In an attempt to understand how cork and vascular cambium coordinate lateral growth, we identified a set of LOB DOMAIN transcription factors, LBD1, LBD3, LBD4 and LBD11, which regulate radial growth by specifically controlling cellular growth (Ye et al. Current Biology 2021). Tissue specific manipulation of LBDs is thus our prime approach to assess how the two lateral meristem coordinate mechanistically radial growth. In addition to this, we are also studying the downstream action of LBDs. We discovered that LBDs regulate large number of genes encoding primary cell wall modifying enzymes. Among them we identified four PECTIN LYASE LIKE (PLL) genes. Quadruple knockout led to reduced secondary growth, altered pectin content in cell wall and stiffer cells wall, similar to the lbd mutants. Thus, LBDs promote radial growth at least in part by upregulating four PLL genes, which regulated pectin composition in the primary cell wall, and this in turn soften the wall, which then enables accelerated cell growth. We are finalizing this work and submitting it in December 2024.
As planned in the project proposal, we carried out lineage tracing and detailed marker analysis during cork cambium development. We discovered that phloem parenchyma cells initially originated from vascular cambium cells gradually transforms to become periderm cells. This transformation was confirmed with single cell RNA sequencing analysis (scRNAseq). This transition can be accelerated wit jasmonic acid treatment or wounding. Additionally, scRNAseq analysis enabled us to identify novel cork and vascular cambium factors. We generated more than 100 transgenic YFP reporter lines to verify all the clusters in the scRNAseq data, and this way we were able to generate detailed cell types atlas of Arabidopsis root cambium. This paper is now under revision.
As a follow up of the Smetana et al paper (Nature 2019), in which we identified plant hormone auxin defining the stem cell organizer of the cambium, we developed methods to manipulate auxin distribution within the cambium. We discovered that another plant hormone, gibberellic acid, promote polar auxin transport and thus distribution of auxin within the cambium. We showed that broadness of auxin within the cambium determines whether cambium stem cell preferentially produce xylem or phloem (Mäkilä et al. Nature Plants 2023).
Finally, we showed that TDIF ligand-activated PXY receptors promote the expression of CAMBIUM-EXPRESSED AINTEGUMENTA-LIKE (CAIL) transcription factors to define cambium stem cell identity in the Arabidopsis root. By sequestrating the phloem-originated TDIF, xylem-expressed and auxin-induced PXY confines the TDIF signaling front, resulting in the activation of CAIL expression and stem cell identity in only a narrow domain. This was studied by combining experimentation and computational modelling (ten Tusscher lab). Thus, our findings show how signals emanating from cells on opposing sides ensure robust yet dynamically adjustable positioning of a bifacial stem cell layer. This work was published very recently in Science (2024).
