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

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

Effect of mutating the SAUR63 clade

A) Genomic map showing positions of genes and locations of mutations from CRISPR/Cas9 mutagenesis in the 9x-saur mutant based on the TAIR10 Arabidopsis genome annotation. The first sgRNA directed cuts in both SAUR61 and SAUR64 (green arrows), creating a deletion between them (green bar) and leaving behind a hybrid gene with a frameshift at the junction site (symbolized by a green X). The second sgRNA directed cuts in the remaining genes (blue arrows, with lighter blue indicating slight mismatches between the sgRNA and the genome), leading to deletions (blue bars) and/or frameshift mutations (blue X’s). SAUR gene names are abbreviated as S61 etc. SAUR61-SAUR68 are on chromosome 1 and SAUR75 is on chromosome 5. B,C) 5-day-old seedlings grown on 1x MS/1% Suc medium in long days. Scale bar, 1 mm. D) Hypocotyl lengths of seedlings grown for 4d in short days on 0.5x MS medium. n, 27 (wild type), 22 (9x-saur). E) Cotyledon area of seedlings grown on vertically oriented plates for 6d on MS/1% Suc medium. n, 16 (wild type), 22 (9x-saur). Graphs show means ± s.d. No statistical differences were detected between wild-type and 9x-saur mutant measurements by t-test. F) Sequences of guide RNAs used for CRISPR/Cas9-mediated mutagenesis, wild-type genes, and mutant alleles present in the 9x-saur mutant. Underlines indicate PAM motif adjacent to guide RNA target site, and any mismatches to the guide RNA sequence. Uppercase bold letters indicate insertion mutations. All alleles create frameshift mutations except for saur75-1, which has an in-frame deletion of 13 amino acids in the SAUR domain.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

Primers used [Dataset]

1 table.S3 Table. Primers used.Peer reviewe

Seedling phenotypes of plants expressing SAUR63:X fusion proteins

A,B) Hypocotyl length (A) and hypocotyl tortuosity index [B, 1 –(distance between ends)/(contour length)]) of seedlings of indicated genotypes grown for 3d in darkness on plates with 0.5x MS medium. C,D) Root length (C) and root tortuosity index (D) of seedlings of indicated genotypes grown for 4d in long days on plates with 1x MS 1% Sucrose medium. E,F) Root length (E) and root tortuosity index (F) of seedlings of indicated genotypes grown for 4d in short days on plates with 0.5x MS medium. Graphs show means ± s.d. Letters in graphs indicate statistical classes based on Tukey’s Honestly Significant Difference test. n from left to right: Panels A,B: 51, 24, 26, 28, 23, 25, 25; Panels C,D: 34, 17, 23, 14, 19, 15, 16; Panels E,F: 43, 21, 18, 17, 16, 15, 17. The same genotypes were measured in panels A-F, with genotype designations shown only in panels E and F. G,H,I) Hypocotyl epidermal cells of seedlings of indicated genotypes grown in short days for 2 days, visualized with the ML1:RFP shoot epidermis plasma membrane marker. Shown are z-stack confocal image projections of the near side of the hypocotyl. Scale bar, 0.1 mm. S2 Fig shows measurements of cell sizes in this experiment.Peer reviewe