Abstract
Plants synthesize carbohydrates in photosynthetic tissues, with the majority of plants transporting sucrose to non-photosynthetic tissues to sustain growth and development. While the anatomical, biochemical, and physiological processes regulating sucrose long-distance transport are well characterized, little is known concerning the genes controlling whole-plant carbohydrate partitioning. To identify loci influencing carbon export from leaves, we screened mutagenized maize plants for phenotypes associated with reduced carbohydrate transport, including chlorosis and excessive starch and soluble sugars in leaves. Carbohydrate partitioning defective1 (Cpd1) was identified as a semi-dominant mutant exhibiting these phenotypes. Phloem transport experiments suggested that the hyperaccumulation of starch and soluble sugars in the Cpd1/+ mutant leaves was due to inhibited sucrose export. Interestingly, ectopic callose deposits were observed in the phloem of mutant leaves, and probably underlie the decreased transport. In addition to the carbohydrate hyperaccumulation phenotype, Cpd1/+ mutants overaccumulate benzoxazinoid defense compounds and exhibit increased tolerance when attacked by aphids. However, double mutant studies between Cpd1/+ and benzoxazinoid-less plants indicate that the ectopic callose and carbon hyperaccumulation are independent of benzoxazinoid production. Based on the formation of callose occlusions in the developing phloem, we hypothesize that the cpd1 gene functions early in phloem development, thereby impacting whole-plant carbohydrate partitioning.
Highlights
The transport of carbohydrates from photosynthetic source tissues to non-photosynthetic sink tissues is crucial for plant growth, development, and yield (Lalonde et al, 2003; Slewinski and Braun, 2010a; Braun et al, 2014; Yadav et al, 2015; Julius et al, 2017)
Phloem transport experiments suggested that the hyperaccumulation of starch and soluble sugars in the Carbohydrate partitioning defective1 (Cpd1)/+ mutant leaves was due to inhibited sucrose export
In Cpd1/+ mutants, both bundle sheath (BS) and M cells contained abundant starch, with the BS cells showing substantial staining (Fig. 3D). These results demonstrate that the mutant leaves hyperaccumulate starch, suggesting that Cpd1/+ mutants are severely limited in the ability to remobilize stored carbon
Summary
The transport of carbohydrates from photosynthetic source tissues (leaves) to non-photosynthetic sink tissues (e.g. roots, reproductive organs, and stem) is crucial for plant growth, development, and yield (Lalonde et al, 2003; Slewinski and Braun, 2010a; Braun et al, 2014; Yadav et al, 2015; Julius et al, 2017). To test directly whether phloem transport of sucrose is altered in the mutant, 14C-labeled sucrose was applied to an abraded region at the tip of a mature source leaf of an intact plant to observe its transport in both wild-type and Cpd1/+ mutant leaves.
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