Disruption of auxin transport is associated with aberrant leaf development in maize

Abstract

Despite recent progress, the mechanisms governing shoot morphogenesis in higher plants are only partially understood. Classical physiological studies have suggested that gradients of the plant growth regulator auxin may play a role in controlling tissue differentiation in shoots. More recent molecular genetic studies have also identified knotted1 like homeobox (knox) genes as important regulators of shoot development. The maize (Zea mays L.) mutant rough sheath2 (rs2) displays ectopic expression of at least three knox genes and consequently conditions a range of shoot and leaf phenotypes, including aberrant vascular development, ligular displacements, and dwarfism (R. Schneeberger, M. Tsiantis, M. Freeling, J.A. Langdale [1998] Development 125: 2857-2865). In this report, we show that rs2 mutants also display decreased polar auxin transport in the shoot. We also demonstrate that germination of wild-type maize seedlings on agents known to inhibit polar auxin transport mimics aspects of the rs2 mutant phenotype. The phenotype elaborated in inhibitor-treated plants is not correlated with ectopic KNOX protein accumulation.

Figure 1

Perturbed auxin transport in mesocotyls of rs2 mutant seedlings. Acropetal transport (black bars) and basipetal (white bars) transport of exogenously supplied C-14 IAA in wild-type and mutant (rs2-twd) seedlings. In wild-type plants, a significant difference is seen between the acropetal and basipetal measurements, demonstrating the presence of active polar basipetal transport mechanisms. In mutant plants, no significant difference is seen between basipetal and acropetal measurements, suggesting that the ability to basipetally transport auxin is significantly reduced in mutant tissue.

Figure 2

Effects of PAT inhibitors on maize seedling growth. Dwarfism and twisting: A and C, Plants on the left have been germinated on control medium and allowed to grow in sterile pots for 2 weeks; plants on the right have been germinated in the presence of 28 μm TIBA and grown for the same time. B, The plant on the left is a wild-type sibling of the rs2 mutant plant shown on the right. Displaced ligule formation: D, Seedling leaf of an untreated wild-type plant. White arrow points to the ligule. Leaf twisting and aberrant ligular formation in a TIBA-treated wild-type plant (E) and in a rs2 mutant plant (F). White arrows point to the non-discrete ligular boundary. Hypertrophic vascularization: G, Vascular morphology of a lateral vein in an untreated wild-type seedling. Vascular hypertrophy seen in a TIBA-treated plant (H) and in a rs2 mutant (I) plant. Xylem and phloem are labeled X and P, respectively. Gray lines indicate the edge of the phloem in each case. Bar = 30 μm.

Figure 3

Precocious axillary bud development in rs2 mutant seedlings. A, Median section through the apical region of a 10-d-old wild-type seedling. Bar = 1 mm. B, Median section through the apical region of a 10-d-old rs2 mutant seedling. Arrowheads point toward the axillary buds. Size bar = 1 mm. C, Number of axillary buds developed in 10-d-old wild-type (wt) and rs2 mutant seedlings.

Figure 4

KNOX protein accumulation patterns in TIBA-treated wild-type plants. A, Median section through the apical region of a 10-d-old untreated wild-type seedling. B, Median section through the apical region of a 10-d-old TIBA-treated wild-type seedling. Arrows denote the position of axillary meristems. C, Transverse section through the apical region of a 10-d-old TIBA-treated wild-type seedling. Bars = 100 μm.