TheSalixSmSPR1 Involved in Light-Regulated Cell Expansion by Modulating Microtubule Arrangement

Abstract

Arrangement 28 29 Running title: SPR1 function in MT Elongation and polymerization 30 31 Highlight: Function of microtubule-associated protein SPR1 is directly related to light, and crucial to 32 the balance of tubulin polymerization 33 34Abstract: 35 Light signaling and cortical microtubule (MT) arrays are essential to the anisotropic growth of plant 36 cells. Microtubule-associated proteins (MAPs) function as regulators that mediate plant cell expansion 37 or elongation by altering the arrangements of the MT arrays. However, current understanding of the 38 molecular mechanism of MAPs in relation to light to regulate cell expansion or elongation is limited. 39 Here, we show that the microtubule-associated protein SPR1 is involved in light-regulated directional 40 cell expansion by modulating microtubule elongation in Salix matsudana. Overexpression of SmSPR1 in 41 Arabidopsis results in right-handed helical orientation of hypocotyls in dark-grown etiolated seedlings, 42 whereas the phenotype of transgenic plants was indistinguishable from those of wild-type plants under 43 light conditions. Phenotypic characterization of the transgenic plants showed reduced anisotropic growth 44 and left-handed helical MT arrays in etiolated hypocotyl cells. Protein interaction assays revealed that 45 SPR1, CSN5A (subunits of COP9 signalosome, a negative regulator of photomorphogenesis), and 46 ELONGATED HYPOCOTYL 5 (HY5, a transcription factor that promotes photomorphogenesis) 47 interacted with each other in vivo. The phenotype of Arabidopsis AtSPR1-overexpressing transgenic 48 lines was similar to that of SmSPR1-overexpressing transgenic plants, and overexpression of Salix 49 SmSPR1 can rescue the spr1 mutant phenotype, thereby revealing the function of SPR1 in plants.50 51 Ketwords: Salix matsudana, microtubule, microtubule-associated proteins, protein interaction, light 52 regulation, cell expansion and elongation, SPR153 54 Introduction 55 Plant cells exhibit different development patterns, ranging from skotomorphogenic development in 56 darkness to photomorphogenesis. Cell proliferation of skotomorphogenesis in dark-grown seedlings 57does not significantly change, whereas the hypocotyl rapidly elongates in one direction, and this highly 58 directional cell expansion results in the elongation of this specific organ. This process is accompanied by 59 an apical hook, which is attenuated by pro-plastid differentiation that is essential to the initiation of a 60 newly germinated seedling to push through the soil. In contrast, when seedlings are exposed to light, 61 hypocotyl elongation is limited, petioles and cotyledons open, pro-plasmids develop into chloroplasts, 62 and roots elongate (And and Deng, 1996; Gendreau et al., 1997). Previous studies have demonstrated 63 that many factors mediate cell expansion and elongation in the dark and light, which include multiple 64 photoreceptors, plant hormones, and transcription factors (Castillon et al., 2007; Galvã o and Fankhauser, 65 2015). These studies have largely concern...