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gene activation, microtubules, morphogenesis, plant transformation, symbiosis
Abstract:
Microtubules are thought to be major determinants of plant morphogenesis, through effects on planes of cell
division and on directions of differential cell expansion. In differentiation and redifferentiation processes, tubulin
expression may prove a useful early indicator of cell activity. We examined the expression and localization of the
pea -tubulin gene TubA1 in situ and in transgenic alfalfa (Medicago sativa) to explore its use as a probe for plant
development, and as a test case for correct developmental expression between two legume species commonly compared
for studies of symbiosis with Rhizobium. The TubA1 mRNA was more abundant in root tips and immature
leaves than in other tissues of pea. The promoter of TubA1 was fused to -glucuronidase (GUS) to analyze -
tubulin expression in transgenic alfalfa. Transient assays indicated that the TubA1 gene is transcribed at moderate
levels compared to the cauliflower mosaic virus (CaMV) 35S promoter. Histochemical staining for GUS activity
confirmed a correlation between TubA1 expression and cell division in nodules, roots and leaves. TubA1 promoter
activity was first detected in the inner cortex of the root between 18 h and 24 h after spot inoculation with Rhizobium
meliloti. Expression of a c-myc epitope fused to the carboxy-terminus of TubA1 resulted in an incorporation into
the microtubular cytoskeleton, demonstrating the effectiveness of at least one epitope tag in creating functional
tubulin fusions.
