Defects in meristem maintenance, cell division, and cytokinin signaling are early responses in the boron deficient maize mutant tassel-less1.

Meristems house the stem cells needed for the developmental plasticity observed in adverse environmental conditions and are crucial for determining plant architecture. Meristem development is particularly sensitive to deficiencies of the micronutrient boron, yet how boron integrates into meristem development pathways is unknown. We addressed this question using the boron-deficient maize mutant, tassel-less1 (tls1). Reduced boron uptake in tls1 leads to a progressive impairment of meristem development that manifests in vegetative and reproductive defects. We show, that the tls1 tassel phenotype (male reproductive structure) was partially suppressed by mutations in the CLAVATA1 (CLV1)-ortholog, thick tassel dwarf1 (td1), but not by other mutants in the well characterized CLV-WUSCHEL pathway, which controls meristem size. The suppression of tls1 by td1 correlates with altered signaling of the phytohormone cytokinin. In contrast, mutations in the meristem maintenance gene knotted1 (kn1) enhanced both vegetative and reproductive defects in tls1. In addition, reduced transcript levels of kn1 and cell cycle genes are early defects in tls1 tassel meristems. Our results show that specific meristem maintenance and hormone pathways are affected in tls1, and suggest that reduced boron levels induced by tls1 are the underlying cause of the observed defects. We, therefore, provide new insights into the molecular mechanisms affected by boron deficiency in maize, leading to a better understanding of how genetic and environmental factors integrate during shoot meristem development.

Transport of boron by the tassel-less1 aquaporin is critical for vegetative and reproductive development in maize.

The element boron (B) is an essential plant micronutrient, and B deficiency results in significant crop losses worldwide. The maize (Zea mays) tassel-less1 (tls1) mutant has defects in vegetative and inflorescence development, comparable to the effects of B deficiency. Positional cloning revealed that tls1 encodes a protein in the aquaporin family co-orthologous to known B channel proteins in other species. Transport assays show that the TLS1 protein facilitates the movement of B and water into Xenopus laevis oocytes. B content is reduced in tls1 mutants, and application of B rescues the mutant phenotype, indicating that the TLS1 protein facilitates the movement of B in planta. B is required to cross-link the pectic polysaccharide rhamnogalacturonan II (RG-II) in the cell wall, and the percentage of RG-II dimers is reduced in tls1 inflorescences, indicating that the defects may result from altered cell wall properties. Plants heterozygous for both tls1 and rotten ear (rte), the proposed B efflux transporter, exhibit a dosage-dependent defect in inflorescence development under B-limited conditions, indicating that both TLS1 and RTE function in the same biological processes. Together, our data provide evidence that TLS1 is a B transport facilitator in maize, highlighting the importance of B homeostasis in meristem function.

Synergistic interaction of CLAVATA1, CLAVATA2, and RECEPTOR-LIKE PROTEIN KINASE 2 in cyst nematode parasitism of Arabidopsis.

Plant-parasitic cyst nematodes secrete CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (CLE)-like effector proteins. These proteins act as ligand mimics of plant CLE peptides and are required for successful nematode infection. Previously, we showed that the CLV2/CORYNE (CRN) heterodimer receptor complex is required for nematode CLE signaling. However, there was only a partial reduction in nematode infection when this signaling was disrupted, indicating that there might be additional nematode CLE receptors. In this study, we demonstrate that CLV1 and RECEPTOR-LIKE PROTEIN KINASE 2/TOADSTOOL2 (RPK2), two additional receptors that can transmit the CLV3 signal independent of CLV2/CRN for shoot apical meristem maintenance, also play a role in nematode CLE perception. Localization studies showed that both receptors are expressed in nematode-induced syncytia. Infection assays with clv1 and rpk2 single mutants revealed a decrease in both nematode infection and syncytium size. Significantly, further reduction in nematode infection was observed when rpk2 was combined with clv1 and clv2 mutants. Taken together, our results indicate that parallel signaling pathways involving CLV1, CLV2, and RPK2 are important for nematode parasitism.

The barren stalk2 Gene Is Required for Axillary Meristem Development in Maize.

