Although the size of an organism is a defining feature, little is known about the mechanisms that set the final size of organs and whole organisms. Here we describe Arabidopsis DA1, encoding a predicted ubiquitin receptor, which sets final seed and organ size by restricting the period of cell proliferation. The mutant protein encoded by the da1-1 allele has a negative activity toward DA1 and a DA1-related (DAR) protein, and overexpression of a da1-1 cDNA dramatically increases seed and organ size of wild-type plants, identifying this small gene family as important regulators of seed and organ size in plants. Supplemental Many experiments suggest that organs possess intrinsic information about their final size and grow until they reach a final predetermined mass (Conlon and Raff 1999;Day and Lawrence 2000), but the mechanisms setting the limits of growth are not well characterized despite their central importance. Recently, a key pathway suppressing cell proliferation during organogenesis has been identified (Dong et al. 2007) that is conserved in insects and mammals. However, many of the factors regulating organ size in animals have no obvious counterparts in plants, suggesting that the control of plant organ size involves novel mechanisms. Although external cues such as light, day length, and temperature influence plant growth and adapt sessile plants to their prevailing environment, the final size of plant seeds and determinate organs is reasonably constant within a given species, whereas interspecific seed and organ size variation is remarkably large, demonstrating that developing seeds and organs also possess intrinsic information about their final size (Tsukaya 2006). The mechanisms that establish the final size of seeds and organs and mediate environmental inputs into growth are poorly understood, despite their fundamental importance and relevance to crop plant improvement.Plant organ growth occurs by an initial proliferative phase in which cell numbers increase while their size remains fairly constant, followed by dramatic cell size increases that cease when the set size of the organ is reached. Increases in cell ploidy occur during later stages of organ growth that can be associated with the final size of cells (Sugimoto-Shirasu and Roberts 2003). In leaves, the transition from cell proliferation to cell expansion follows cell cycle arrest fronts that move from the tip to the base (Donnelly et al. 1999). Modulation of the time and location of cell proliferation arrest (Nath et al. 2003;Dinneny et al. 2004;Disch et al. 2006;White 2006) have been established as key regulatory points during leaf and petal formation that set final organ size and establish its shape. Interaction between organs also influences seed size. Reduced maternal integument size reduces final seed size (Garcia et al. 2005), and reduced endosperm proliferation arrests cell elongation in the integument (Garcia et al. 2003).The growth regulator auxin promotes growth through ARGOS (Hu et al. 2003), which mediates expression of AINTEGUMEN...

SummaryPlants both produce and utilize carbohydrates and have developed mechanisms to regulate their sugar status and co-ordinate carbohydrate partitioning. High sugar levels result in a feedback inhibition of photosynthesis and an induction of storage processes. We used a genetic approach to isolate components of the signalling pathway regulating the induction of starch biosynthesis. The regulatory sequences of the sugar inducible ADP-glucose pyrophosphorylase subunit ApL3 were fused to a negative selection marker. Of the four impaired sucrose induction (isi) mutants described here, two (isi1 and isi2) were speci®c to this screen. The other two mutants (isi3 and isi4) showed additional phenotypes associated with sugarsensing screens that select for seedling establishment on high-sugar media. The isi3 and isi4 mutants were found to be involved in the abscisic acid signalling pathway. isi3 is allelic to abscisic acid insensitive4 (abi4), a gene encoding an Apetala2-type transcription factor; isi4 was found to be allelic to glucose insensitive1 (gin1) previously reported to reveal cross-talk between ethylene and glucose signalling. Here we present an alternative interpretation of gin1 as an allele of the ABA-de®cient mutant aba2. Expression analysis showed that ABA is unable to induce ApL3 gene expression by itself, but greatly enhances ApL3 induction by sugar. Our data suggest a major role for ABA in relation to sugar-signalling pathways, in that it enhances the ability of tissues to respond to subsequent sugar signals.

Summary Control of organ size and shape by cell proliferation and cell expansion is a fundamental developmental process, but the mechanisms that set the size and shape of determinate organs are largely unknown in plants. Molecular, genetic, cytological and biochemical approaches were used to characterize the roles of the Arabidopsis thaliana G protein γ subunit (AGG3) gene in organ growth. Here, we describe A. thaliana AGG3, which promotes petal growth by increasing the period of cell proliferation. Both the N‐terminal region and the C‐terminal domains of AGG3 are necessary for the function of AGG3. By contrast, analysis of a series of AGG3 derivatives with deletions in specific domains showed that the deletion of any of these domains cannot completely abolish the function of AGG3. The GFP‐AGG3 fusion protein is localized to the plasma membrane. The predicted transmembrane domain plays an important role in the plasma membrane localization of AGG3. Genetic analyses revealed that AGG3 action requires a functional G protein α subunit (GPA1) and G protein β subunit (AGB1). Our findings demonstrate that AGG3, GPA1 and AGB1 act in the same genetic pathway to influence organ size and shape in A. thaliana.

