SAUR63 stimulates cell growth at the plasma membrane

In plants, regulated cell expansion determines organ size and shape. Several members of the family of redundantly acting Small Auxin Up RNA (SAUR) proteins can stimulate plasma membrane (PM) H+-ATPase proton pumping activity by inhibiting PM-associated PP2C.D phosphatases, thereby increasing the PM electrochemical potential, acidifying the apoplast, and stimulating cell expansion. Similarly, Arabidopsis thaliana SAUR63 was able to increase growth of various organs, antagonize PP2C.D5 phosphatase, and increase H+-ATPase activity. Using a gain-of-function approach to bypass genetic redundancy, we dissected structural requirements for SAUR63 growth-promoting activity. The divergent N-terminal domain of SAUR63 has a predicted basic amphipathic α-helix and was able to drive partial PM association. Deletion of the N-terminal domain decreased PM association of a SAUR63 fusion protein, as well as decreasing protein level and eliminating growth-promoting activity. Conversely, forced PM association restored ability to promote H+-ATPase activity and cell expansion, indicating that SAUR63 is active when PM-associated. Lipid binding assays and perturbations of PM lipid composition indicate that the N-terminal domain can interact with PM anionic lipids. Mutations in the conserved SAUR domain also reduced PM association in root cells. Thus, both the N-terminal domain and the SAUR domain may cooperatively mediate the SAUR63 PM association required to promote growth

SAUR63 stimulates cell growth at the plasma membrane

International audienceIn plants, regulated cell expansion determines organ size and shape. Several members of the family of redundantly acting Small Auxin Up RNA (SAUR) proteins can stimulate plasma membrane (PM) H + -ATPase proton pumping activity by inhibiting PM-associated PP2C.D phosphatases, thereby increasing the PM electrochemical potential, acidifying the apoplast, and stimulating cell expansion. Similarly, Arabidopsis thaliana SAUR63 was able to increase growth of various organs, antagonize PP2C.D5 phosphatase, and increase H + -ATPase activity. Using a gain-of-function approach to bypass genetic redundancy, we dissected structural requirements for SAUR63 growth-promoting activity. The divergent N-terminal domain of SAUR63 has a predicted basic amphipathic α-helix and was able to drive partial PM association. Deletion of the N-terminal domain decreased PM association of a SAUR63 fusion protein, as well as decreasing protein level and eliminating growth-promoting activity. Conversely, forced PM association restored ability to promote H + -ATPase activity and cell expansion, indicating that SAUR63 is active when PM-associated. Lipid binding assays and perturbations of PM lipid composition indicate that the N-terminal domain can interact with PM anionic lipids. Mutations in the conserved SAUR domain also reduced PM association in root cells. Thus, both the N-terminal domain and the SAUR domain may cooperatively mediate the SAUR63 PM association required to promote growth

NAAIRS mutants of SAUR63 and root tortuosity index

1 table.S2 Table. NAAIRS mutants of SAUR63 and root tortuosity index.Peer reviewe

Localization of SAUR63:YFP:HA in root meristem epidermal cells compared to control WAVE lines

A-D)P35S:SAUR63:YFP:HA. E-H)P35S:SAUR6326-142:YFP:HA. I-L)PUBQ10:WAVE 138Y expressing a PM-localized YFP fusion protein. M-P) PUBQ10:WAVE 1Y expressing a cytoplasmically-localized YFP fusion protein. Q-T) PUBQ10:WAVE 9Y expressing a YFP fusion protein localized to the tonoplast. Shown are fluorescence confocal microscopy images of YFP (green, A,E,I,M,Q), FM4-64 membrane staining (magenta, B,F,J,N,R), and both channels together (C,G,K,O,S) with vertical yellow lines indicating locations of quantitation of fluorescence intensity signals, scaled to the maximum signal along the line (D,H,L,P,T). Image color channel brightnesses were adjusted for visibility. Scale bar, 20 μm.Peer reviewe