The diversity of plant architecture is determined by axillary meristems (AMs). AMs are produced from small groups of stem cells in the axils of leaf primordia and generate vegetative branches and reproductive inflorescences. Previous studies identified genes critical for AM development that function in auxin biosynthesis, transport, and signaling. barren stalk1 (ba1), a basic helix-loop-helix transcription factor, acts downstream of auxin to control AM formation. Here, we report the cloning and characterization of barren stalk2 (ba2), a mutant that fails to produce ears and has fewer branches and spikelets in the tassel, indicating that ba2 functions in reproductive AM development. Furthermore, the ba2 mutation suppresses tiller growth in the teosinte branched1 mutant, indicating that ba2 also plays an essential role in vegetative AM development. The ba2 gene encodes a protein that co-localizes and heterodimerizes with BA1 in the nucleus. Characterization of the genetic interaction between ba2 and ba1 demonstrates that ba1 shows a gene dosage effect in ba2 mutants, providing further evidence that BA1 and BA2 act together in the same pathway. Characterization of the molecular and genetic interaction between ba2 and additional genes required for the regulation of ba1 further supports this finding. The ba1 and ba2 genes are orthologs of rice genes, LAX PANICLE1 (LAX1) and LAX2, respectively, hence providing insights into pathways controlling AMs development in grasses.

Hormone signaling in plant development.

Hormone signaling plays diverse and critical roles during plant development. In particular, hormone interactions regulate meristem function and therefore control formation of all organs in the plant. Recent advances have dissected commonalities and differences in the interaction of auxin and cytokinin in the regulation of shoot and root apical meristem function. In addition, brassinosteroid hormones have recently been discovered to regulate root apical meristem size. Further insights have also been made into our understanding of the mechanism of crosstalk among auxin, cytokinin, and strigolactone in axillary meristems.

CLAVATA signaling pathway receptors of Arabidopsis regulate cell proliferation in fruit organ formation as well as in meristems.

The CLAVATA1 (CLV1), CLV2, and CORYNE (CRN) receptors in Arabidopsis thaliana maintain cell proliferation in shoot apical meristems by restricting expression of the transcription factor WUSCHEL (WUS). Previously characterized receptor mutants generate extra fruit and floral organs that are proposed to arise from enlarged floral meristems (FMs). We identified new alleles in clv1, clv2, and crn and found that most mutants produce only extra fruit organs and generate FMs of similar dimensions as wild type. Characterization of gynoecium development in receptor mutants revealed increased cell proliferation and ectopic fruit organ initiation after FM termination. These regions of increased cell division also display expanded expression of the cell proliferation-promoting transcription factor SHOOTMERISTEMLESS (STM), similar to the expansion of WUS expression in the shoot apical meristems of strong clv1 mutants. We also examined genetic interactions between the ERECTA (ER) and BARELY ANY MERISTEM 1 (BAM1) receptor-like kinases and CLV pathway receptors. Our results suggest a model in which CLV1/BAM1 and CLV2/CRN complexes act in separate, parallel pathways in shoot meristems, while the CLV1, CLV2, and CRN receptors function together in a linear pathway during fruit development. These results demonstrate the importance of regulating cell proliferation in plants that undergo organogenesis throughout their life cycle.

Intragenic suppression of a trafficking-defective brassinosteroid receptor mutant in Arabidopsis.

The cell surface receptor kinase BRASSINOSTEROID-INSENSITIVE-1 (BRI1) is the major receptor for steroid hormones in Arabidopsis. Plants homozygous for loss-of-function mutations in BRI1 display a reduction in the size of vegetative organs, resulting in dwarfism. The recessive bri1-5 mutation produces receptors that do not accumulate to wild-type levels and are retained mainly in the endoplasmic reticulum. We have isolated a dominant suppressor of the dwarf phenotype of bri1-5 plants. We show that this suppression is caused by a second-site mutation in BRI1, bri1-5R1. The bri1-5R1 mutation partially rescues the phenotypes of bri1-5 in many tissues and enhances bri1-5 phenotypes above wild-type levels in several other tissues. We demonstrate that the phenotypes of bri1-5R1 plants are due to both increased cell expansion and increased cell division. To test the mechanism of bri1-5 suppression, we assessed whether the phenotypic suppression in bri1-5R1 was dependent on ligand availability and the integrity of the signaling pathway. Our results indicate that the suppression of the dwarf phenotypes associated with bri1-5R1 requires both BR biosynthesis and the receptor kinase BRI1-ASSOCIATED KINASE-1 (BAK1). Finally, we show that bri1-5R1 partially restores the accumulation and plasma membrane localization of BRI1. Collectively, our results point toward a model in which bri1-R1 compensates for the protein-folding abnormalities caused by bri1-5, restoring accumulation of the receptor and its delivery to the cell surface.