Accumulation kinetics of cotton late embryogenesis-abundant mRNAs and storage protein mRNAs: coordinate regulation during embryogenesis and the role of abscisic acid.

The accumulation of total RNA transcripts of 18 late embryo-abundant (Lea) gene families, each encoding two closely related Lea mRNAs, was measured in cotyledon total RNA during embryogenesis and germination of Gossypium hirsutum L. by RNA dot hybridization. Transcript abundance of the three storage protein families was also followed. The Lea mRNAs belong to only two related groups of commonly regulated mRNAs. The transcript level of each of the 6 members of Class I has two transient maxima during early maturation and a maximum level at 3 days prior to desiccation. The transcript level of each of the 12 members of Class II increases abruptly in late maturation with a maximum concentration at 3 days before desiccation (Class IIA) or at desiccation (Class IIB). Several patterns of early accumulation also exist within Class II, some overlapping with those of storage protein or Lea Class I mRNAs. The concentrations of Lea mRNAs increase at least 10- to 1700-fold during embryogenesis and decline 15- to 220-fold during the first day of germination. Earlier studies indicated that most Lea mRNAs, but no storage protein mRNAs, are induced in excised embryos exposed to abscisic acid (ABA). Free (+)ABA was measured during embryogenesis using a monoclonal antibody-based ELISA. The ABA concentration shows maxima correlating with the maxima in early Class I Lea mRNA abundance. However, ABA declines during the accumulation of Class I and Class II Lea mRNAs in late embryogenesis. Consequently, ABA could be an endogenous regulator of Class I Lea mRNAs during early maturation but is not the primary regulator of Lea mRNAs during late maturation. In contrast, cotyledon water potential exhibits a decrease that correlates with the late induction of Lea mRNAs. These findings are consistent with the hypothesis that low water potential is the endogenous regulator of some ABA-inducible mRNAs. The cessation of vascular flow to the embryo may also be involved in the disappearance of storage protein mRNAs and the late induction of Lea mRNAs.