An MYB-family transcription factor with suber powers.

Abstract

The root endodermis is the primary boundary between the inner vascular tissue and the outer cortex, operating as an apoplasmic barrier for selective nutrient uptake and defence. How is the production of endodermal barriers regulated during development? New research in The Plant Journal (Cohen et al., 2020) describes a transcription factor that controls the establishment of the endodermal suberin lamellae. The discovery is an important milestone in understanding the formation of this hydrophobic barrier and its role in maintaining plant fitness. The root endodermal barrier requires the coordinated action of two hydrophobic polymers, lignin and suberin. Lignin is a building block for the Casparian strip, a ring-like structure that surrounds the walls of mature, fully elongated endodermal cells. The suberin lamellae appears later in development, at first as a ‘patchy’ zone, where endodermal cells are suberized intermittently, and later as a ‘continuous’ zone, when cells are fully differentiated (Barberon, 2017). Whereas the Casparian strip serves as an apoplastic barrier, the suberin lamellae limits solute transports intracellularly. Despite the important role that suberization plays at the root endodermis, the regulation of this process is not well understood. In a previous study, Asaph Aharoni and his team discovered two Arabidopsis transcription factors (MYB107 and MYB9) that play a role in suberin monomer biosynthesis and transport, as well as in their subsequent polymerization in the seed outer integument layer (Lashbrooke et al., 2016). In this follow-up study, the team discovered and characterized a related MYB-type transcription factor, MYB39, that regulates root endodermis suberization. The transcription factor, aptly named SUBERMAN, was able to activate the entire suberin metabolism machinery when expressed heterologously in Nicotiana tabacum (tobacco), even in leaves, which naturally do not deposit suberin lamellae in cell walls. Altering the expression of SUBERMAN resulted not only in changes in the deposition patterns of suberin, but also affected the nutrient uptake capacity of the root. The work comes from Aharoni’s lab at the Weizmann Institute (Rehovot, Israel), in collaboration with scientists at the Horticultural Plant Biology and Metabolomics Center in Fujian Agriculture and Forestry University, China. Aharoni’s team has long focused on diverse aspects of plant metabolism, including the production, regulation, and role of secondary metabolites in plant development and stress responses. Hagai Cohen, first author of the paper, was a postdoctoral fellow in Aharoni’s group, and now directs his own lab at the Agricultural Research Organization (ARO, Volcani Center, Israel). Cohen’s group investigates plant lipophilic barriers and their role in pathogen response. Cohen et al. (2020) demonstrates that SUBERMAN triggers the expression of not only suberin biosynthesis genes, but also of genes required to assemble and deposit these unique structures at their correct subcellular sites (see Figure). The authors point out that SUBERMAN probably cooperates with other still-unknown transcription factors to execute the necessary steps for establishing the suberin lamellae. Their current work on root transcriptomics and other approaches has highlighted additional candidate genes with potential roles in suberin metabolism, which the team plans to investigate further to determine their role in suberin deposition in different plant tissues and cell types. The results from Cohen et al. (2020) open the way for dissecting the molecular mechanisms involved in establishing a major root barrier. According to Cohen and Aharoni, in the future, their discoveries could be used to implement metabolic engineering strategies aimed at designing novel barriers in roots or other plant parts. These barriers could make plants more resistant to diverse stress conditions, such as mineral deficiency or pathogen invasion. Other research groups are also likely to recruit SUBERMAN to the rescue. For example, one group is trying to engineer plants to overproduce suberin to sequester carbon from the atmosphere and thereby slow down climate change.