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

Abstract

The tie-dyed2 (tdy2) mutant of maize (Zea mays) displays variegated green and yellow leaves. Intriguingly, the yellow leaf tissues hyperaccumulate starch and sucrose, the soluble sugar transported long distance through the phloem of veins. To determine the molecular basis for Tdy2 function, we cloned the gene and found that Tdy2 encodes a callose synthase. RNA in situ hybridizations revealed that in developing leaves, Tdy2 was most highly expressed in the vascular tissue. Comparative expression analysis with the vascular marker maize PINFORMED1a-yellow fluorescent protein confirmed that Tdy2 was expressed in developing vein tissues. To ascertain whether the defect in tdy2 leaves affected the movement of sucrose into the phloem or its long-distance transport, we performed radiolabeled and fluorescent dye tracer assays. The results showed that tdy2 yellow leaf regions were defective in phloem export but competent in long-distance transport. Furthermore, transmission electron microscopy of tdy2 yellow leaf regions showed incomplete vascular differentiation and implicated a defect in cell-to-cell solute movement between phloem companion cells and sieve elements. The disruption of sucrose movement in the phloem in tdy2 mutants provides evidence that the Tdy2 callose synthase functions in vascular maturation and that the vascular defects result in impaired symplastic trafficking into the phloem translocation stream.

Symplastic Phloem Loading in Poplar

“…The primers used for quantification are listed in Supplemental Table S2. Callose staining followed the procedure in Slewinski et al, 2012.…”

Sucrose Transporter ZmSut1 Expression and Localization Uncover New Insights into Sucrose Phloem Loading

“…Although apoplasmic phloem loading in maize is physiologically and anatomically well established Heyser et al, 1978;Slewinski et al, 2009Slewinski et al, , 2012, the exact role of ZmSut1 in this process is not well defined. Specifically, does ZmSUT1 mediate Suc phloem loading?…”

Increased Expression of a Phloem Membrane Protein Encoded by NHL26 Alters Phloem Export and Sugar Partitioning in Arabidopsis

“…In maize, the tie dyed1 (Tdy1) mutant is impaired in a regulatory process that affects loading and consequently has a general defect of sugar export (Braun et al, 2006;Ma et al, 2008Ma et al, , 2009). The maize Tdy2 mutant, in which a callose synthase is defective, also displays impaired phloem transport (Slewinski et al, 2012). The Arabidopsis pho3 mutant, which has a weak allele of suc2, has a deficiency of the Suc transporter SUC2 (Gottwald et al, 2000;Zakhleniuk et al, 2001), which controls the influx of Suc into the CC-SE complex.…”