The Roles Of Gibberellin In Cell Wall Compositional Changes And Arabidopsis Root Elongation

Bioactive gibberellins(GAs)are diterpene plant hormones that are involved in various developmental processes throughout the life cycle of the plant,from seed germination,through leaf expansion,stem elongation,flower induction and development,to fruit set and seed development.Several studies have shown that GA level or signal affects plant response to biotic stress and abiotic stress.The evidences for the role of GA in these processes indicate that GAs has more physiological effects than we could expect.Cell wall protects cell from biotic and abiotic stress conditions,such as wounding and pathogen infection,it is also one of the most important limiting factors for cell elongation and cell proliferation.Cells must double in size prior to dividing but cannot do so independently,because they are physically restrained by adjacent tissues with which they share cell walls.Gibberellin controls cell elongation and cell proliferation in Arabidopsis.There should have some interaction between cell wall and gibberellin.Gibberellin-deficient mutant gal-3 has a shorter stem than wild type.Exogenous GA3 application could completely restore stem elongation.Biomass in gibberellin-deficient plants is less than that in normal plants.Cell wall polysaccharides content in wild-type and gibberellin-deficient plants were tested.The content of cellulose,hemicelluose,pectin and lignin were lower in gibberellin-deficient Arabidopsis plants than that in wild-type.Exogenous GA3 application could restore the content of cellulose,hemicelluose,pectin and lignin in gibberellin-deficient Arabidopsis plants to the wild-type plants levels.Histochemical analysis was conducted using phloroglucin-HCl,ruthenium red and hydroxylamine-ferric chloride to detect lignin,highly methyl-esterified pectins and unesterified pectins,respectively.Data from histochemical analysis were consistent with chemical analysis.A set of cellulose,hemicelluose,pectin and were analysis in gibberellin-deficient and wild-type Arabidopsis plants.The expression of lignin biosynthetic genes(4CL,COMT,CCR,CAD and CCoAOMT),homogalacturonan biosynthetic genes(GAUT1,GAUT7 and GAUT8),xylan biosynthetic genes(FRA8,IRX9,IRX10 and IRX14),xyloglucan biosynthetic genes(XTT1,XTT2 and XTT5)and cellulose synthase(CESA4,CESA7 and CESA8)were lower in gibberellin-deficient Arabidopsis plants as compared with wild-type plants.In addition,PME activity was lower in gibberellin-deficient plants than wild-type plants.Exogenous GA3 application could restore the level of PME activity in gibberellin-deficient plants to the wild-type plants level.These results suggest that gibberellin modulate the compositional changes of cell wall in Arabidopsis.Pectin methylesterases(PMEs)are involved,directly and indirectly,in diverse physiological processes associated with both vegetative and reproductive plant development.However,the influence of gibberellin and light on PME activity is poorly understood.This study was conducted to investigate the effect of gibberellin and light on PME activity in two varieties of peas(low gibberellin content named dwarf pea,high gibberellin content named tall pea).PME activity was lower in dwarf pea than in tall pea.Exogenous GA3 application enhanced PME activity in both dwarf pea and tall pea.Exogenous GA3 application could restore the level of PME activity in dwarf pea to the tall pea level.Further experiments showed that the effect was dose dependent.Dark-grown peas had a lower PME activity than light-grown peas.There was a decreasing trend of PME activity when etiolated seedlings were transferred into light.This trend lasted for at least for 24 hours.Applied GA3 repressed light-induced inhibition of PME activity.PME activity of etiolated seedling was partially decreased when paclobutrazol,a GA biosynthesis inhibitor,was applied.These results suggested that gibberellin might interact with light in the regulation of PME activity.Variation in cell elongation is of value in studying the interactions of phytohormones.Both auxin and gibberellin are involved in cell elongation.Auxins influence the biosynthesis and signaling of gibberellins(GAs).GAs-induced changes in auxin transport have been observed in several developmental processes,such as lateral root formation and adventitious rooting.However,the influence of GAs on auxins in root elongation is poorly understood.This study was conducted to investigate the effect of GA3 on Arabidopsis root elongation in the presence of auxin.Root elongation was inhibited in roots treated with both IAA and GA3,compared to IAA alone.The effect was dose dependent.Further experiments showed that GA3 can modulate auxin signaling based on the root elongation in auxin-signaling mutants and the expression of auxin-responsive reporters.The GA3-enhanced inhibition of root elongation observed in the wild-type was not found in the auxin signaling mutants tir1-1 and axr1-5.GA3 increased DR5::GUS expression in the root meristem and elongation zones,and IAA2::GUS in the columella.The DR5rev::GFP signal was enhanced in columella cells of the root caps and the elongation zone in GA3-treated seedling roots.A reduction was observed in the stele of PAC-treated roots.We also examined the effect of GA3 on auxin transport.The enhanced responsiveness caused by GA3 was not observed in the auxin influx mutant auxl-7 or efflux mutant eir1-1.Additional molecular data demonstrated that GA3 could promote auxin transport via AUX1 and PIN proteins.However,GA3-induced PIN gene expression did not fully explain GA-enhanced PIN protein accumulation.These results suggest that GA3 is involved in the auxin-mediated primary root elongation by modulating auxin signaling and transport,and,thus,enhances root responsiveness to exogenous IAA.Our study indicates that gibberellin might be involved in other developmental processes through modulating auxin metabolism.