Altered pectin composition in primary cell walls of korrigan, a dwarf mutant of Arabidopsis deficient in a membrane-bound endo-1,4-beta-glucanase

Korrigan (kor) is a dwarf mutant of Arabidopsis thaliana (L.) Heynh. that i s deficient in a membrane-bound endo-1,4-beta -glucanase. The effect of the mutation on the pectin network has been studied in kor by microscopical te chniques associated with various probes specific for different classes of p ectic polysaccharides. The localisation of native crystalline cellulose was also examined using the cellobiohydrolase I-gold probe. The investigations were focused on the external cell walls of the epidermis, a cell layer tha t, in a number of plant species, has been shown to be growth limiting. Anio nic sites associated with pectic polymers were quantified using the cationi c gold probe. Homogalacturonans were quantified using polyclonal anti-polyg alacturonic acid/rhamnogalacturonan I antibodies recognising polygalacturon ic acid, and monoclonal JIM7 and JIM5 antibodies recognising homogalacturon ans with a high or low degree of methyl-esterification, respectively. Rhamn ogalacturonans were quantified with two monoclonal antibodies, LM5, recogni sing beta -1,4 galactan side chains of rhamnogalacturonan I, and CCRCM2. Ou r results show a marked increase in homogalacturonan epitopes and a decreas e in rhamnogalacturonan epitopes in kor compared to the wild type. A substa ntial decrease in cellobiohydrolase I-gold labelling was also observed in t he mutant cell walls. These findings demonstrate that a deficiency in an en do-1,4-beta -glucanase, which is in principle not directly implicated in pe ctin metabolism, can induce important changes in pectin composition in the primary cell wall. The changes indicate the existence of feedback mechanism s controlling the synthesis and/or deposition of pectic polysaccharides in primary cell walls.