The same mobile protein governs seed size and infloresence structure

A staggering proportion of the human diet is derived from seeds. Grains such as rice or wheat are staple crops for much of the planet, so being able to breed plants that produce larger grains is a major goal for many botanists and plant breeders. But what factors control seed size? Research recently published in Nature Plants by Bin Zhang and colleagues has found a role for a gene TERMINAL FLOWER1 (TFL1) in the plant Arabidopsis thaliana. The gene encodes a mobile protein that influences the endosperm, a major component of the seed that provides a store of nutrients for the seed embryo when it germinates.

Seeds, in flowering plants, come from the fertilized flowers. “In flowering plants, seed development is initiated by double fertilization,” said Hao Yu, head of the lab conducting the research, in an email to Botany One. “Before fertilization, an ovule contains an unfertilized egg cell and double unfertilized central cells, which are enclosed by an integument. After fertilization, the haploid egg cell, haploid central cells, and maternal diploid integument develop into the diploid embryo, triploid endosperm, and diploid seed coat, respectively. Therefore, the embryo, endosperm, and seed coat all contribute to the final seed size in flowering plants.”

TFL1 was already known to control the time of flowering in a plant. However, Yu’s team noticed that the tfl1 phenotype of Arabidopsis had large seeds, and that the TFL1 mRNA (the messenger RNA molecule) and the TFL1 protein were localized in different places in the seed. This apparent movement of the protein suggested that TFL1’s role in a seed was not entirely straightforward…Yu’s team faced three challenges. The first was identifying the genomic fragment that produced TFL1’s effect, this allowed the scientists to tag TFL1, enabling them to trace its role around the plant. “The second challenge was to overcome the technical barrier of detecting TFL1 protein through optimizing various approaches, since its endogenous expression level is extremely low,” said Yu.

Finally the team had to image the results in the microscope – not a trivial task. After fertilization, the primary endosperm nucleus undergoes nuclear divisions withoutany cell walls being formed – creating a very delicate ‘bag of nuclei’. As Yu points out “During the sectioning process, the free-nuclear endosperm is easily damaged and dropped from glass slides. Moreover, the endosperm is spatially divided into three domains, including micropylar, peripheral and chalazal endosperm. Because of these features, we had to test with many sections to obtain the ideal ones with a maximal inclusion of these intact structures.”

Quite how the TFL1 protein  moves through the plant to send these signals is not yet clear. In the seed, TFL1 is transported by RAN proteins and stabilises the ABI5 protein but in the shoot, TFL1 exists in a very different environment. As Yu says

“In the shoot, TFL1 mRNA is expressed in the center of the inflorescence meristem, whereas TFL1 protein moves to the outer cells, and accumulates in the entire meristem to repress the expression of floral meristem-identity genes LFY and AP1,” said Yu. “However, as ABI5 and RAN loss-of-function Arabidopsis mutants do not exhibit inflorescence architecture phenotypes, the regulators for TFL1 trafficking in seeds and shoots could be different.”

The next step in understanding what TFL1 is doing will be to examine what role it plays in signalling between the maternal plant and the seed endosperm. It’s the localization of TFL1 in the chalazal area of the endosperm – the part of the endosperm next to the plant’s nutrient conducting tissue – that suggests this is a useful line of research. As Yu says “So it could serve as a key passage to mediate the flux of signals or nutrients from the maternal tissues to the whole endosperm and embryo. Thus, generation of TFL1 mRNA in the chalazal endosperm and its mobile protein may allow mother plants to mediate seed development in response to the uptake of maternal signals”

Using Arabidopsis thaliana, for this work allows Yu’s team to investigate complex systems like TFL1 signalling in a well-understood model plant – botanists use Arabidopsis like a biomedical scientist would use a lab rat. An additional benefit is that the Arabidopsis genome is so well understood that it is often possible to find gene orthologs in other species, such as crop plants; orthologs are genes that share a common ancestor and may have a similar function. But being able to trace these connections allows researchers to discover processes in Arabidopsis, but to develop applications in crop plants. However, this requires teamwork across a wide variety of methods. “Skills in performing physiological, genetic, molecular and biochemical experiments are required,” said Yu. “To develop these skills, diligence, perseverance and establishing good learning habits are important.”

You can read the paper, via ReadCube for free in Nature Plants.