Callose Syndromes

 Cellulose-callose syndrome
When we attempt to synthesize cellulose (1,4--glucan) from UDP-glucose with plant plasma membranes, callose is predominantly formed instead. Although cellulose formed in plants is the most abundant biopolymer on Earth, predominant formation of callose from UDP-glucose by the membranes is a major side product (symptom). This is characteristic of this “syndrome,” which has interested plant biologists ever since the first attempt was made to synthesize cellulose from UDP-glucose using plant enzymes. Callose synthase is often used as a marker for the plasma membrane and is suggested to be an altered form of cellulose synthase. In fact, callose synthase observed at the end of fibrils forms a complex with a doughnut-shaped structure, 20 to 30 nm in diameter, which is similar to the terminal complex for cellulose synthases (1). Recently, studies of the normalization of cellulose synthesis resulted in successful cellulose biosynthesis, which was different from callose synthesis. In addition, the gene for cellulose synthase (CesA) is also different from that of callose synthase (Csl).

Microtubule-callose syndrome
Parallelism of microtubules to callose fibrils and loss of control over fibril orientation following microtubule disruption with propyzamide are characteristics of a “syndrome” that was observed in the glucan synthesis from UDP-glucose in the sheets of the plasma membrane (2). Callose synthase may bind to microtubules or microtubule-associated proteins directly or indirectly via other components of the enzyme complex. This syndrome is related to the fact that cellulose microfibrils are deposited parallel to cortical microtubules in plants.

Wound response-callose syndrome
Callose deposition induced by physiological stress, chemical and mechanical wounds, and pathogen infection of plant cells are characteristics of this “syndrome,” in which callose papillae barriers are believed to physically protect and impede pathogen attacks on plant cells. Although AtCSL5 (AtCalS12) is responsible for callose synthase related to wound response, its knockout mutant did not show a pathogen-induced callose formation but instead gained a phenotype of resistance to pathogens. Nishimura et al. (3) suggest that callose could be an induced defense response with negative feedback regulation by which callose synthesis suppresses the pathway of salicylic acid-dependent disease resistance. The salicylic- and pathogen-responsive genes are upregulated in the mutants, and these genes are hyperinduced after infection. In addition, Jacobs et al. (4) suggest that callose protects fungi during pathogenesis either by facilitating nutrient uptake or by preventing host-pathogen interactions.

Of the many problems associated with callose, the major question remains its actual function in plants.

Takahisa Hayashi (Research Institute for Sustainable Humanosphere, Kyoto University)
References (1) Hayashi T, Read SM, Bussell J, Thelen M, Lin FC, Brown Jr RM, Delmer DP: UDP-Glucose:(13)--glucan synthases from mung bean and cotton. Plant Physiol, 83, 1054-1062, 1987
(2) Hirai N, Sonobe S, Hayashi T: In situ synthesis of -glucan microfibrils on tobacco plasma membrane sheets. Proc Natl Acad Sci USA, 95, 15102-15106, 1998
(3) Nishimura MT, Stein M, Hou BH, Vogel JP, Edwards H, Somerville SC: Loss of a callose synthase results in salicylic acid-dependent disease resistance. Science, 301, 969-972, 2003
(4) Jacobs AK, Lipka V, Burton RA, Panstruga R, Strizhov N, Schulze-Lefert P, Fincher GB: An Arabidopsis callose synthase, GSL5, is required for wound and papillary callose formation. Plant Cell, 15, 2503-2513, 2003
Jun. 30, 2005

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