The final size of plant organs is determined by a combination of cell proliferation and cell expansion. Leaves account for a large part of above-ground biomass and provide energy to complete the plant's life cycle. Although the final size of leaves is remarkably constant under fixed environmental conditions, several genes have been described to enhance leaf growth when their expression is modulated. In Arabidopsis (Arabidopsis thaliana), mutations in DA1 and BB increase leaf size, an effect that is synergistically enhanced in the double mutant. Here, we show that overexpression of a dominant-negative version of DA1 enhances leaf size in a broad range of natural accessions of this species, indicating a highly conserved role of this protein in controlling organ size. We also found that during early stages of development, leaves of da1-1 and bb/eod1-2 mutants were already larger than the isogenic Col-0 wild type, but this phenotype was triggered by different cellular mechanisms. Later during development, da1-1 and bb/eod1-2 leaves showed a prolonged longevity, which was enhanced in the double mutant. Conversely, ectopic expression of DA1 or BB restricted growth and promoted leaf senescence. In concert, shortly upon induction of DA1 and BB expression, several marker genes for the transition from proliferation to expansion were highly up-regulated. Additionally, multiple genes involved in maintaining the mitotic cell cycle were rapidly down-regulated and senescence genes were strongly up-regulated, particularly upon BB induction. With these results, we demonstrate that DA1 and BB restrict leaf size and promote senescence through converging and different mechanisms.Arabidopsis (Arabidopsis thaliana) leaf growth is determined by different cellular and developmental events that are regulated by complex gene networks. Over the past years, numerous genes have been identified to regulate one or more of these events Hepworth and Lenhard, 2014). Leaves are formed at the flanks of the shoot apical meristem and grow exclusively through cell proliferation during the earliest stages of development (Kalve et al., 2014;Tsukaya, 2013). Later, cells stop dividing at the tip of the leaf, and a cell cycle arrest front proceeds in a basipetal direction (Andriankaja et al., 2012;Donnelly et al., 1999;Kazama et al., 2010). Down-regulation of the SAMBA gene has been shown to result in larger leaf primordia during the earliest stages of development (Eloy et al., 2012). The duration of the cell proliferation phase is extended in plants overexpressing AINTEGUMENTA (ANT), ANGUSTIFOLIA3 (AN3), GROWTH REGULATING FACTOR3 (GRF3), GRF5, and miR319 (also known as JAGGED AND WAVY [JAW]; Debernardi et al., 2014;Gonzalez et al., 2010;Horiguchi et al., 2005;Mizukami and Fischer, 2000;Palatnik et al., 2003;Vercruyssen et al., 2015), leading to the formation of larger leaves. During the transition from cell division to cell expansion, chloroplasts start differentiating (Andriankaja et al., 2012), allowing the leaves to become an energy source for sink t...

Potassium (K) is an important plant macronutrient that has various functions throughout the whole plant over its entire life span. Cytokinins (CKs) are known to regulate macronutrient homeostasis by controlling the expression of nitrate, phosphate and sulfate transporters. Although several studies have described how CKs signal deficiencies for some macronutrients, the roles of CKs in K signaling are poorly understood. CK content has been shown to decrease under K-starved conditions. Specifically, a CK-deficient mutant was more tolerant to low K than wild-type; however, a plant with an overaccumulation of CKs was more sensitive to low K. These results suggest that K deprivation alters CK metabolism, leading to a decrease in CK content. To investigate this phenomenon further, several Arabidopsis lines, including a CK-deficient mutant and CK receptor mutants, were analyzed in low K conditions using molecular, genetic and biochemical approaches. ROS accumulation and root hair growth in low K were also influenced by CKs. CK receptor mutants lost the responsiveness to K-deficient signaling, including ROS accumulation and root hair growth, but the CK-deficient mutant accumulated more ROS and exhibited up-regulated expression of HAK5, which is a high-affinity K uptake transporter gene that is rapidly induced by low K stress in ROS- and ethylene-dependent manner in response to low K. From these results, we conclude that a reduction in CK levels subsequently allows fast and effective stimulation of low K-induced ROS accumulation, root hair growth and HAK5 expression, leading to plant adaptation to low K conditions.

Although phytohormones such as gibberellins are essential for many conserved aspects of plant physiology and development, plants vary greatly in their responses to these regulatory compounds. Here, we use genetic perturbation of endogenous gibberellin levels to probe the extent of intraspecific variation in gibberellin responses in natural accessions of Arabidopsis (Arabidopsis thaliana). We find that these accessions vary greatly in their ability to buffer the effects of overexpression of GA20ox1, encoding a rate-limiting enzyme for gibberellin biosynthesis, with substantial differences in bioactive gibberellin concentrations as well as transcriptomes and growth trajectories. These findings demonstrate a surprising level of flexibility in the wiring of regulatory networks underlying hormone metabolism and signaling.