A Causal Gene for Seed Dormancy on Wheat Chromosome 4A Encodes a MAP Kinase Kinase.

Seed germination under the appropriate environmental conditions is important both for plant species survival and for successful agriculture. Seed dormancy, which controls germination time, is one of the adaptation mechanisms and domestication traits [1]. Seed dormancy is generally defined as the absence of germination of a viable seed under conditions that are favorable for germination [2]. The seed dormancy of cultivated plants has generally been reduced during domestication [3]. Bread wheat (Triticum aestivum L.) is one of the most widely grown crops in the world. Weak dormancy may be an advantage for the productivity due to uniform emergence and a disadvantage for the risks of pre-harvest sprouting (PHS), which decreases grain quality and yield [4]. A number of quantitative trait loci (QTLs) controlling natural variation of seed dormancy have been identified on various chromosomes [5]. A major QTL for seed dormancy has been consistently detected on chromosome 4A [6-13]. The QTL was designated as a major gene, Phs1, which could be precisely mapped within a 2.6 cM region [14]. Here, we identified a mitogen-activated protein kinase kinase 3 (MKK3) gene (designated TaMKK3-A) by a map-based approach as a candidate gene for the seed dormancy locus Phs1 on chromosome 4A in bread wheat. Complementation analysis showed that transformation of a dormant wheat cultivar with the TaMKK3-A allele from a nondormant cultivar clearly reduced seed dormancy. Cultivars differing in dormancy had a single nonsynonymous amino acid substitution in the kinase domain of the predicted MKK3 protein sequence, which may be associated with the length of seed dormancy.

Characterization of three homoeologous cDNAs encoding chloroplast-targeted aminolevulinic acid dehydratase in common wheat.

In the tetrapyrrole biosynthetic pathway of higher plants, 5-aminolevulinic acid (ALA) is metabolized by ALA dehydratase (ALAD). Here, we isolated ALAD1 cDNA from common wheat (Triticum aestivum L.) and its diploid progenitors, and produced transgenic tobacco plants expressing the wheat ALAD1 gene. The ALAD1 genes were highly conserved among wheat relatives, and three homoeologous loci of wheat ALAD1 (TaALAD1) were equally transcribed in common wheat. A transient expression assay of a TaALAD1-GFP (green fluorescent protein) fusion protein suggested that TaALAD1 is localized in chloroplasts. Overexpression of TaALAD1 in transgenic tobacco resulted in a significant increase in ALAD activity in leaves. Moreover, the transgenic tobacco showed vigorous growth and increased survival rate on medium containing ALA at herbicidal concentrations. These results indicate that wheat ALAD1 has catalytic activity in metabolizing ALA in plastids, and that ectopic expression of TaALAD1 in transgenic plants increases their tolerance to ALA application at high concentrations.