SEC1A is a major Arabidopsis Sec1/Munc18 gene in vesicle trafficking during pollen tube tip growth

Arabidopsis Sar1b is critical for pollen tube growth

Pollen tube growth is an essential step leading to reproductive success in flowering plants, in which vesicular trafficking plays a key role. Vesicular trafficking from endoplasmic reticulum to the Golgi apparatus is mediated by the coat protein complex II (COPII). A key component of COPII is small GTPase Sar1. Five Sar1 isoforms are encoded in the Arabidopsis genome and they show distinct while redundant roles in various cellular and developmental processes, especially in reproduction. Arabidopsis Sar1b is essential for sporophytic control of pollen development while Sar1b and Sar1c are critical for gametophytic control of pollen development. Because functional loss of Sar1b and Sar1c resulted in pollen abortion, whether they influence pollen tube growth was unclear. Here we demonstrate that Sar1b mediates pollen tube growth, in addition to its role in pollen development. Although functional loss of Sar1b does not affect pollen germination, it causes a significant reduction in male transmission and of pollen tube penetration of style. We further show that membrane dynamics at the apex of pollen tubes are compromised by Sar1b loss-of-function. Results presented provide further support of functional complexity of the Sar1 isoforms.

The hydroxyproline O-arabinosyltransferase FIN4 is required for tomato pollen intine development

The pollen grain cell wall is a highly specialized structure composed of distinct layers formed through complex developmental pathways. The production of the innermost intine layer, composed of cellulose, pectin and other polymers, is particularly poorly understood. Here we demonstrate an important and specific role for the hydroxyproline O-arabinosyltransferase (HPAT) FIN4 in tomato intine development. HPATs are plant-specific enzymes which initiate glycosylation of certain cell wall structural proteins and signaling peptides. FIN4 was expressed throughout pollen development in both the developing pollen and surrounding tapetal cells. A fin4 mutant with a partial deletion of the catalytic domain displayed significantly reduced male fertility in vivo and compromised pollen hydration and germination in vitro. However, fin4 pollen that successfully germinated formed morphologically normal pollen tubes with the same growth rate as the wild-type pollen. When we examined mature fin4 pollen, we found they were cytologically normal, and formed morphologically normal exine, but produced significantly thinner intine. During intine deposition at the late stages of pollen development we found fin4 pollen had altered polymer deposition, including reduced cellulose and increased detection of pectin, specifically homogalacturonan with both low and high degrees of methylesterification. Therefore, FIN4 plays an important role in intine formation and, in turn pollen hydration and germination and the process of intine formation involves dynamic changes in the developing pollen cell wall.

The triticale mature pollen and stigma proteomes - assembling the proteins for a productive encounter

Triticeae crops are major contributors to global food production and ensuring their capacity to reproduce and generate seeds is critical. However, despite their importance our knowledge of the proteins underlying Triticeae reproduction is severely lacking and this is not only true of pollen and stigma development, but also of their pivotal interaction. When the pollen grain and stigma are brought together they have each accumulated the proteins required for their intended meeting and accordingly studying their mature proteomes is bound to reveal proteins involved in their diverse and complex interactions. Using triticale as a Triticeae representative, gel-free shotgun proteomics was used to identify 11,533 and 2977 mature stigma and pollen proteins respectively. These datasets, by far the largest to date, provide unprecedented insights into the proteins participating in Triticeae pollen and stigma development and interactions. The study of the Triticeae stigma has been particularly neglected. To begin filling this knowledge gap, a developmental iTRAQ analysis was performed revealing 647 proteins displaying differential abundance as the stigma matures in preparation for pollination. An in-depth comparison to an equivalent Brassicaceae analysis divulged both conservation and diversification in the makeup and function of proteins involved in the pollen and stigma encounter. SIGNIFICANCE: Successful pollination brings together the mature pollen and stigma thus initiating an intricate series of molecular processes vital to crop reproduction. In the Triticeae crops (e.g. wheat, barley, rye, triticale) there persists a vast deficit in our knowledge of the proteins involved which needs to be addressed if we are to face the many upcoming challenges to crop production such as those associated with climate change. At maturity, both the pollen and stigma have acquired the protein complement necessary for their forthcoming encounter and investigating their proteomes will inevitably provide unprecedented insights into the proteins enabling their interactions. By combining the analysis of the most comprehensive Triticeae pollen and stigma global proteome datasets to date with developmental iTRAQ investigations, proteins implicated in the different phases of pollen-stigma interaction enabling pollen adhesion, recognition, hydration, germination and tube growth, as well as those underlying stigma development were revealed. Extensive comparisons between equivalent Triticeae and Brassiceae datasets highlighted both the conservation of biological processes in line with the shared goal of activating the pollen grain and promoting pollen tube invasion of the pistil to effect fertilization, as well as the significant distinctions in their proteomes consistent with the considerable differences in their biochemistry, physiology and morphology.

