Local maternal control of seed size by KLUH/CYP78A5-dependent growth signaling.

Seed development in plants involves the coordinated growth of the embryo, endosperm, and maternal tissue. Several genes have been identified that influence seed size by acting maternally, such as AUXIN RESPONSE FACTOR2, APETALA2, and DA1. However, given the lack of gain-of-function effects of these genes on seed size, it is unclear whether their activity levels are limiting in WT plants and whether they could thus be used to regulate seed size in development or evolution. Also, whether the altered seed sizes reflect local gene activity or global physiological changes is unknown. Here, we demonstrate that the cytochrome P450 KLUH (KLU) regulates seed size. KLU acts locally in developing flowers to promote seed growth, and its activity level is limiting for seed growth in WT. KLU is expressed in the inner integument of developing ovules, where it non-cell autonomously stimulates cell proliferation, thus determining the growth potential of the seed coat and seed. A KLU-induced increase in seed size leads to larger seedlings and higher relative oil content of the seeds. Genetic analyses indicate that KLU acts independently of other tested maternal factors that influence integument cell proliferation. Thus, the level of KLU-dependent growth factor signaling determines size in ovules and seeds, suggesting this pathway as a target for crop improvement.

Control of plant organ size by KLUH/CYP78A5-dependent intercellular signaling.

Plant organs grow to characteristic sizes that are genetically controlled. In animals, signaling by mobile growth factors is thought to be an effective mechanism for measuring primordium size, yet how plants gauge organ size is unclear. Here, we identify the Arabidopsis cytochrome P450 KLUH (KLU)/CYP78A5 as a stimulator of plant organ growth. While klu loss-of-function mutants form smaller organs because of a premature arrest of cell proliferation, KLU overexpression leads to larger organs with more cells. KLU promotes organ growth in a non-cell-autonomous manner, yet it does not appear to modulate the levels of known phytohormones. We therefore propose that KLU is involved in generating a mobile growth signal distinct from the classical phytohormones. The expression dynamics of KLU suggest a model of how the arrest of cell proliferation is coupled to the attainment of a certain primordium size, implying a common principle of size measurement in plants and animals.

Growing up to one's standard.

Plant organs grow to characteristic sizes and shapes that are dictated by the plant's genotype and the identity of the organ. Significant progress has been made in identifying and characterizing regulatory factors that promote organ growth, which act either on cell proliferation or on cell expansion. Their activity is antagonized by repressors of growth that limit organ size. Although the way in which that genes determine the identity of an organ modify its growth patterns is still unclear, initial links between growth regulators and patterning activities are being uncovered. As for the differences in organ size and shape between plant species, studies of natural variation are beginning to shed light on the underlying molecular changes.

The E3 ubiquitin ligase BIG BROTHER controls arabidopsis organ size in a dosage-dependent manner.

Organ growth up to a species-specific size is tightly regulated in plants and animals. Final organ size is remarkably constant within a given species, suggesting that a species-specific size checkpoint terminates organ growth in a coordinated and timely manner. Phytohormones influence plant organ size, but their precise functions in size control are unclear because of their pleiotropic and complex developmental roles. The Arabidopsis transcription factors AINTEGUMENTA and JAGGED promote organ growth by maintaining cellular proliferation potential. Loss of the Antirrhinum transcription factor CINCINNATA causes leaf overgrowth, yet also leads to a highly abnormal leaf shape. Thus, no dedicated factor that limits the final size of plant organs has been isolated. Here, we identify the novel RING-finger protein BIG BROTHER (BB) as a repressor of plant organ growth. Small changes in BB expression levels substantially alter organ size, indicating a central regulatory role for BB in growth control. Recombinant BB protein has E3 ubiquitin-ligase activity that is essential for its in vivo function, suggesting that BB acts by marking cellular proteins for degradation. Our data indicate that plants limit the duration of organ growth and ultimately organ size by actively degrading critical growth stimulators.