January 2025 TPJ Editor choice: Special transport: the transcription factor SHORT-ROOT is involved in assimilate accumulation in the chalazal seed coat in Arabidopsis and soybean

Highlighted publication:

SHORT-ROOT specifically functions in the chalazal region to modulate assimilate partitioning into seeds
https://doi.org/10.1111/tpj.17096

Special transport: the transcription factor SHORT-ROOT is involved in assimilate accumulation in the chalazal seed coat in Arabidopsis and soybean

Plant seeds are a main food source for human nutrition by providing nutrients, including sugars, oils, proteins and ions. Seeds contain the plant embryo, and a nourishing tissue, the endosperm, in which nutrients and starch are stored. Both are surrounded by the seed coat, which promotes first dormancy and then germination, and acts as interface against the environment. The seed is connected to the maternal tissue through the funiculus.

It is not known how assimilates from maternal tissues are selectively transported and stored in the seeds, as the vasculature terminate at the junction between the ovule and the funiculus in most plant species. The chalazal seed coat (CZSC) is the group of cells connecting the funiculus and the seed. There are indications that the CZSC is involved in active nutrient transport, and Li and colleagues found that the transcription factor SHORT-ROOT (SHR) is specifically expressed in the seed coat. Therefore, they investigated its function in seed development in Arabidopsis and soybean.

The authors found that the cell walls of the CZSC in Arabidopsis and the corresponding region in soybean seeds were thickened, enriched in lipid substances and highly suberized during the globular embryo stage. A comparison of the CZSC and the remaining seed coat transcriptomes showed an enrichment of genes involved in the biosynthesis of cutin, suberin and wax.

The suberization of the CZSC suggested a zonal function of the seed coat, forming a barrier to promote selective nutrient transport and accumulation. In suberin-deficient Arabidopsis mutants, the authors observed increased phosphorus, potassium, and magnesium levels. Transcriptome analysis revealed the expression of specific nutrient and sugar transporters in the CZSC and soybean counter-palisade cells, highlighting the role of the CZSC in active transport.

By transcriptome analysis, reporter lines and RNA in situ hybridization analyses, the authors identified SHR as specifically expressed in the CZSC of Arabidopsis and soybean. Arabidopsis shr mutants had less suberin in the CSCZ. ‘Non-invasive Micro-test Technology’ indicated a directional flow of K⁺ and NO^-3 in the seed funiculus. It showed that ion efflux from the wild type seeds was much higher than that from shr, suggesting that shr lacks a barrier to prevent ion leakage.

In addition to SHR, the authors found that many MYB family genes were specifically expressed in the CZSC, including AtMYB36, a key regulator of root endodermal differentiation and suberization. In the root endodermis, AtMYB36 acts downstream of AtSHR to mediate suberization, and a similar regulation took place in the CZSC. AtMYB36 in turn was able to activate the promoters of phosphorus and sucrose transporters.

The findings of Li et al. suggest that the ability of the CZSC region to accumulate assimilates in the seed depends on the formation of suberin lamellae and the specific expression of transporters, both regulated by the AtSHR-AtMYB network. The suberin barrier and active transport within the CZSC region are crucial not only for assimilate accumulation, but also for seed development. The authors hope that their findings will contribute to research aimed at improving seed quality in dicot crop plants. Enhancing the suberin barrier could improve nutrient accumulation, but it must be coordinated with the enhancement of transporters specifically expressed in the chalazal zone. With these advancements, it could be possible to improve the nutrient content of dicotyledonous seeds.