Plants' autotrophic source organs capture the energy and carbon and transform those into the sugars which later on are mobilized to a heterotrophic developing sink. Scientists are trying to understand this nutrient mobilization pathway which could help improve crop yield. Recently, researchers discovered how an RNA-binding protein JULGI can keep phloem development in check and the plants lacking this protein can produce more fruits by leveraging resources towards the developing sink.

 Plants are equipped with vasculature consisting of xylem and phloem tissues to transport water, mineral and nutrients to distant organs. Phloem is a continuum for resource distribution between source and sink organs, but as of the year 2022, the role of phloem transport capacity in regulating plant yield was largely unknown. Prof Ildoo Hwang’s research group at POSTECH identified a genetically and functionally conserved negative regulator of phloem development and transport in tomato (Solanum lycopersicum L.) named JULGI (SlJUL). 

 SIJUL is an RNA-binding protein that binds the 5’UTR region of its target mRNA SlSMXL5, a positive regulator of phloem development and suppresses its translation. To paint a complete picture of the association between phloem strength, the resource mobilization process and the yield, they manipulated phloem cell numbers in a precise manner by employing different gene editing tools. Their research which appears in The Plant Biotechnology Journal, 2022 establishes that the increment in phloem cell number confers spare transport capacity. The evidence from their research efforts suggests that the suppression of SlJUL increases the number of phloem cells and sucrose transport, enhancing the commitment of photosynthates and thereby leads to a significantly increased fruit biomass per plant and sugar content in tomato. On the contrary, a complete loss-of-function in SlJUL resulted in the prolific phloem tissue but compromised plant yield associated with severe growth retardation.

 Research from Prof Hwang’s group shows that if plants make too much phloem at the wrong times it can severely restrict yield, so they control production with phloem moderation. With this study, the potential for enhancing resource flux to sink organs has emerged as an innovative strategy to improve crop productivity beyond the limitations of primary productivity.

[Reference] Nam et al., (2022) “ JULGI-mediated increment in phloem transport capacity relates to fruit yield in tomato, Plant Biotechnology Journal, http://doi.org/10.1111/pbi.13831

[Main Author] Hoyoung Nam (POSTECH), Ildoo Hwang (POSTECH)

* Contact : Professor Ildoo Hwang (ihwang@postech.ac.kr)