A shrunken-2 Transgene Increases Maize Yield by Acting in Maternal Tissues to Increase the Frequency of Seed Development

Abstract

The maize (Zea mays) shrunken-2 (Sh2) gene encodes the large subunit of the rate-limiting starch biosynthetic enzyme, ADPglucose pyrophosphorylase. Expression of a transgenic form of the enzyme with enhanced heat stability and reduced phosphate inhibition increased maize yield up to 64%. The extent of the yield increase is dependent on temperatures during the first 4 d post pollination, and yield is increased if average daily high temperatures exceed 33°C. As found in wheat (Triticum aestivum) and rice (Oryza sativa), this transgene increases maize yield by increasing seed number. This result was surprising, since an entire series of historic observations at the whole-plant, enzyme, gene, and physiological levels pointed to Sh2 playing an important role only in the endosperm. Here, we present several lines of evidence that lead to the conclusion that the Sh2 transgene functions in maternal tissue to increase seed number and, in turn, yield. Furthermore, the transgene does not increase ovary number; rather, it increases the probability that a seed will develop. Surprisingly, the number of fully developed seeds is only ;50% of the number of ovaries in wild-type maize. This suggests that increasing the frequency of seed development is a feasible agricultural target, especially under conditions of elevated temperatures.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

“…Most efforts involved the over-expression of AGPase via seed-specific promoters in major cereals [166][167][168][169][170][171]. However, in all of the above cases, the cause of yield gains was unexpected; increased seed number (rather than seed size) through reduced ovule abortion, or increased biomass of maternal tissues.…”

Enhancing the heat stability and kinetic parameters of the maize endosperm ADP-glucose pyrophosphorylase using iterative saturation mutagenesis

“…In practice, however, the number of sequence variants at the DNA level (64 517 for the gene studied here) means that the required library sizes vastly outstrip the ability of molecular biology to construct them and the ability of practical screening methods to evaluate them. Even with the necessarily sparse exploration of sequence space, such efforts have yielded modestly-improved variants of AGPase [3]; however, it is highly unlikely that the ''best'' possible variant has been identified by these studies. We have therefore chosen a more targeted strategy to improve the heat stability of maize AGPase.…”

“…Increasing the thermostability of this enzyme therefore has great agronomic importance [3]. Previously, variants of this enzyme were transformed into maize [3,4], potato [5], wheat [6,7] and rice [8,9] and yield increases in seed number or weight were obtained. Recently, an evolution-directed approach was applied to maize endosperm AGPase.…”

“…The heat lability of this AGPase is often linked to grain loss during hot weather (reviewed in [2]). Increasing the thermostability of this enzyme therefore has great agronomic importance [3]. Previously, variants of this enzyme were transformed into maize [3,4], potato [5], wheat [6,7] and rice [8,9] and yield increases in seed number or weight were obtained.…”

High throughput transformation of a Sorghum cDNA library for rice improvement

“…Increasing evidence has documented that alien genes from other plants can be employed for rice improvement. Previous reports show that a gene encoding the large subunit of AGPase as a ratelimiting enzyme in starch synthesis, maize shrunken-2, can increase rice yield by 23 % (Hannah et al 2012). Zmhdz10, a drought-induced HD-Zip I transcription factor overexpressed in both rice and Arabidopsis leads to enhanced drought and salt tolerance (Zhao et al 2014).…”