Morphogenesis requires the coordination of cellular behaviors along developmental axes. In plants, gradients of growth and differentiation are typically established along a single longitudinal primordium axis to control global organ shape. Yet, it remains unclear how these gradients are locally adjusted to regulate the formation of complex organs that consist of diverse tissue types. Here we combine quantitative live imaging at cellular resolution with genetics, and chemical treatments to understand the formation of Arabidopsis thaliana female reproductive organ (gynoecium). We show that, contrary to other aerial organs, gynoecium shape is determined by two orthogonal, time-shifted differentiation gradients. An early mediolateral gradient controls valve morphogenesis while a late, longitudinal gradient regulates style differentiation. Local, tissue-dependent action of these gradients serves to fine-tune the common developmental program governing organ morphogenesis to ensure the specialized function of the gynoecium.

Plant organs have evolved into diverse shapes for specialized functions despite emerging as simple protrusions at the shoot apex. Cauline leaves serve as photosynthetic organs and protective structures for emerging floral buds. However, the growth patterns underlying this dual function remain unknown. Here, we investigate the developmental dynamics shaping Arabidopsis (Arabidopsis thaliana) cauline leaves underlying their functional diversification from other laminar organs. We show that cauline leaves display a significant delay in overall elongation compared to rosette leaves. Using live imaging, we reveal that their functional divergence hinges on early modulation of the timing of cell differentiation and cellular growth rates. In contrast to rosette leaves and sepals, cell differentiation is delayed in cauline leaves, fostering extended proliferation, prolonged morphogenetic activity, and growth redistribution within the organ. Notably, cauline leaf growth is transiently suppressed during the early stages, keeping the leaf small and unfolded during the initiation of the first flowers. Our findings highlight the unique developmental timing of cauline leaves, underlying their shift from an early protective role to a later photosynthetic function.

Callus formation and shoot regeneration are naturally triggered by stress and damage to the plant. They are also key components of tissue culture, which is crucial for gene editing, transformation, propagation, and other technologies in many species. Thus, the study of callus formation and shoot regeneration provides valuable insights into plant development. The transcription factor ENHANCER OF SHOOT REGENERATION 2/DORNROSCHEN-LIKE/BOLITA/SUPPRESSOR OF PHYTOCHROME B-4 2 (ESR2/DRNL/BOL/SOB2) promotes green callus formation in roots as well as shoot regeneration when overactive, and the phythormone cytokinin plays a prominent role in both the processes. Yet, the role of ESR2 in the cytokinin pathway has not been previously described in Arabidopsis (Arabidopsis thaliana). We found that cytokinin content and the expression of the cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE 5 (IPT5) are greater in plants with high ESR2 activity. ESR2 also regulates the cytokinin signaling repressor ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6), and surprisingly, ESR2-stimulated green callus formation requires IPT5 and AHP6. Therefore, ESR2 promotes both cytokinin biosynthesis and the inhibition of cytokinin signaling and, paradoxically, requires a combination of these 2 effects for green callus induction. These findings provide a foundation to better understand the processes involved in tissue reprograming toward callus formation and the role of ESR2 in callus formation and shoot regeneration.

During in vitro maize plant regeneration somatic cells change their normal fate and undergo restructuring to generate pluripotent cells able to originate new plants. Auxins are essential to achieve such plasticity. Their physiological effects are mediated by auxin response factors (ARFs) that bind auxin responsive elements within gene promoters. Small trans-acting (ta)-siRNAs, originated from miR390-guided TAS3 primary transcript cleavage, target ARF3/4 class (tasiR-ARFs). Here we found that TAS3b precursor as well as derived tasiR-ARFbD5 and tasiR-ARFbD6 display significantly lower levels in non-embryogenic callus (NEC), while TAS3g, miR390 and tasiR-ARFg are more abundant in the same tissue. However, Argonaute (AGO7) and leafbladeless 1 (LBLl) required for tasiR-ARF biogenesis showed significantly higher transcript levels in EC suggesting limited tasiR-ARF biogenesis in NEC. The five maize ARFs targeted by tasiR-ARFs were also significantly enriched in EC and accompanied by higher auxin accumulation with punctuate patterns in this tissue. At hormone half-reduction and photoperiod implementation, plant regeneration initiated from EC with transient TAS3g, miR390 and tasiR-ARFg increase. Upon complete hormone depletion, TAS3b became abundant and derived tasiR-ARFs gradually increased at further regeneration stages. ZmARF transcripts targeted by tasiR-ARFs, as well as AGO7 and LBL1 showed significantly lower levels during regeneration than in EC. These results indicate a dynamic tasiR-ARF mediated regulation throughout maize in vitro plant regeneration.

Morphogenesis requires the coordination of cellular behaviors along developmental axes ¹ . In plants, gradients of growth and differentiation are typically established along a single longitudinal primordium axis to control organ shaping ² . Here we combine quantitative live-imaging at cellular resolution with genetics, chemical treatments, and modeling to understand the formation of Arabidopsis thaliana female reproductive organ (gynoecium). We show that, contrary to other aerial organs, gynoecium shape is determined by two competing differentiation gradients positioned along two orthogonal axes. An early mediolateral gradient, dependent on meristematic activity in the medial domain, controls the valve morphogenesis while simultaneously restricting an auxin-dependent, longitudinal gradient to the style. This gradient competition serves to finetune the common developmental program governing organ morphogenesis to ensure the specialized function of the gynoecium ^3,4 .