Expression of the pea (Pisum sativum L.) alpha-tubulin gene TubA1 is correlated with cell division activity

Microtubules are thought to be major determinants of plant morphogenesis, t hrough effects on planes of cell division and on directions of differential cell expansion. In differentiation and redifferentiation processes, tubuli n expression may prove a useful early indicator of cell activity. We examin ed the expression and localization of the pea alpha-tubulin gene TubA1 in s itu and in transgenic alfalfa (Medicago sativa) to explore its use as a pro be for plant development, and as a test case for correct developmental expr ession between two legume species commonly compared for studies of symbiosi s with Rhizobium. The TubA1 mRNA was more abundant in root tips and immatur e leaves than in other tissues of pea. The promoter of TubA1 was fused to b eta-glucuronidase (GUS) to analyze alpha-tubulin expression in transgenic a lfalfa. Transient assays indicated that the TubA1 gene is transcribed at mo derate levels compared to the cauliflower mosaic virus (CaMV) 35S promoter. Histochemical staining for GUS activity confirmed a correlation between Tu bA1 expression and cell division in nodules, roots and leaves. TubA1 promot er 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 incorpor ation into the microtubular cytoskeleton, demonstrating the effectiveness o f at least one epitope tag in creating functional tubulin fusions.