ESCRT components ISTL1 andLIP5 are required for tapetal function and pollen viability.

Pollen wall assembly is crucial for pollen development and plant fertility. The durable biopolymer sporopollenin and the constituents of the tryphine coat are delivered to developing pollen grains by the highly coordinated secretory activity of the surrounding tapetal cells. The role of membrane trafficking in this process, however, is largely unknown. In this study, we used Arabidopsis thaliana to characterize the role of two late-acting endosomal sorting complex required for transport (ESCRT) components, ISTL1 and LIP5, in tapetal function. Plants lacking ISTL1 and LIP5 form pollen with aberrant exine patterns, leading to partial pollen lethality. We found that ISTL1 and LIP5 are required for exocytosis of plasma membrane and secreted proteins in the tapetal cells at the free microspore stage, contributing to pollen wall development and tryphine deposition. Whereas the ESCRT machinery is well known for its role in endosomal trafficking, the function of ISTL1 and LIP5 in exocytosis is not a typical ESCRT function. The istl1 lip5 double mutants also show reduced intralumenal vesicle concatenation in multivesicular endosomes in both tapetal cells and developing pollen grains as well as morphological defects in early endosomes/trans-Golgi networks, suggesting that late ESCRT components function in the early endosomal pathway and exocytosis.

Functional Overlap of Long-Chain Acyl-CoA Synthetases in Arabidopsis.

Long-chain acyl-CoA synthetases (LACSs) play diverse and essential roles in lipid metabolism. The genomes of model eukaryotic organisms encode multiple LACS genes, and the substrate specificities of LACS homologs often overlap substantially. Homologous LACSs tend to differ in their expression patterns, localizations, and, by extension, the metabolic pathways to which they contribute. The Arabidopsis genome encodes a family of nine LACS genes, which have been characterized largely by reverse genetic analysis of mutant phenotypes. Because of redundancy, distinguishing the contributions of some Arabidopsis LACS genes has been challenging. Here, we have attempted to clarify the functions of LACSs that functionally overlap by synopsizing the results of previous work, isolating a suite of higher-order mutants that were previously lacking, and analyzing oil, wax, cutin, cuticle permeability, fertility and growth phenotypes. LACS1, LACS2, LACS4, LACS8 and LACS9 all affect cuticular lipid metabolism, but have different precise roles. Seed set, seed weight and storage oil amounts of higher-order lacs1, lacs2, lacs4, lacs8 and lacs9 mutants vary greatly, with these traits subject to different effects of fertility and oil synthesis defects. LACS4, LACS8 and LACS9 have partially redundant roles in development, as lacs4 lacs8 and lacs4 lacs9 double mutants are dwarf. lacs4 lacs8 lacs9 triple mutants were not recovered, and are assumed to be non-viable. Together, these results sketch a complex network of functions and functional interactions within the Arabidopsis LACS gene family.

ABCG transporters are required for suberin and pollen wall extracellular barriers in Arabidopsis.

Effective regulation of water balance in plants requires localized extracellular barriers that control water and solute movement. We describe a clade of five Arabidopsis thaliana ABCG half-transporters that are required for synthesis of an effective suberin barrier in roots and seed coats (ABCG2, ABCG6, and ABCG20) and for synthesis of an intact pollen wall (ABCG1 and ABCG16). Seed coats of abcg2 abcg6 abcg20 triple mutant plants had increased permeability to tetrazolium red and decreased suberin content. The root system of triple mutant plants was more permeable to water and salts in a zone complementary to that affected by the Casparian strip. Suberin of mutant roots and seed coats had distorted lamellar structure and reduced proportions of aliphatic components. Root wax from the mutant was deficient in alkylhydroxycinnamate esters. These mutant plants also had few lateral roots and precocious secondary growth in primary roots. abcg1 abcg16 double mutants defective in the other two members of the clade had pollen with defects in the nexine layer of the tapetum-derived exine pollen wall and in the pollen-derived intine layer. Mutant pollen collapsed at the time of anther desiccation. These mutants reveal transport requirements for barrier synthesis as well as physiological and developmental consequences of barrier deficiency.