The ZmMYB84‐ZmPKSB regulatory module controls male fertility through modulating anther cuticle—pollen exine trade‐off in maize anthers

Abstract

Summary Anther cuticle and pollen exine are two crucial lipid layers that ensure normal pollen development and pollen–stigma interaction for successful fertilization and seed production in plants. Their formation processes share certain common pathways of lipid biosynthesis and transport across four anther wall layers. However, molecular mechanism underlying a trade‐off of lipid‐metabolic products to promote the proper formation of the two lipid layers remains elusive. Here, we identified and characterized a maize male‐sterility mutant pksb, which displayed denser anther cuticle but thinner pollen exine as well as delayed tapetal degeneration compared with its wild type. Based on map‐based cloning and CRISPR/Cas9 mutagenesis, we found that the causal gene (ZmPKSB) of pksb mutant encoded an endoplasmic reticulum (ER)‐localized polyketide synthase (PKS) with catalytic activities to malonyl‐CoA and midchain‐fatty acyl‐CoA to generate triketide and tetraketide α‐pyrone. A conserved catalytic triad (C171, H320 and N353) was essential for its enzymatic activity. ZmPKSB was specifically expressed in maize anthers from stages S8b to S9‐10 with its peak at S9 and was directly activated by a transcription factor ZmMYB84. Moreover, loss function of ZmMYB84 resulted in denser anther cuticle but thinner pollen exine similar to the pksb mutant. The ZmMYB84‐ZmPKSB regulatory module controlled a trade‐off between anther cuticle and pollen exine formation by altering expression of a series of genes related to biosynthesis and transport of sporopollenin, cutin and wax. These findings provide new insights into the fine‐tuning regulation of lipid‐metabolic balance to precisely promote anther cuticle and pollen exine formation in plants.

A Systematic Investigation of Lipid Transfer Proteins Involved in Male Fertility and Other Biological Processes in Maize

“…Anther cuticles and pollen walls are composed of complex substances, including cutin, wax, and sporopollenin [ 47 ]. The precursors of sporopollenin, cutin, and wax are synthesized in anther tapetum, and catalyzed by a series of lipid metabolism-related GMS gene-encoding enzymes, such as ZmMs25/ZmFAR1 encoding a plastid-localized fatty acyl reductase [ 48 , 49 ], ZmMs33 encoding a glycerol-3-phosphate acyltransferase [ 50 , 51 , 52 , 53 ], ZmMs30 encoding a GDSL lipase [ 54 ], and ZmPKSB encoding a polyketide synthase [ 55 ]. Then, the lipid precursors are transported into the pollen surface and anther outer wall by ABCGs such as ZmABCG26 and ZmMs13/ZmABCG2a [ 48 , 56 , 57 ], LTPs such as Zmms44 [ 32 ], and other transporters [ 47 ].…”

CRISPR/Cas9‐based genome editing of 14 lipid metabolic genes reveals a sporopollenin metabolon ZmPKSB‐ZmTKPR1‐1/‐2 required for pollen exine formation in maize

“…First, ZmPKSB has a trade-off function on anther cuticle and pollen exine formation (Liu et al, 2022), but not for ZmTKPR1-1/-2. All nine cutin/wax-related genes were significantly upregulated in pksb anthers (Liu et al, 2022), but four of these genes (ZmKCAS-4, ZmGL8, ZmCER8 and ZmABCG2a) were significantly down-regulated in tkpr1-1/-2 anthers (Figure 7e). Correspondingly, the total contents of anther cutin and wax in tkpr1-1/-2 anthers were significantly lower than those in pksb anthers (Figure 7c,d).…”

“…ZmPKSB, ZmTKPR1-1 and ZmTKPR1-2 interact and form a multienzyme complex Growing evidence has proved that various proteins involved in lipid biosynthetic pathways can form dynamic protein-protein and protein-lipid interactomes or metabolons (Nakamura, 2017). Since ZmPKSB (Liu et al, 2022), ZmTKPR1-1 and -2 exhibit similar expression patterns during anther development and their encoded proteins are localized in the ER (Figures 4c-e and S6), we speculated that ZmPKSB, ZmTKPR1-1 and -2 may interact and form a complex to conduct their functions in maize anther development.…”

“…Second, since ZmPKSB and ZmTKPR1-1/-2 are located upstream and downstream of a sequential enzymatic reaction (Figure 6c,e), respectively, their mutations may lead to the accumulation of different intermediate lipid products. For example, according to the phenylpropanoid pathway for biosynthesis of sporopollenin precursors (Wan et al, 2020), loss-offunction of ZmPKSB probably causes excessive accumulation of midchain-and x-OH fatty acyl-CoA substrates (Liu et al, 2022).…”

The World Trends and Patterns of Grain Loss and Waste Research and Their Implications

“…Consequently, while the agricultural sector focuses on "source development," it is equally vital to prioritize "resource conservation." 22,23 This entails minimizing GLW while making optimal use of the resources that underpin the food system. Embracing this approach has become an imperative choice for society as a whole.…”