Altered Germination and Subcellular Localization Patterns for PUB44/SAUL1 in Response to Stress and Phytohormone Treatments

BACKGROUND: In plants, the ubiquitin-proteasome system is emerging as a significant regulatory system throughout the plant lifecycle. The ubiquitination of a target protein requires the sequential actions of the E1, E2 and E3 enzymes, with the latter E3 enzyme conferring target selection in this process. There are a large number of predicted E3 enzymes in plant genomes, and very little is known about the functions of many of these predicted genes. Here we report here an analysis of two closely-related members of the Arabidopsis Plant U-box (PUB) family of E3 ubiquitin ligases, PUB43 and PUB44. PRINCIPAL FINDINGS: Homozygous pub44/pub44 mutant seedlings were found displayed a seedling lethal phenotype and this corresponded with widespread cell death lesions throughout the cotyledons and roots. Interestingly, heterozygous PUB44/pub44 seedlings were wild-type in appearance yet displayed intermediate levels of cell death lesions in comparison to pub44/pub44 seedlings. In contrast, homozygous pub43/pub43 mutants were viable and did not show any signs of cell death despite the PUB43 gene being more highly expressed than PUB44. The PUB44 mutants are not classical lesion mimic mutants as they did not have increased resistance to plant pathogens. We also observed increased germination rates in mutant seeds for both PUB44 and PUB43 under inhibitory concentrations of abscisic acid. Finally, the subcellular localization of PUB44 was investigated with transient expression assays in BY-2 cells. Under varying conditions, PUB44 was observed to be localized to the cytoplasm, plasma membrane, or nucleus. CONCLUSIONS: Based on mutant plant analyses, the Arabidopsis PUB43 and PUB44 genes are proposed to function during seed germination and early seedling growth. Given PUB44's ability to shuttle from the nucleus to the plasma membrane, PUB44 may be active in different subcellular compartments as part of these biological functions

Role of autophagy in plant nutrient deficiency

One of the environmental stresses frequently encountered by plants is nutrient deficiency. Therefore, reuse of valuable cellular nutrients is an important trait in nutrient use efficiency (NUE). High NUE is a desired trait in plants at all developmental steps to reach maximum potentials with minimum inputs. Two highly conserved evolutionary mechanisms are responsible for protein turnover at the cellular level, the ubiquitin-proteasome system (UPS) and the autophagy pathway. Generally, UPS recycles short-lived regulatory proteins while autophagy recycles long-lived proteins, protein aggregates or organelles. The proteins, which are destined for degradation, are marked by a special polypeptide tag, ubiquitin. The features of this tag, as well as activity of ubiquitinating and deubiquitinating enzymes, are determinants that allocate the protein into one or the other degradation systems. Apart from the common subset of over 30 proteins required for the “core autophagy”, there exist selective autophagy cargo receptors. These proteins perform the quality control function by recognizing ubiquitinated cargoes (ready for degradation) and linking them to the autophagy machinery. Adequate knowledge of the processes of selective autophagy will be beneficial for agricultural production and the environment by delivering the methods and means for obtaining crops with improved NUE, higher yield and better stress tolerance