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Inorganic phosphate (Pi) is one of the most important nutrients for plant growth. Under Pi starvation, intracellular components are degraded for Pi recycling in plant cells. Macroautophagy/autophagy is a process for vacuolar degradation of cytoplasmic components including organelles, but it is still unclear whether this process is involved in plant growth and Pi recycling during Pi starvation. Recently, we reported that the degradation of endoplasmic reticulum (ER) by selective autophagy, termed reticulophagy, contributes to Pi recycling and is an important stress response in the early phase of Pi starvation. During this phase, oxidized lipids are accumulated in the plant cell in an iron ion-dependent manner, and this accumulation causes ER stress which induces reticulophagy. As a result, the Pi contents are maintained at a sufficient level during early Pi starvation, suppressing any late Pi starvation responses, such as membrane lipid remodeling. Thus, we proposed that ER stress-induced reticulophagy is an important Pi salvage system during the early phase of Pi starvation in plants.

Pexophagy plays a pivotal role in the selective removal of ROS-generating peroxisomes, which protects plants from oxidative damage during photosynthesis under high-intensity light.

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is “Pi recycling” in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling.

To preserve germination ability, plant seeds must be protected from environmental stresses during the storage period. Here, we demonstrate that autophagy, an intracellular degradation system, maintains seed germination ability in Arabidopsis thaliana. The germination ability of long-term (>5 years) stored dry seeds of autophagy-defective (atg) mutant and wild-type (WT) plants was compared. Long-term stored (old) seeds of atg mutants showed lower germination ability than WT seeds, although short-term stored (new) seeds of atg mutants did not show such a phenotype. After removal of the seed coat and endosperm from old atg mutant seeds, the embryos developed into seedlings. Autophagic flux was maintained in endosperm cells during the storage period, and autophagy defect resulted in the accumulation of oxidized proteins and accelerated endosperm cell death. Consistent with these findings, the transcripts of genes, ENDO-β-MANNANASE 7 and EXPANSIN 2, which are responsible for degradation/ remodeling of the endosperm cell wall during germination, were reduced in old atg mutant seeds. We conclude that autophagy maintains endosperm quality during seed storage by suppressing aging-dependent oxidative damage and cell death, which allows the endosperm to perform optimal functions during germination, i.e., cell wall degradation/remodeling, even after long-term storage. [ABSTRACT FROM AUTHOR]