The psychedelic genes of maize redundantly promote carbohydrate export from leaves

Abstract

Whole-plant carbohydrate partitioning involves the assimilation of carbon in leaves and its translocation to nonphotosynthetic tissues. This process is fundamental to plant growth and development, but its regulation is poorly understood. To identify genes controlling carbohydrate partitioning, we isolated mutants that are defective in exporting fixed carbon from leaves. Here we describe psychedelic (psc), a new mutant of maize (Zea mays) that is perturbed in carbohydrate partitioning. psc mutants exhibit stable, discrete chlorotic and green regions within their leaves. psc chlorotic tissues hyperaccumulate starch and soluble sugars, while psc green tissues appear comparable to wild-type leaves. The psc chlorotic and green tissue boundaries are usually delineated by larger veins, suggesting that translocation of a mobile compound through the veins may influence the tissue phenotype. psc mutants display altered biomass partitioning, which is consistent with reduced carbohydrate export from leaves to developing tissues. We determined that the psc mutation is unlinked to previously characterized maize leaf carbohydrate hyperaccumulation mutants. Additionally, we found that the psc mutant phenotype is inherited as a recessive, duplicate-factor trait in some inbred lines. Genetic analyses with other maize mutants with variegated leaves and impaired carbohydrate partitioning suggest that Psc defines an independent pathway. Therefore, investigations into the psc mutation have uncovered two previously unknown genes that redundantly function to regulate carbohydrate partitioning in maize.

Figure 1.—

psc leaves display chlorotic regions that hyperaccumulate starch. (A) Wild-type (WT) leaves have uniform green coloration. (B) Wild-type leaves cleared and IKI stained showing that the leaf contains little starch after the dark period. (C) psc leaves display chlorotic and green tissues. (D) psc leaves cleared and IKI stained showing that chlorotic regions hyperaccumulate starch after the dark period, whereas green regions do not. Free-hand leaf cross-sections of (E) wild-type, (F) psc green, and (G) psc chlorotic tissues. psc chlorotic tissue hyperaccumulates starch in both the bundle sheath and mesophyll cells (arrow in G), whereas wild-type and psc green tissues contain trace amounts of starch in the bundle sheath cells (arrowheads in E and F) after the dark period. Bar, 100 μm for panels E–G.

Figure 2.—

Starch accumulation precedes chlorosis in psc mutant leaves. (A and C) Leaf photographs. (B and D) Same leaves cleared and IKI stained. (A and B) Wild type. (C and D) psc mutant. Arrow points to a region in a psc mutant leaf that has hyperaccumulated starch and progressed to chlorosis, whereas the arrowhead shows a region that has begun to hyperaccumulate starch but has not yet progressed to chlorosis.

Figure 3.—

The psc chlorotic leaf phenotype does not progressively expand. Photographs of the same psc mutant leaf over time. (A) Two days after leaf emergence. (B) Day 7. (C) Day 12. (D) Day 17.

Figure 4.—

High light is required for psc chlorotic tissue formation. (A and C) Wild type (WT). (B and D) psc mutant. (A and B) Leaves from plants grown under 2100 μmol m⁻² sec⁻¹ light. (C and D) Leaves from plants grown under 400 μmol m⁻² sec⁻¹ light.

Figure 5.—

Large and intermediate veins frequently delineate the boundary between psc chlorotic and green tissue. (A) psc mutant leaf showing a chlorotic and green tissue boundary at a large vein (arrow) as well as at an intermediate vein (arrowhead). (B) Cleared and IKI-stained cross-section of the boundary between chlorotic and green tissues marked with an arrow in A revealed it is located at a large vein. (C) Cleared and IKI-stained cross-section of the boundary between chlorotic and green tissues indicated with an arrowhead in A revealed it is located at an intermediate vein. (D) Lower magnification view of a cleared and IKI-stained cross-section of a chlorotic-green tissue boundary located at a large vein (left). (E) Abaxial view of D. In D and E, note the gradient of starch accumulation progressing into the chlorotic tissue (right). Bars, 100 μm.

Figure 6.—

psc mutants have reduced growth of vegetative and reproductive tissues. (A) Photograph of mature psc (right) and wild-type sibling (left) field grown plants. (B) psc tassels (right) display diminished growth compared to wild-type tassels (left). (C) psc ears (right) display reduced size and yield compared to wild-type ears (left).

Figure 7.—

psc and tdy1 exhibit an additive genetic interaction. (A) Photograph of mature wild-type (WT), psc, tdy1, and psc; tdy1 double mutant plants. (B) Leaves from wild type, psc, tdy1, and psc; tdy1 double mutants. (C) Same leaves as shown in B, cleared and IKI stained. (D) Close-up of overlapping tdy1 and psc chlorotic tissues from a psc; tdy1 double mutant. Arrow indicates longitudinally streaked psc chlorotic tissue accumulating anthocyanins. Arrowhead indicates tdy1 chlorotic region. Asterisk indicates severely chlorotic tissue expressing both the psc and tdy1 chlorotic phenotypes and accumulating anthocyanins.