Pectin Synthesis and Pollen Tube Growth in Arabidopsis Involves Three GAUT1 Golgi-Anchoring Proteins: GAUT5, GAUT6, and GAUT7

Abstract

The major cell wall pectic glycan homogalacturonan (HG) is crucial for plant growth, development, and reproduction. HG synthesis occurs in the Golgi and is catalyzed by members of the galacturonosyltransferase (GAUT) family with GAUT1 being the archetypal and best studied family member. In Arabidopsis suspension culture cells and tobacco leaves, the Golgi localization of Arabidopsis GAUT1 has been shown to require protein-protein interactions with its homolog GAUT7. Here we show that in pollen tubes GAUT5 and GAUT6, homologs of GAUT7, also target GAUT1 to the Golgi apparatus. Pollen tube germination and elongation in double homozygous knock-out mutants ( gaut5 gaut6 , gaut5 gaut7 , and gaut6 gaut7 ) are moderately impaired, whereas gaut5 −/− gaut6 −/− gaut7 +/− triple mutant is severely impaired and male infertile. Amounts and distributions of methylesterified HG in the pollen tube tip were severely distorted in the double and heterozygous triple mutants. A chimeric protein comprising GAUT1 and a non-cleavable membrane anchor domain was able to partially restore pollen tube germination and elongation and to reverse male sterility in the triple mutant. These results indicate that GAUT5, GAUT6, and GAUT7 are required for synthesis of native HG in growing pollen tubes and have critical roles in pollen tube growth and male fertility in Arabidopsis.

Recent Advances in Understanding the Roles of Pectin as an Active Participant in Plant Signaling Networks

“…Pectins—structurally complex polysaccharides including HG, RG-I, and RG-II—are synthesized in the Golgi apparatus and secreted into the cell wall [ 10 , 14 ]. HG, which is composed of approximately 65% pectin, is synthesized by the well-characterized glucuronosyltransferase (GAUT) family of enzymes [ 15 , 16 ]. GAUT transfers galacturonic acid (GalA) from uridine-diphosphate-GalA (UDP-GalA) onto HG acceptors [ 17 ].…”

“…GAUT1 and GAUT7 form a complex, and the retention of GAUT1 in the Golgi is dependent on GAUT7, suggesting a non-catalytic GAUT1-anchoring role for GAUT7 [ 16 ]. HG synthesis via GAUT heterocomplexes was recently reported in pollen tubes: GAUT5, GAUT6, and GAUT7 are redundantly required to localize GAUT1 to the Golgi and for HG biosynthesis and male gametophyte development [ 15 ]. The gaut5 and gaut7 single mutants, the double mutants ( gaut5 gaut6 , gaut5 gaut7 , and gaut6 gaut7 ), and the heterozygous triple mutant ( gaut5 gaut6 gaut7 +/− ) show increasingly severe defects in pollen tube elongation, resulting in male infertility in the heterozygous triple mutant [ 15 ].…”

“…HG synthesis via GAUT heterocomplexes was recently reported in pollen tubes: GAUT5, GAUT6, and GAUT7 are redundantly required to localize GAUT1 to the Golgi and for HG biosynthesis and male gametophyte development [ 15 ]. The gaut5 and gaut7 single mutants, the double mutants ( gaut5 gaut6 , gaut5 gaut7 , and gaut6 gaut7 ), and the heterozygous triple mutant ( gaut5 gaut6 gaut7 +/− ) show increasingly severe defects in pollen tube elongation, resulting in male infertility in the heterozygous triple mutant [ 15 ]. The specialization of different GAUT family members for scaffolding and/or catalytic functions may contribute to the ability of plant cells to generate complex and variable cell walls.…”

Holistic insights from pollen omics: co-opting stress-responsive genes and ER-mediated proteostasis for male fertility

“…HG is synthesized by several Golgi-associated galacturonosyl transferases (GAUT) in pollen, and loss of GAUT activity results in impaired pollen germination and tube elongation ( Atmodjo et al, 2013 ). For example, GAUT1 interacts with either GAUT5, 6, or 7 to mediate activity, and male infertility is severe in triple mutants ( gaut5,6,7 ; Lund et al, 2020 ). These four GAUT genes are expressed in developing pollen (uninucleate microspore [UNM], BCP, and TCP) and functionally distinct from two others, GAUT13 and GAUT14, which are preferentially expressed in MP and in PTs ( Figure 3B ).…”

Influence of osmotic condition on secondary cell wall formation of xylem vessel cells induced by the master transcription factor VND7

“…In cell morphogenesis, the cell wall plays a crucial role in elongation and expansion of growing cells. For instance, pollen tube cells that elongate by tip growth actively synthesize and secrete pectin polysaccharides (PCW components) into the tip region of the cells (Chebli et al 2012;Lund et al 2020). The homogalacturonan (HG) domain of pectin polysaccharides is highly methylesterified at the tip region, whereas a lower degree of methylesterifed HG is preferentially distributed along the remainder of the pollen tube PCW in Arabidopsis and tobacco pollen tubes (Bosch et al 2005;Mravec et al 2017).…”