Arabidopsis SMALL AUXIN UP RNA63 promotes hypocotyl and stamen filament elongation

Abstract

SUMMARYAuxin regulates plant growth and development in part by activating gene expression. Arabidopsis thaliana SMALL AUXIN UP RNAs (SAURs) are a family of early auxin-responsive genes with unknown functionality. Here, we show that transgenic plant lines expressing artificial microRNA constructs (aMIR-SAUR-A or -B) that target a SAUR subfamily (SAUR61-SAUR68 and SAUR75) had slightly reduced hypocotyl and stamen filament elongation. In contrast, transgenic plants expressing SAUR63:GFP or SAUR63:GUS fusions had long hypocotyls, petals and stamen filaments, suggesting that these protein fusions caused a gain of function. SAUR63:GFP and SAUR63:GUS seedlings also accumulated a higher level of basipetally transported auxin in the hypocotyl than did wild-type seedlings, and had wavy hypocotyls and twisted inflorescence stems. Mutations in auxin efflux carriers could partially suppress some SAUR63:GUS phenotypes. In contrast, SAUR63:HA plants had wild-type elongation and auxin transport. SAUR63:GFP protein had a longer half-life than SAUR63:HA. Fluorescence imaging and microsomal fractionation studies revealed that SAUR63:GFP was localized mainly in the plasma membrane, whereas SAUR63:HA was present in both soluble and membrane fractions. Low light conditions increased SAUR63:HA protein turnover rate. These results indicate that membrane-associated Arabidopsis SAUR63 promotes auxin-stimulated organ elongation.

The tissue-specific and developmentally regulated expression patterns of theSAUR41subfamily ofSMALL AUXIN UP RNAgenes

“…8 We found that SAUR41 displayed distinctive expression patterns in Arabidopsis roots; it was normally restricted to the quiescent center and cortex/endodermis initials of primary root meristems. 9 During lateral root development, SAUR41 was also expressed in prospective stem cell niches of lateral root primordial and in expanding endodermal cells surrounding the lateral root primordia.…”

“…9 In the meantime, SAUR36, another clade III SAUR gene of Arabidopsis has been identified as a positive regulator of leaf senescence and a negative regulator of leaf size. 10 In an independent study, SAUR63 localized primarily to the plasma membrane, 7,8 whereas SAUR36 targeted the nucleus.…”

“…Keywords: SAUR, Arabidopsis, auxin transport, cell expansion, root meristem patterning, stomatal development More recent work has suggested that SAUR19 (clade II) and SAUR63 (clade I) promote cell expansion through the modulation of auxin transport. 7,8 Most recently, we presented our work on SAUR41, a clade III SAUR gene with a distinctive expression pattern in root meristems. 9 Overexpression of SAUR41 from the CaMV 35S promoter led to pleiotropic auxin-related phenotypes, including long hypocotyls, increased vegetative biomass and lateral root development in young seedlings and expanded petals and twisted inflorescence stems in adult plants.…”

Arabidopsis SAURs are critical for differential light regulation of the development of various organs

“…Previous studies have shown that ectopic expression of several SAUR family genes resulted in elongated hypocotyls in the light and that plants overexpressing SAUR19 exhibited opened cotyledons in the dark (23)(24)(25). However, several other SAURs have been suggested to inhibit cell expansion in certain organs (26)(27)(28).…”

“…Based on microarray analysis, about half of SAURs are up-regulated by auxin treatment in Arabidopsis (22). Overexpression of several distinct SAUR genes resulted in increased growth of cotyledons, hypocotyls, or roots, respectively (23)(24)(25), whereas a few SAURs have been suggested to negatively regulate cell growth in specific organs (26)(27)(28). Recently, it has been found that SAURs could interact with D-clade type 2C protein phosphatase (PP2C) proteins to inhibit their phosphatase activities, therefore activating…”