Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis    

Abstract

One isoform of callose synthase, Glucan Synthase-Like7 (GSL7), is tightly coexpressed with two isoforms of sucrose synthase (SUS5 and SUS6) known to be confined to phloem sieve elements in Arabidopsis (Arabidopsis thaliana). Investigation of the phenotype of gsl7 mutants of Arabidopsis revealed that the sieve plate pores of stems and roots lack the callose lining seen in wild-type plants. Callose synthesis in other tissues of the plant appears to be unaffected. Although gsl7 plants show only minor phenotypic alterations during vegetative growth, flowering stems are reduced in height and all floral parts are smaller than those of wild-type plants. Several lines of evidence suggest that the reduced growth of the inflorescence is a result of carbohydrate starvation. Levels of sucrose, hexoses, and starch are lower in the terminal bud clusters of gsl7 than in those of wild-type plants. Transcript levels of "starvation" genes expressed in response to low sugars are elevated in the terminal bud clusters of gsl7 plants, at the end of the night, and during an extended night. Pulse-chase experiments with 14 CO 2 show that transport of assimilate in the flowering stem is much slower in gsl7 mutants than in wild-type plants. We suggest that the callose lining of sieve plate pores is essential for normal phloem transport because it confers favorable flow characteristics on the pores.

A mutation in the catalytic subunit of the glycosylphosphatidylinositol transamidase disrupts growth, fertility and stomata formation in Arabidopsis.

“…Whether those developmental processes are regulated by specific GPI-APs or through changes in callose deposition is a question for the future. The callose lining in sieve plate pores is essential for normal phloem transport and as a result for the movement of FLOWERING LOCUS T (Barratt et al, 2011;Rinne et al, 2011).…”

Tie-dyed2Encodes a Callose Synthase That Functions in Vein Development and Affects Symplastic Trafficking within the Phloem of Maize Leaves

“…Other recent research reported that the AtGSL7 gene functioned to synthesize callose in the phloem sieve plates (Barratt et al, 2011;Xie et al, 2011). To examine callose deposition at sieve plates and in response to wounding, we performed aniline blue staining.…”

CHOLINE TRANSPORTER-LIKE1 is required for sieve plate development to mediate long-distance cell-to-cell communication

“…Callose has been shown to be involved in the regulation of the sieve pore diameter. In cals7 mutants reduced callose deposition at the sieve plate leads to a reduced number of open pores per sieve plate, smaller diameter sieve pores and impaired phloem transport 22,23 . But also over accumulation of callose, as shown by the phloem-specific expression of a gain of function mutant of CalS3, inhibits phloem conductivity 24 , suggesting that callose deposition at the sieve plate has to be tightly regulated.…”