SEC1A and SEC6 synergistically regulate pollen tube polar growth

Pollen tube polar growth is a key physiological activity for angiosperms to complete double fertilization, which is highly dependent on the transport of polar substances mediated by secretory vesicles. The exocyst and Sec1/Munc18 (SM) proteins are involved in the regulation of the tethering and fusion of vesicles and plasma membranes, but the molecular mechanism by which they regulate pollen tube polar growth is still unclear. In this study, we found that loss of function of SEC1A, a member of the SM protein family in Arabidopsis thaliana, resulted in reducing pollen tube growth and a significant increase in pollen tube width. SEC1A was diffusely distributed in the pollen tube cytoplasm, and was more concentrated at the tip of the pollen tube. Through co-immunoprecipitation-mass spectrometry screening, protein interaction analysis and in vivo microscopy, we found that SEC1A interacted with the exocyst subunit SEC6, and they mutually affected the distribution and secretion rate at the tip of the pollen tube. Meanwhile, the functional loss of SEC1A and SEC6 significantly affected the distribution of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex member SYP125 at the tip of the pollen tube, and led to the disorder of pollen tube cell wall components. Genetic analysis revealed that the pollen tube-related phenotype of the sec1a sec6 double mutant was significantly enhanced compared with their respective single mutants. Therefore, we speculated that SEC1A and SEC6 cooperatively regulate the fusion of secretory vesicles and plasma membranes in pollen tubes, thereby affecting the length and the width of pollen tubes.

VPS45 is required for both diffuse and tip growth of Arabidopsis thaliana cells

INTRODUCTION

VPS45 belongs to the Sec1/Munc18 family of proteins, which interact with and regulate Qa-SNARE function during membrane fusion. We have shown previously that Arabidopsis thaliana VPS45 interacts with the SYP61/SYP41/VTI12 SNARE complex, which locates on the trans-Golgi network (TGN). It is required for SYP41 stability, and it functions in cargo trafficking to the vacuole and in cell expansion. It is also required for correct auxin distribution during gravitropism and lateral root growth.

RESULTS

As vps45 knockout mutation is lethal in Arabidopsis, we identified a mutant, vps45-3, with a point mutation in the VPS45 gene causing a serine 284-to-phenylalanine substitution. The VPS45-3 protein is stable and maintains interaction with SYP61 and SYP41. However, vps45-3 plants display severe growth defects with significantly reduced organ and cell size, similar to vps45 RNAi transgenic lines that have reduced VPS45 protein levels. Root hair and pollen tube elongation, both processes of tip growth, are highly compromised in vps45-3. Mutant root hairs are shorter and thicker than those of wild-type plants, and are wavy. These root hairs have vacuolar defects, containing many small vacuoles, compared with WT root hairs with a single large vacuole occupying much of the cell volume. Pollen tubes were also significantly shorter in vps45-3 compared to WT.

DISCUSSION

We thus show that VPS45 is essential for proper tip growth and propose that the observed vacuolar defects lead to loss of the turgor pressure needed for tip growth.