Antibodies

1 table.S5 Table. Antibodies.Peer reviewe

Epistasis among SAUR63 and PP2C.D5 lines, and phenotypes related to PM H+-ATPase activity

A) Appearance of genotypes used for crosses presented in A and in Fig 2C and 2D, grown for 6d in long days in the presence of sucrose. Scale bar, 3 mm. B) Hypocotyl lengths of seedlings of indicated genotypes grown for 4d in short days in the absence of sucrose. Plants measured were F1 progeny of crosses of transgenic lines with each other or with wild-type Columbia, and were hemizygous for the indicated transgene(s). Fig 2C shows a subset of this data. A replicate experiment gave similar results. C) Appearance of PEST:SAUR63:CerFP:HA and wild-type seedlings grown with estradiol and in the absence or presence of 15 mM LiCl. Seedlings were grown for 3d under control conditions, and then transferred to plates with estradiol and with or without 15 mM LiCl, and grown for an additional 3d before imaging. Dots mark positions of root tips at the time of transfer to estradiol plates. Scale bar, 5 mm. D) Root growth of indicated genotypes in the absence (open bars) or presence (closed bars) of 15 mM LiCl. Seedlings were grown without LiCl for 5d, transferred to plates containing 0 or 15 mM LiCl, and root growth over the next three days was measured. E) HPTS fluorescence ratios around root cells of indicated genotypes. Data are pooled from measurements taken on three different days, each normalized to the average of wild-type values on those days. Graphs show means ± s.d. Letters in graphs indicate statistical classes based on Tukey’s Honestly Significant Difference test. n, from left to right: panel B: 28, 24, 24, 30, 26, 27, 26, 25, 26, 25, 24, 20, 24, 26, 25; panel D: 21, 27, 16, 23, 19, 25, 20, 30, 18, 28, 21, 30, 23, 31, 18, 29; panel E: 26, 13, 14, 16.Peer reviewe

Localization of SAUR63:YFP:HA variants in Nicotiana benthamiana leaf cells

1 figure.A-L) Confocal images showing YFP fluorescence of indicated SAUR63:YFP:HA variants expressed in transiently transformed N. benthamiana leaves. Scale bar, 20 μm.Peer reviewe

Seedling phenotypes of plants expressing SAUR63:YFP:HA variants

A) Cotyledon area of seedlings of indicated genotypes grown on vertically oriented plates for 7d in long days in the absence (open bars) or presence (filled bars) of 1% sucrose. B) Root length of seedlings of indicated genotypes grown on vertically oriented plates for 4d in long days in the absence (open bars) or presence (filled bars) of 1% sucrose. C,F) Hypocotyl lengths (E) and hypocotyl tortuosity index [F, 1 –(distance between ends)/(contour length)] of seedlings grown on vertically oriented plates for 3d in darkness without sucrose. D) Cotyledon areas of P35S:SAUR63 and P35S:CBL11-12:SAUR6326-142 seedlings grown for 6d on vertically oriented MS 1% Suc plates. E) Hypocotyl lengths of P35S:SAUR63 and P35S:CBL11-12:SAUR6326-142 seedlings grown for 4d on vertically oriented 0.5x MS plates. Panels D and E show data for the three homozygous single-locus P35S:SAUR63 lines that differed most from wild type among seven lines analyzed. Graphs show means ± s.d. Letters in graphs indicate statistical classes based on Tukey’s Honestly Significant Difference test. n, from left to right: panel A: 26, 20, 25, 15, 22, 23, 24, 20, 22, 23, 19, 19, 21, 19, 21, 20; panel B: 19, 16, 18, 14, 20, 20, 21, 16, 21, 17, 19, 16, 18, 15, 16, 17; panel D: 77, 18, 17, 17, 19, 20; panel E: 119, 25, 24, 27, 21, 24; panels C and F: 35, 36, 45, 43, 47, 38, 41, 47.Peer reviewe

Primers used [Dataset]

1 table.S3 Table. Primers used.Peer reviewe

Clones for transformation

1 table.S4 Table. Clones for transformation.Peer reviewe