Gravity sensing and responses in the coordination of the shoot gravitropic setpoint angle.

Gravity is one of the fundamental environmental cues that affect plant development. Indeed, the plant architecture in the shoots and roots is modulated by gravity. Stems grow vertically upward, whereas lateral organs, such as the lateral branches in shoots, tend to grow at a specific angle according to a gravity vector known as the gravitropic setpoint angle (GSA). During this process, gravity is sensed in specialised gravity-sensing cells named statocytes, which convert gravity information into biochemical signals, leading to asymmetric auxin distribution and driving asymmetric cell division/expansion in the organs to achieve gravitropism. As a hypothetical offset mechanism against gravitropism to determine the GSA, the anti-gravitropic offset (AGO) has been proposed. According to this concept, the GSA is a balance of two antagonistic growth components, that is gravitropism and the AGO. Although the nature of the AGO has not been clarified, studies have suggested that gravitropism and the AGO share a common gravity-sensing mechanism in statocytes. This review discusses the molecular mechanisms underlying gravitropism as well as the hypothetical AGO in the control of the GSA.

The boundary-expressed EPIDERMAL PATTERNING FACTOR-LIKE2 gene encoding a signaling peptide promotes cotyledon growth during Arabidopsis thaliana embryogenesis.

The shoot organ boundaries have important roles in plant growth and morphogenesis. It has been reported that a gene encoding a cysteine-rich secreted peptide of the EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) family, EPFL2, is expressed in the boundary domain between the two cotyledon primordia of Arabidopsis thaliana embryo. However, its developmental functions remain unknown. This study aimed to analyze the role of EPFL2 during embryogenesis. We found that cotyledon growth was reduced in its loss-of-function mutants, and this phenotype was associated with the reduction of auxin response peaks at the tips of the primordia. The reduced cotyledon size of the mutant embryo recovered in germinating seedlings, indicating the presence of a factor that acted redundantly with EPFL2 to promote cotyledon growth in late embryogenesis. Our analysis suggests that the boundary domain between the cotyledon primordia acts as a signaling center that organizes auxin response peaks and promotes cotyledon growth.

A Peptide Pair Coordinates Regular Ovule Initiation Patterns with Seed Number and Fruit Size.

Ovule development in Arabidopsis thaliana involves pattern formation, which ensures that ovules are regularly arranged in the pistils to reduce competition for nutrients and space. Mechanisms underlying pattern formation in plants, such as phyllotaxis, flower morphogenesis, or lateral root initiation, have been extensively studied, and genes controlling the initiation of ovules have been identified. However, the fundamental patterning mechanism that determines the spacing of ovule anlagen within the placenta remained unexplored. Using natural variation analysis combined with quantitative trait locus analysis, we found that the spacing of ovules in the developing gynoecium and fruits is controlled by two secreted peptides, EPFL2 and EPFL9 (also known as Stomagen), and their receptors from the ERECTA (ER) family that act from the carpel wall and the placental tissue. We found that a signaling pathway controlled by EPFL9 acting from the carpel wall through the LRR-receptor kinases ER, ERL1, and ERL2 promotes fruit growth. Regular spacing of ovules depends on EPFL2 expression in the carpel wall and in the inter-ovule spaces, where it acts through ERL1 and ERL2. Loss of EPFL2 signaling results in shorter gynoecia and fruits and irregular spacing of ovules or even ovule twinning. We propose that the EPFL2 signaling module evolved to control the initiation and regular, equidistant spacing of ovule primordia, which may serve to minimize competition between seeds or facilitate equal resource allocation. Together, EPFL2 and EPFL9 help to coordinate ovule patterning and thereby seed number with gynoecium and fruit growth through a set of shared receptors.

Gravity-Sensing Tissues for Gravitropism Are Required for "Anti-Gravitropic" Phenotypes of Lzy Multiple Mutants in Arabidopsis.

Plant posture is controlled by various environmental cues, such as light, temperature, and gravity. The overall architecture is determined by the growth angles of lateral organs, such as roots and branches. The branch growth angle affected by gravity is known as the gravitropic setpoint angle (GSA), and it has been proposed that the GSA is determined by balancing two opposing growth components: gravitropism and anti-gravitropic offset (AGO). The molecular mechanisms underlying gravitropism have been studied extensively, but little is known about the nature of the AGO. Recent studies reported the importance of LAZY1-LIKE (LZY) family genes in the signaling process for gravitropism, such that loss-of-function mutants of LZY family genes resulted in reversed gravitropism, which we term it here as the "anti-gravitropic" phenotype. We assume that this peculiar phenotype manifests as the AGO due to the loss of gravitropism, we characterized the "anti-gravitropic" phenotype of Arabidopsis lzy multiple mutant genetically and physiologically. Our genetic interaction analyses strongly suggested that gravity-sensing cells are required for the "anti-gravitropic" phenotype in roots and lateral branches. We also show that starch-filled amyloplasts play a significant role in the "anti-gravitropic" phenotype, especially in the root of the lzy multiple mutant.

Expression of a kinase-dead form of CPK33 involved in florigen complex formation causes delayed flowering.

Regulation of flowering time is crucial for reproductive success of plants. FLOWERING LOCUS T (FT) protein is a central component of florigen and forms a ternary complex with 14-3-3 and FD, a basic leucine zipper transcription factor, in the shoot apex and promotes flowering. This complex formation requires phosphorylation of threonine residue at position 282 of FD. A calcium-dependent protein kinase CPK33 is responsible for the phosphorylation. However, possibly due to functional redundancy among calcium-dependent protein kinases, impact of the loss of CPK33 reported in the previous study was rather limited. Here, we report that expression of a kinase-dead form of CPK33 caused a clear delayed-flowering phenotype, supporting for an important role of CPK33 in florigen function through FD phosphorylation.

Calcium-dependent protein kinases responsible for the phosphorylation of a bZIP transcription factor FD crucial for the florigen complex formation.

Appropriate timing of flowering is critical for reproductive success and necessarily involves complex genetic regulatory networks. A mobile floral signal, called florigen, is a key molecule in this process, and flowering locus T (FT) protein is its major component in Arabidopsis. FT is produced in leaves, but promotes the floral transition in the shoot apex, where it forms a complex with a basic region/leucine-zipper (bZIP) transcription factor, FD. Formation of the florigen complex depends on the supposed phosphorylation of FD; hitherto, however, the responsible protein kinase(s) have not been identified. In this study, we prepared protein extracts from shoot apices of plants around the floral transition, and detected a protein kinase activity that phosphorylates a threonine residue at position 282 of FD (FD T282), which is a crucial residue for the complex formation with FT via 14-3-3. The kinase activity was calcium-dependent. Subsequent biochemical, cellular, and genetic analyses showed that three calcium-dependent protein kinases (CDPKs) efficiently phosphorylate FD T282. Two of them (CPK6 and CPK33) are expressed in shoot apical meristem and directly interact with FD, suggesting they have redundant functions. The loss of function of one CDPK (CPK33) resulted in a weak but significant late-flowering phenotype.