Abstract
Protein ubiquitination is a posttranslational regulatory process essential for plant growth and interaction with the environment. E3 ligases, to which the seven in absentia (SINA) proteins belong, determine the specificity by selecting the target proteins for ubiquitination. SINA proteins are found in animals as well as in plants, and a small gene family with highly related members has been identified in the genome of rice (Oryza sativa), Arabidopsis (Arabidopsis thaliana), Medicago truncatula, and poplar (Populus trichocarpa). To acquire insight into the function of SINA proteins in nodulation, a dominant negative form of the Arabidopsis SINAT5 was ectopically expressed in the model legume M. truncatula. After rhizobial inoculation of the 35S:SINAT5DN transgenic plants, fewer nodules were formed than in control plants, and most nodules remained small and white, a sign of impaired symbiosis. Defects in rhizobial infection and symbiosome formation were observed by extensive microscopic analysis. Besides the nodulation phenotype, transgenic plants were affected in shoot growth, leaf size, and lateral root number. This work illustrates a function for SINA E3 ligases in a broad spectrum of plant developmental processes, including nodulation.
Highlights
Protein ubiquitination is a posttranslational regulatory process essential for plant growth and interaction with the environment
RING domain-independent dimerization of the Arabidopsis SINAT5 dimer is essential for Ub E3 ligase activity, and the dominantnegative effect of SINAT5DN is due to the formation of inactive heterodimers between SINAT5 and SINAT5DN (Xie et al, 2002)
To investigate the role of seven in absentia (SINA) proteins in nodule development, M. truncatula SINAT5 homologs were isolated by TBLASTX analyses with the M. truncatula Gene Index (MTGI) of the expressed sequence tag database from The Institute for Genomic Research and the available genomic data
Summary
Protein ubiquitination is a posttranslational regulatory process essential for plant growth and interaction with the environment. The name of the protein family is derived from the first member that was characterized, the Drosophila melanogaster SINA that regulates photoreceptor differentiation by targeting the transcription factor Tramtrack for proteasomal degradation (Carthew and Rubin, 1990; Li et al, 1997; Cooper, 2007). Only the SINAT5 gene of Arabidopsis (Arabidopsis thaliana) has been functionally characterized It attenuates the auxin-induced lateral root (LR) formation, because ectopic expression results in a lower LR number (Xie et al, 2002), while ectopic expression of a dominant-negative Cys-49/Ser RING domain mutant of SINAT5 (SINAT5DN) causes more LRs than in wild-type plants. SINAT5 affects LR formation by targeting the NAC1 transcription factor (a member of the NO APICAL MERISTEM/CUP-SHAPED COTYLEDON family) for degradation (Xie et al, 2000, 2002). The SINAT2 gene of Arabidopsis has been shown to interact with the transcription factor AtRAP2.2 involved in carotenogenesis (Welsch et al, 2007)
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