction was mostly blocked in one of the cross-sections. Thus, it appears that the simultaneous expansion of embolized conduit clusters may be required to induce a large-scale embolism across the functional xylem. Consequently, xylem dysfunction in infected trees may be closely related to both the distribution and the number of PWN in the pine stem.

Highlights

In pine wilt disease (PWD), embolized tracheids arise after virulent pine wood nematodes (PWN), Bursaphelenchus xylophilus, invade the resin canal of pine tree; infected pine trees die from significant loss of xylem water conduction
The virulent PWN significantly multiply in the pine stem (Mamiya, 1975; Kiyohara and Suzuki, 1978; Kiyohara and Bolla, 1990), and the leaf-water potential abruptly decreases, inducing death (Hashimoto, 1976; Ikeda et al, 1990; Fukuda et al, 1992a)
On August 4, the xylem water distribution did not change in cross-sections below 23 cm, some wound-induced black areas were observed in cross-sections from 0 to 22 cm

Summary

RESULTS

In the cross-sectional MR images of pine stems, we could identify five different tissues: bark, cambium, the current year’s annual ring, the previous year’s annual ring, and pith. An inoculation wound cutting the edge of the current-year xylem was clearly observed in some cross-sections on July 19 (Fig. 2, arrowhead). On August 4, the xylem water distribution did not change in cross-sections below 23 cm, some wound-induced black areas (embolism) were observed in cross-sections from 0 to 22 cm. In these cross-sections, new white areas (callus and wound xylem) were laid by the neighboring cambium that had started to cover the wound (Fig. 2, small arrow). In all cross-sections on September 1, an embolism of crescent shape was observed just beneath the newly formed xylem (Fig. 2, large arrows). The base leaf-water potential hardly changed on any measurement date, and the lowest value was obtained on July 26 (20.45 MPa)

Monitoring of Embolism Development in the

DISCUSSION

Plant Materials and Nematode Inoculation

Computation of the Embolized Area in Xylem in MR Images

LITERATURE CITED