Veciana Nil, Martín Guiomar, Monte Elena
Centre for Research in Agricultural Genomics (CRAG) CSIC-IRTA-UAB-UB, Campus UAB, Bellaterra, Barcelona, Spain.
Department of Biology, Healthcare and the Environment, Faculty of Pharmacy and Food Sciences, University of Barcelona, Barcelona, 08028, Spain.
Plant J. 2025 May;122(4):e70196. doi: 10.1111/tpj.70196.
Despite its crucial role during seedling deetiolation, cotyledon expansion has been largely overlooked, with hypocotyl elongation favored as the primary phenotypic readout in light signaling research. Here, we investigate how cotyledon expansion is regulated during seedling establishment and reveal that light-induced cotyledon expansion involves a rapid switch in growth direction - from longitudinal in darkness to transversal upon initial light exposure. Using PIFq- and phyA/phyB-deficient Arabidopsis mutants, we demonstrate that this switch is repressed by PIFs in the dark and promoted by phytochromes under red light. Notably, expansion is antagonistically regulated in the light by GUN1-mediated plastid retrograde signaling. Cotyledon expansion involves rapid epidermis cell expansion, transitioning from rectangular in darkness to characteristic lobed cells in light. Importantly, our findings show that mesophyll extension is driven not only by cell enlargement but also by palisade cell division, consistent with an enrichment of cell cycle-related genes that are antagonistically regulated by the PIF/phy system and retrograde signaling in the cotyledon. Finally, using mutant lines expressing PIF1 and phyB specifically in the epidermis, we establish that epidermal expansion can drive palisade cell growth, while mesophyll cell division is predominantly regulated by light at the tissue-specific level. This study provides a novel framework for investigating cotyledon expansion during seedling deetiolation, incorporating tissue-level regulation. We propose that cotyledons serve as an excellent model for studying morphogenesis and organ geometry, which in plants is governed by directional cell growth.
尽管子叶扩展在幼苗去黄化过程中起着关键作用，但在很大程度上被忽视了，在光信号研究中，下胚轴伸长作为主要的表型指标更受青睐。在这里，我们研究了幼苗建立过程中子叶扩展是如何被调控的，并揭示光诱导的子叶扩展涉及生长方向的快速转变——从黑暗中的纵向生长转变为初次光照后的横向生长。利用PIFq以及phyA/phyB缺失的拟南芥突变体，我们证明这种转变在黑暗中被PIFs抑制，在红光下被光敏色素促进。值得注意的是，在光照条件下，GUN1介导的质体逆向信号传导对抗性地调节子叶扩展。子叶扩展涉及表皮细胞的快速扩张，从黑暗中的长方形细胞转变为光照下特有的叶状细胞。重要的是，我们的研究结果表明，叶肉扩展不仅由细胞增大驱动，还由栅栏细胞分裂驱动，这与子叶中细胞周期相关基因的富集一致，这些基因受到PIF/phy系统和逆向信号传导的对抗性调节。最后，使用在表皮中特异性表达PIF1和phyB的突变系，我们确定表皮扩展可以驱动栅栏细胞生长，而叶肉细胞分裂主要在组织特异性水平上受光调节。这项研究为研究幼苗去黄化过程中的子叶扩展提供了一个新的框架，纳入了组织水平的调控。我们认为子叶是研究形态发生和器官几何形状的一个优秀模型，在植物中，形态发生和器官几何形状由定向细胞生长控制。
