The canonical E2Fs together with RETINOBLASTOMA-RELATED are required to establish quiescence during plant development.

Maintaining stable and transient quiescence in differentiated and stem cells, respectively, requires repression of the cell cycle. The plant RETINOBLASTOMA-RELATED (RBR) has been implicated in stem cell maintenance, presumably by forming repressor complexes with E2F transcription factors. Surprisingly we find that mutations in all three canonical E2Fs do not hinder the cell cycle, but similarly to RBR silencing, result in hyperplasia. Contrary to the growth arrest that occurs when exit from proliferation to differentiation is inhibited upon RBR silencing, the e2fabc mutant develops enlarged organs with supernumerary stem and differentiated cells as quiescence is compromised. While E2F, RBR and the M-phase regulatory MYB3Rs are part of the DREAM repressor complexes, and recruited to overlapping groups of targets, they regulate distinct sets of genes. Only the loss of E2Fs but not the MYB3Rs interferes with quiescence, which might be due to the ability of E2Fs to control both G1-S and some key G2-M targets. We conclude that collectively the three canonical E2Fs in complex with RBR have central roles in establishing cellular quiescence during organ development, leading to enhanced plant growth.

Polyamine Metabolism Is Involved in the Direct Regeneration of Shoots from Arabidopsis Lateral Root Primordia.

Plants can be regenerated from various explants/tissues via de novo shoot meristem formation. Most of these regeneration pathways are indirect and involve callus formation. Besides plant hormones, the role of polyamines (PAs) has been implicated in these processes. Interestingly, the lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems by exogenous cytokinin application. In this system, no callus formation takes place. We report that the level of PAs, especially that of spermidine (Spd), increased during meristem conversion and the application of exogenous Spd improved its efficiency. The high endogenous Spd level could be due to enhanced synthesis as indicated by the augmented relative expression of PA synthesis genes (AtADC1,2, AtSAMDC2,4, AtSPDS1,2) during the process. However, the effect of PAs on shoot meristem formation might also be dependent on their catabolism. The expression of Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) was shown to be specifically high during the process and its ectopic overexpression increased the LRP-to-shoot conversion efficiency. This was correlated with Spd accumulation in the roots and ROS accumulation in the converting LRPs. The potential ways how PAO5 may influence direct shoot organogenesis from Arabidopsis LRPs are discussed.

Arabidopsis NAP-related proteins (NRPs) contribute to the coordination of plant growth, developmental rate, and age-related pathogen resistance under short days.

Plant nucleosome assembly protein-related proteins (NRPs) are histone chaperons involved in nucleosome turnover. Despite this basic cellular function, the Arabidopsis nrp1-1 nrp2-1 knock out mutant has been reported to exhibit only mild seedling root phenotypes and to significantly affect the expression of only few hundred genes Zhu et al. (2006). Here we report that NRP loss-of-function as well as the ectopic overexpression of At NRP1 significantly affected the growth, development, and the pathogen response of Arabidopsis plants under short day conditions. The nrp1-1 nrp2-1 mutant grew faster and flowered weeks earlier than the wild type and the overexpressor. The latter developed slower and flowered at a lower number of leaves than the mutant and the wild type. Moreover, the mutant was more sensitive, the overexpressor was more tolerant to pathogen-induced necrosis correlating with their more adult and juvenile character, respectively. Transcriptomic comparison of mature non-bolting plants agreed with the phenotypes. The presented and other published data indicate that although NRPs might not be absolutely required for normal plant growth and development, their level needs to be controlled to allow the epigenetic coordination of metabolic, growth, defence and developmental processes during the acclimation to unfavourable growth conditions such as short days.