plant surface microbiology.pdf

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Frank B. Dazzo
response is less structured, but accumulates in a cylindrical zone surrounding
the root 2–4 mm from the root tip. These microscopical observations suggest
that Rhizobium responds chemotactically in situ to different, multiple chemical
gradients in the external environment surrounding the clover root.
3.4 Root Hair Alterations Affecting Their Dynamic Growth Extension
and Primary Host Infection
Quantitative microscopy has played a major role in analyzing the developmental
morphology of white clover root hairs to elucidate the mechanisms of
rhizobial CLOS action in modulating the growth dynamics and symbiont
infectibility of these target host cells (Dazzo et al. 1996a).We performed timelapse
video microscopy of axenic seedling roots treated with nanomolar concentrations
of wild-type R. leguminosarum bv. trifolii CLOSs and grown geotropically
under microbiologically controlled conditions, followed by a
quantitative time-series image analysis of individual root hair growth in the
acquired video-recorded images at 4-s resolution (Dazzo et al. 1996a). This
analysis indicated that the earliest discernible root hair deformations occur
within 2.12±0.65 h after application of the wild-type CLOS, and that the morphological
basis of the dominant type of CLOS-induced Had is a short-range
alteration in direction of polar extension growth of the root hair tip rather
than distortion of an already elongated root hair wall, resulting in a redirection
of tip growth that deviates from the medial axis of the root hair cylinder.
Further studies of quantitative microscopy indicated that CLOS action
extends the growing period of active root hair elongation for ~ 5.2 h beyond
its normal duration without affecting the elongation rate per se (~19 mm/h),
resulting in mature root hairs that are on average about 100 mm longer. This
extended growth period predictably increases the duration in which the root
hair’s “window of infectibility” remains open before cessation of growth. Consistent
with this hypothesis, CLOS action was shown by polarized light
microscopy to induce localized isotropic alterations in the otherwise
anisotropic, ordered crystalline architecture of root hair walls and shown by
phase contrast light microscopy to significantly increase the number of
potential infection sites and promote their infectibility by wild-type R. leguminosarum
bv. trifolii (Dazzo et al. 1996a). These studies gave new information
on the mechanisms of CLOS action that participate in activating root hair
infectibility in the Rhizobium-legume symbiosis.