Expansins expression is associated with grain size dynamics in wheat (Triticum aestivum L.)

Abstract

Grain weight is one of the most important components of cereal yield and quality. A clearer understanding of the physiological and molecular determinants of this complex trait would provide an insight into the potential benefits for plant breeding. In the present study, the dynamics of dry matter accumulation, water uptake, and grain size in parallel with the expression of expansins during grain growth in wheat were analysed. The stabilized water content of grains showed a strong association with final grain weight (r(2)=0.88, P <0.01). Grain length was found to be the trait that best correlated with final grain weight (r(2)=0.98, P <0.01) and volume (r(2)=0.94, P <0.01). The main events that defined final grain weight occurred during the first third of grain-filling when maternal tissues (the pericarp of grains) undergo considerable expansion. Eight expansin coding sequences were isolated from pericarp RNA and the temporal profiles of accumulation of these transcripts were monitored. Sequences showing high homology with TaExpA6 were notably abundant during early grain expansion and declined as maturity was reached. RNA in situ hybridization studies revealed that the transcript for TaExpA6 was principally found in the pericarp during early growth in grain development and, subsequently, in both the endosperm and pericarp. The signal in these images is likely to be the sum of the transcript levels of all three sequences with high similarity to the TaExpA6 gene. The early part of the expression profile of this putative expansin gene correlates well with the critical periods of early grain expansion, suggesting it as a possible factor in the final determination of grain size.

Fig. 1.

Volume (filled circles), dry weight (open circles), water content (filled squares), and grain dimensions [length (open squares), width (filled triangles), and height (open triangles)] of grain position 2 in growing seasons 1 (A, D) and 2 (B, E) and grain position 3 (C, F) in season 2.

Fig. 2.

Relationship between grain weight and stabilized grain length of grain position 2 in season 1 and positions 1–4 in growing season 2.

Fig. 3.

Expression profile of fragments encoding expansins sequences of wheat grain pericarps. Four expression patterns were identified: A. Variable expression (no single peak was identified); B. Expression decrease gradually; C. Peak to 61°Cd after anthesis; D. Peak to 186 °Cd after anthesis. Total RNA (2 ng) from grain position 2 collected between 24 °Cd and 325 °Cd after anthesis (2–20 DAA) in growing season 1 was used as a template for RT-PCR analysis by gene-specific primers. The PCR products were separated on a 1% agarose gel and stained with ethidium bromide.

Fig. 4.

Expression profile of fragments encoding expansin sequences of wheat grain pericarp. Total RNA (2 ng) from grain positions 2 (A) and 3 (B) collected between 41 °Cd and 470 °Cd after anthesis (6–34 DAA) in growing season 2 was used as a template for RT-PCR analysis by primers. The PCR products were separated on a 1% agarose gel and stained with ethidium bromide.

Fig. 5.

In situ localization of expansin mRNA on transverse sections of wheat grains (grain position 2) at 5 (A, B) and 10 (C, D) DAA. (A, C) Hybridization with a gene-specific TaExpA6 antisense probe. (B, D) Hybridization with a sense probe. In (A) and (C), arrows identify cells showing hybridization signals. For grains, the following cell types are annotated: pericarp (p), endosperm (e), nucellar projection (np), integuments (in).