Fumie Omori

Variation for root aerenchyma formation in flooded and non-flooded maize and teosinte seedlings

Morphological and anatomical factors such as aerenchyma formation in roots and the development of adventitious roots are considered to be amongst the most important developmental characteristics affecting flooding tolerance. In this study we investigated the lengths of adventitious roots and their capacity to form aerenchyma in three- and four-week-old seedlings of two maize (Zea mays ssp. mays, Linn.) inbred accessions, B64 and Na4, and one teosinte, Z. nicaraguensis Iltis & Benz (Poaceae), with and without a flooding treatment. Three weeks after sowing and following a seven day flooding treatment, both maize and teosinte seedlings formed aerenchyma in the cortex of the adventitious roots of the first three nodes. The degree of aerenchyma formation in the three genotypes increased with a second week of flooding treatment. In drained soil, the two maize accessions failed to form aerenchyma. In Z. nicaraguensis, aerenchyma developed in roots located at the first two nodes three weeks after sowing. In the fourth week, aerenchyma developed in roots of the third node, with a subsequent increase in aerenchyma in the second node roots. In a second experiment, we investigated the capacity of aerenchyma to develop in drained soil. An additional three teosinte species and 15 maize inbred lines, among them a set of flooding-tolerant maize lines, were evaluated. Evaluations indicate that accessions of Z. luxurians (Durieu & Asch. Bird) and two maize inbreds, B55 and Mo20W, form aerenchyma when not flooded. These materials may be useful genetic resources for the development of flooding-tolerant maize accessions.

Relationship between constitutive root aerenchyma formation and flooding tolerance in Zea nicaraguensis

Background and aimsThe teosinte Zea nicaraguensis, which is adapted to frequently flooded lowlands, is considered a valuable germplasm resource for the development of flooding-tolerant maize. This species can form constitutive root aerenchyma under well-drained conditions. The objectives of this study were to screen Z. nicaraguensis accessions for the capacity to form constitutive aerenchyma, to obtain progeny with differing degrees of aerenchyma formation, and to compare the flooding tolerance of these progeny.MethodsWe evaluated constitutive aerenchyma formation in the root cortex of seedlings of eight accessions and several segregating populations of Z. nicaraguensis. We also evaluated flooding tolerance in lines selected for high or low degrees of constitutive aerenchyma formation.ResultsSeedlings of the eight accessions showed an extremely wide and continuous range of variation in aerenchyma formation. By phenotypic selection within two accessions, we obtained lines with either high or low degrees of constitutive aerenchyma formation. The lines selected for a higher degree of formation showed relatively high flooding tolerance evaluated by shoot dry weight ratio (flooded:control) than those with a lower degree of formation.ConclusionsA greater capacity to form constitutive aerenchyma can enhance flooding tolerance.

A major locus involved in the formation of the radial oxygen loss barrier in adventitious roots of teosinte Zea nicaraguensis is located on the short-arm of chromosome 3

A radial oxygen loss (ROL) barrier in roots of waterlogging-tolerant plants promotes oxygen movement via aerenchyma to the root tip, and impedes soil phytotoxin entry. The molecular mechanism and genetic regulation of ROL barrier formation are largely unknown. Zea nicaraguensis, a waterlogging-tolerant wild relative of maize (Zea mays ssp. mays), forms a tight ROL barrier in its roots when waterlogged. We used Z. nicaraguensis chromosome segment introgression lines (ILs) in maize (inbred line Mi29) to elucidate the chromosomal region involved in regulating root ROL barrier formation. A segment of the short-arm of chromosome 3 of Z. nicaraguensis conferred ROL barrier formation in the genetic background of maize. This chromosome segment also decreased apoplastic solute permeability across the hypodermis/exodermis. However, the IL and maize were similar for suberin staining in the hypodermis/exodermis at 40 mm and further behind the root tip. Z. nicaraguensis contained suberin in the hypodermis/exodermis at 20 mm and lignin at the epidermis. The IL with ROL barrier, however, did not contain lignin in the epidermis. Discovery of the Z. nicaraguensis chromosomal region responsible for root ROL barrier formation has improved knowledge of this trait and is an important step towards improvement of waterlogging tolerance in maize.