Lack of phosphoserine phosphatase activity alters pollen and tapetum development in Arabidopsis thaliana.

Formation of mature pollen grain, an essential process for the reproduction of higher plants, is affected in lines that are deficient in the enzymes of the phosphorylated pathway of serine biosynthesis (PPSB). Mutants of phosphoserine phosphatase (PSP), the enzyme that catalyses the last step of PPSB, are embryo-lethal. When they are complemented with a construct carrying PSP1 cDNA under the control of the 35S promoter (psp1.1 35S:PSP1), which is poorly expressed in anther tissues, plants display a wild-type phenotype, but are male-sterile. The pollen from the psp1.1 35S:PSP1 lines are shrunken and unviable. Here we report the morphological alterations that appear in the psp1.1 35S:PSP1 lines during microspore development. We show that the pollen wall from these lines presents a normal exine layer, but a shrunken and collapsed shape. Lack of PSP activity also affects oil bodies formation in the tapetosomes of tapetal cells which, in turn, may influence microspore pollen coat formation. All these results highlight the important role of the PPSB in the normal development of microspores in Arabidopsis thaliana.

Serine biosynthesis by photorespiratory and non-photorespiratory pathways: an interesting interplay with unknown regulatory networks.

Photorespiration is a primary metabolic pathway, which, given its energy costs, has often been viewed as a wasteful process. Despite having reached the consensus that one important function of photorespiration is the removal of toxic metabolite intermediates, other possible functions have emerged, and others could well emerge in the future. As a primary metabolic pathway, photorespiration interacts with other routes; however the nature of these interactions is not well known. One of these interacting pathways could be the biosynthesis of serine, since this amino acid is synthesised through photorespiratory and non-photorespiratory routes. At present, the exact contribution of each route to serine supply in different tissues and organs, their biological significance and how pathways are integrated and/or regulated remain unknown. Here, we review the non-photorespiratory serine biosynthetic pathways, their interactions with the photorespiratory pathway, their putative role in plants and their biotechnological interest.