Specificity of expression of TaCKX family genes in developing plants of wheat and their co-operation within and among organs

Abstract

Multigene families of CKX genes encode cytokinin oxidase/dehydrogenase proteins (CKX), which regulate cytokinin content in organs of developing plants. It has already been documented that some of them play important roles in plant productivity. The presented research is the first step of comprehensive characterization of the bread wheat TaCKX gene family with the goal to select genes determining yield-related traits. The specificity of expression patterns of fifteen formerly annotated members of the TaCKX family was tested in different organs during wheat development. Based on this, the genes were assigned to four groups: TaCKX10, TaCKX5 and TaCKX4, highly specific to leaves; TaCKX3, TaCKX6 and TaCKX11, expressed in various levels through all the organs tested; TaCKX1, TaCKX2.3, TaCKX2.2, TaCKX2.1, TaCKX2.4 and TaCKX2.5 specific to developing spikes and inflorescences; TaCKX9, TaCKX8 and TaCKX7, highly specific to roots. Amplification products of tested genes were mapped to the chromosomes of the A, B or D genome using T. aestivum Ensembl Plants. Based on analysis of TaCKX transcripts as well as encoded amino acids in T. aestivum and Hordeum vulgare the number of CKX genes in wheat was limited to 11 and new numbering of selected TaCKX genes was proposed. Moreover, we found that there were developmental differences in expression of TaCKX in the first and the second spike and expression of some of the genes was daily time dependent. A very high and significant correlation was found between expression levels of TaCKX7 and TaCKX9, genes specific to seedling roots, TaCKX1, TaCKX2.1 and TaCKX2.2, specific to developing spikes, and the group of TaCKX3, 4, 5, 6, 10 and 11, highly expressed in leaves and other organs. The genes also co-operated among organs and were included in two groups representing younger or maturating stages of developing plants. The first group was represented by seedling roots, leaves from 4-week old plants, inflorescences and 0 DAP spikes; the second by developing spikes, 0 DAP, 7 DAP and 14 DAP. The key genes which might determine yield-related traits are indicated and their possible roles in breeding strategies are discussed.

Fig 1. Specificity of TaCKX family gene expression in developing tissues/organs.

The genes are annotated as suggested in Table 1 with the corresponding NCBI accession number in brackets. The relative expression of the genes was measured in: seedling root, well-developed leaf from 4-week-old plant, inflorescence, 0 DAP, 7 DAP and 14 DAP spikes in three cultivars, Kontesa, Ostka, Trappe. The level of expression was related to the mean expression of TaCKX3 in all organs tested set as 1.00. The order of the genes is from the highest to the lowest expression, taking into consideration the specificity of expression. *–significant differences at P<0.05.

Fig 2

TaCKX gene expression in the first (I) and the second (II) developed 7 DAP spike in three cultivars, Kontesa, Ostka, Trappe. The level of expression was related to the mean expression of TaCKX3 set as 1.00. *–significant differences at P<0.05.

Fig 3. Daily time expression of TaCKX family genes in 7 DAP spikes for relatively high expressing genes.

The first spikes were collected at 9:00 am, 12:00 pm and 3:00 pm from three cultivars, Kontesa, Ostka, Trappe. The level of expression was related to the mean expression of TaCKX3 set as 1.00. *–significant differences at P<0.05.

Fig 4. Graphic presentation of co-operation of TaCKX genes in the investigated organs in three cultivars tested.

The genes with the highest expression in tested tissue/organ are at the top of the lists. Correlations of TaCKX gene expression within an organ are marked by lines and among organs by green and white boxes and blue line.