Conditional root expansion mutants of Arabidopsis

Abstract

Regulation of cell expansion is essential to the formation of plant organs. We have characterized 21 mutations, representing six loci, that cause abnormal cell expansion in the root of Arabidopsis thaliana. The phenotype of these mutants is conditional upon the rate of root growth. Calculation of cell volumes indicated that the mutations resulted in defects in either the orientation or the extent of expansion or in both. Analysis of cortical microtubules in the mutants suggested that a shift in the orientation of cell expansion may not be dependent on a change in the orientation of the microtubules. Double mutant combinations resulted in loss of the conditional phenotype suggesting that the genes may act in a similar pathway or encode partially redundant functions.

Fig. 1

Wildtype and CORE seedlings on medium supplemented with 4.5% sucrose, 11 days after germination (DAG). (A) Wild-type WS, (B) lion’s tail-1, (C) cobra-1, (D) quill-1, (E) pom-pom1-1, (F) cudgel-1, (G) pom-pom2-1. Bar, 1 mm.

Fig. 2

Wildtype and CORE single and double mutants grown on medium supplemented with 0.5% sucrose. (A) Wild-type Col, (B) pom-pom1-8, (C) quill-1, (D) lion’s tail-1. Double mutants between (E) pom-pom2-1 and cobra-1, (F) cudgel-1 and pom-pom1-1, (G) cudgel-1 and quill-1, (H) pom-pom2-1 and lion’s tail-1. Bar, 1 mm.

Fig. 3

Optical sections through the roots of seedlings grown on medium supplemented with 0.5% sucrose. Median longitudinal sections through the differentiation zone of (A) wild-type Ws, (B) cobra-3, (C) quill-2, (D) pom-pom1-7, (E) half of the expanded part of the root of lion’s tail-1, (F) unexpanded part of same lion’s tail-1 root, (G) double mutant of cobra-1 and pom-pom2-1, (H) double mutant of cudgel-1 and quill-2, (I) double mutant of cudgel-1 and pom-pom1-1, (J) double mutant of lion’s tail-1 and pom-pom2-1. Bar, 50 μm.

Fig. 4

Optical sections of wildtype and CORE mutant roots of plants grown on medium supplemented with 4.5% sucrose. Median longitudinal sections through the differentiation zone of (A) wild-type Col, (B) lion’s tail-1, (C) cobra-1, (D) quill-1, (E) pom-pom1-1, (F) cudgel-1, (G) pom-pom2-1. Median longitudinal sections through the root tips of (H) wild-type Col, (I) lion’s tail-1, (J) cobra-1, (K) pom-pom1-1 and (L) pom-pom2-1. Bar, 50 μm.

Fig. 5

The effect on cell expansion of a shift from permissive to restrictive conditions. cobra was grown for 8 days on medium supplemented with 0.1% sucrose and root epidermal cells were marked with carbon grains. Seedlings were then transferred to media supplemented with 4.5% sucrose or 0.1% sucrose. (A) cob-1 root tip at 0 time after transfer to 4.5% sucrose. (B) cob-1 at 4 hours after transfer to 4.5% sucrose. (C) cob-1 at 8 hours after transfer to 4.5% sucrose. (D) cob-1 at 12 hours after transfer to 4.5% sucrose. (E) Tracings of root tips shown in A-D with location of carbon grains indicated. Lines connect equivalent regions of the root as evidenced by the presence of distinct carbon grains. Dotted line indicates point above which there is very little abnormal radial expansion. Bar, 100 μm. (F) Tracings of root tips of cob-1 seedlings transferred from 0.1% sucrose to 0.1% sucrose. From left to right, times are 0, 4 and 8 hours after transfer. Bar, 300 μm. Note the carbon grains at the very tip of both sets of roots are on root cap cells and therefore show little movement relative to the tip.

Fig. 6

Optical sections of the root epidermis of seedlings grown on medium supplemented with 4.5% sucrose after immunolocalization of microtubules (A) lion’s tail-1, (B) cobra-1, (C) quill-1, (D) wild-type Col, (E) pom-pom1-1, (F) pom-pom2-1, (G) cudgel-1. Bar, 10 μm.

Fig. 7

Optical sections of CORE double mutants grown on medium supplemented with 4.5% sucrose. Median longitudinal sections through the differentiation zone of double mutants of (A) cudgel-1 and lion’s tail-1, (B) cobra-1 and lion’s tail-1, (C) pom-pom1-1 and lion’s tail-1, (D) pom-pom2-1 and lion’s tail-1, (E) cudgel-1 and quill-2, (F) cudgel-1 and pom-pom1-1. Median longitudinal sections through root tips of double mutants of (G) cudgel-1 and cobra-1, (H) quill-1 and pom-pom1-1, (I) cobra-1 and quill-2, (J) cobra-1 and pom-pom1-10, (K) pom-pom1-1 and pom-pom2-1, (L) cobra-1 and pom-pom2-1. Bar, 50 μm.

Fig. 8

Seedlings of CORE double mutants grown on medium supplemented with 4.5% sucrose approximately 17 DAG. Double mutant of (A) cudgel-1 and lion’s tail-1, (B) quill-3 and lion’s tail-1, (C) pom-pom1-1 and lion’s tail-1, (D) pom-pom2-1 and lion’s tail-1, (E) cudgel-1 and quill-2, (F) cudgel-1 and pom-pom1-1, (G) cudgel-1 and cobra-1, (H) quill-1 and pom-pom1-1, (I) cobra-1 and quill-1, (J) cobra-1 and pom-pom1-10, (K) pom-pom1-1 and pom-pom2-1, (L) cobra-1 and pom-pom2-1. Bar, 1 mm.

Fig. 9

Schematic representation of the three classes of CORE mutants. (A) In lion’s tail cell volume of the outer three layers is reduced as compared to wildtype. (B) In cobra there is approximate conservation of cell volume. Therefore, radial expansion compensates for the reduction in elongation. This leads to a shift in the polarity of expansion. (C) In the other four mutants, represented here by pom-pom1 there is a defect in volume regulation so that the cells attain a greater volume than wildtype. This results from a major increase in radial and circumferential expansion that overcompensates for the reduction in elongation.