Establishment of the chloroplast genetic system in rice during early leaf development and at low temperatures

Abstract

Chloroplasts are the central nodes of the metabolic network in leaf cells of higher plants, and the conversion of proplastids into chloroplasts is tightly coupled to leaf development. During early leaf development, the structure and function of the chloroplasts differ greatly from those in a mature leaf, suggesting the existence of a stage-specific mechanism regulating chloroplast development during this period. Here, we discuss the identification of the genes affected in low temperature-conditional mutants of rice (Oryza sativa). These genes encode factors involved in chloroplast rRNA regulation (NUS1), and nucleotide metabolism in mitochondria, chloroplasts, and cytosol (V2 , V3, ST1). These genes are all preferentially expressed in the early leaf developmental stage P4, and depleting them causes altered chloroplast transcription and translation, and ultimately leaf chlorosis. Therefore, it is suggested that regulation of cellular nucleotide pools and nucleotide metabolism is indispensable for chloroplast development under low temperatures at this stage. This review summarizes the current understanding of these factors and discusses their roles in chloroplast biogenesis.

Keywords: Oryza sativa; chloroplast transcription; low temperature; nucleotide metabolism; translation.

FIGURE 1

(A) Phenotypes of the wild-type (Wt) and virescent mutants (v₁, v₂, and v₃) grown at a restrictive temperature (20°C). (B) Schematic of a rice seedling with a fully expanded third leaf. L1, L2, L3, and L4 indicate the first, second, third, and fourth leaf, respectively. Developmental stages (P0–P6) are also indicated. shoot base, SB corresponds to a 5 mm section from the bottom of the shoot and contains pre-emerged leaves at P0–P3 stages. (C) Plastid gene expression patterns during leaf development (Kusumi et al., 2010a). Horizontal bars indicate the leaf developmental stages.

FIGURE 2

Representation of de novo purine nucleotide synthesis in chloroplasts and cytosol (Sugimoto et al., 2007; Yoo et al., 2009; Kusumi et al., 2011; Nomura et al., 2014). Proteins whose function has been linked to chloroplast development at cold temperature are depicted in red. The names of the mutation for each protein are indicated below. Orange dotted arrows indicate hypothetical routes for membrane transport or enzyme conversions. PRPP, 5-phosphoribosyl 1-pyrophosphate; IMP, 5’-monophosphate; AdS, adenylosuccinat.