Sugars such as sucrose serve dual functions as transported carbohydrates in vascular plants and as signal molecules that regulate gene expression and plant development. Sugar-mediated signals indicate carbohydrate availability and regulate metabolism by co-coordinating sugar production and mobilization with sugar usage and storage. Analysis of mutants with altered responses to sucrose and glucose has shown that signaling pathways mediated by sugars and abscisic acid interact to regulate seedling development and gene expression. Using a novel screen for sugar-response mutants based on the activity of a luciferase reporter gene under the control of the sugar-inducible promoter of the ApL3 gene, we have isolated high sugar-response (hsr) mutants that exhibit elevated luciferase activity and ApL3 expression in response to low sugar concentrations. Our characterization of these hsr mutants suggests that they affect the regulation of sugar-induced and sugar-repressed processes controlling gene expression, growth, and development in Arabidopsis. In contrast to some other sugar-response mutants, they do not exhibit altered responses to ethylene or abscisic acid, suggesting that the hsr mutants may have a specifically increased sensitivity to sugars. Further characterization of the hsr mutants will lead to greater understanding of regulatory pathways involved in metabolite signaling.Sugars such as Glc and Suc regulate many important cellular processes in plants (for review, see Rolland et al., 2002;Rook and Bevan, 2003). The expression of genes involved in photosynthate accumulation, mobilization, and storage is regulated by Glc and Suc (Koch, 1996), and by lightmediated (Neff et al., 2000) and circadian clockmediated (Harmer et al., 2000) signaling mechanisms. This regulatory network serves to integrate the synthesis and use of carbohydrates in different tissues and organs in response to environmental changes and in response to the availability of other nutrients such as nitrogen (Coruzzi and Bush, 2001). The outputs of this regulatory network maintain an optimal dynamic carbohydrate status. For example, in conditions of high carbohydrate demand and if sufficient light energy is available, the regulatory network increases production and mobilization of photosynthate by increasing expression of genes involved in photosynthesis (Koch, 1996), and conversely, when photosynthate is not immediately required, genes involved in starch synthesis (Rook et al., 2001) are activated to maintain a balance between photosynthate supply, demand, and storage. Transport functions respond to photosynthate availability by modulation of Suc transporter gene expression and protein levels (Chiou and Bush, 1998;Vaughn et al., 2002) to integrate carbohydrate sink demand with carbohydrate source production and export.Significant progress is being made in identifying the mechanisms controlling carbohydrate status in plants. Genetic screens in Arabidopsis have identified mutants affecting growth, development, and gene expression responses to Glc, ...

BackgroundWheat is one of the most widely grown crop in temperate climates for food and animal feed. In order to meet the demands of the predicted population increase in an ever-changing climate, wheat production needs to dramatically increase. Spike and grain traits are critical determinants of final yield and grain uniformity a commercially desired trait, but their analysis is laborious and often requires destructive harvest. One of the current challenges is to develop an accurate, non-destructive method for spike and grain trait analysis capable of handling large populations.ResultsIn this study we describe the development of a robust method for the accurate extraction and measurement of spike and grain morphometric parameters from images acquired by X-ray micro-computed tomography (μCT). The image analysis pipeline developed automatically identifies plant material of interest in μCT images, performs image analysis, and extracts morphometric data. As a proof of principle, this integrated methodology was used to analyse the spikes from a population of wheat plants subjected to high temperatures under two different water regimes. Temperature has a negative effect on spike height and grain number with the middle of the spike being the most affected region. The data also confirmed that increased grain volume was correlated with the decrease in grain number under mild stress.ConclusionsBeing able to quickly measure plant phenotypes in a non-destructive manner is crucial to advance our understanding of gene function and the effects of the environment. We report on the development of an image analysis pipeline capable of accurately and reliably extracting spike and grain traits from crops without the loss of positional information. This methodology was applied to the analysis of wheat spikes can be readily applied to other economically important crop species.Electronic supplementary materialThe online version of this article (doi:10.1186/s13007-017-0229-8) contains supplementary material, which is available to authorized users.